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Glossary of Terms

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Terms Used in Arboriculture

Like anything complex, landscape care has its own jargon. If you're seeking clarity about a term tossed around about one of your trees, take a look at our glossary. You're also welcome to contact one of our arborists for clarification anytime.

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

A's

abiotic [ey-bahy-ot-ik]
characterized by the absence of life or living organisms.

abscission [ab-sizh-uhn]
the normal separation of flowers, fruit and leaves from plants.

absorbing roots [ab-sawr-bing roots]
 fine, fibrous roots, usually in the top 12 inches of soil, together with microscopic outgrowths called root hair, that take up water and minerals.

acid [as-id]
having a pH less than 7.0. See alkaline.

actinomycetes [ak-tin-oh-mahy-seets ]
a group of rod-shaped or filamentous soil bacteria similar to fungi that help to break down organic matter and release minerals from the soil.

acuminate [uh-kyoo-muh-nit]
leaves that taper sharply at their tip.

adventitious [ad-vuhn-tish-uhs]
arising abnormally from parts of the root or stem.

adventitious bud [ad-vuhn-tish-uhs buhd]
bud arising from an abnormal place, such as a root.

adventitious roots [ad-vuhn-tish-uhs  roots]
arising from an abnormal place, such as a stem.

aeration [air-ey-shuhn]
providing air to the soil to lessen soil compaction and improve drainage.

air excavator [air eks-kuh-vey-ter]
a hand-held machine that is used to excavate soil with a jet of compressed air. Typically used within the root zone of trees to safely expose roots, or to expose pipes or underground structures. Often referred to as an Air Spade®, the most popular brand.

Air Spade® [air speyd]
popular brand of air excavator.

alkaline [al-kuh-lahyn]
having a pH greater than 7.0. See acid.

allelochemicals [uh-lee-lo-kem-i-kuhls]
substances produced naturally by plants as part of a defense against pests and other plants. May adversely affect the growth and development of other plants.

allelopathy [uh-lee-lop-uh-thee]
a process by which an organism produces biochemicals that influence the growth of other organisms. Some plants, such as the Black Walnut tree (Juglans nigra) and Tree of Heaven (Ailanthus altissima) produce allelochemicals that adversely affect the growth and development of other plants.

alternate [awl-ter-neyt]
in a tree this refers to an arrangement with one leaf or bud at each node, where each subsequent leaf changes sides. Other arrangements are opposite and whorled.

alternate host [awl-ter-neyt hohst]
one of a number of separate obligate hosts to the different life stages of certain pathogens, such as rusts, which must alternate between hosts.

anatomy [uh-nat-uh-mee]
the structure of an organism or any of its parts. Contrast with taxonomy, morphology and physiology.

angiosperm [an-jee-uh-spurm]
a flowering plant. A plant with seeds enclosed in an ovary.The largest phylum of living plants. Contrast with gymnosperm.

annual [an-yoo-uhl
a plant that completes its life cycle in one year.

annual ring [an-yoo-uhl ring]
ring associated with yearly growth, seen in the cross-section of trees. Growth ring.

anthracnose [an-thrak-nohs]:  a group of fungal diseases of trees and plants typically characterized by leaf spots and dead tissue. Anthracnose refers to a broad spectrum of diseases, many of them specific to a single species.  Some forms of anthracnose can be controlled, while others are deadly to the host plant.

anthocyanin [an-thuh-sahy-uh-nin]
any of a class of water-soluble pigments that give flowers colors ranging from red to blue. See carotinoid and xanthophyll.

antidessicant [an-tee-des-i-kuhnt]
a substance applied to leaves to reduce water loss.

apex (pl. apices) [ey-peks; ey-puh-seez]
the growing point of a shoot.

apical [ey-pi-kuhl]
in botany, having to do with the tip of a leaf or stem.

arboriculture [ahr-ber-i-kuhl-cher, ahr-bawr- i-kuhl-cher ]
the study of the cultivation of trees and shrubs.

arborist [ahr-ber-ist]
a tree and shrub care professional trained in diagnosing, preventing and treating tree diseases, controlling pests, pruning and tree removal.

available water [uh-vey-luh-buhl waw-ter]
tree-available water, the amount of groundwater stored in micropores in the soil that is accessible to plants, between the permanent wilting point (too little to sustain life) and field capacity (the largest amount possible for the soil to retain). Also called capillary water. See gravitational water.

axil [ak-sil]
the angle between the upper side of a branch, leaf or petiole and the supporting stem or branch.

axillary bud [ak-suh-ler-ee buhd]
a bud in the axil of a leaf capable of developing into a branch shoot or flower.

 

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B's

backfill [bak-fil]
(1)n.  soil used for refilling an excavated hole. (2) v. to refill an excavated hole.

balled and burlapped (B&B) [bawld  and bur-lapt]
tree or other plant dug and removed from the ground for re-planting, with the roots and soil wrapped in burlap or a similar fabric.

bare root [bair root]
tree or other plant removed from the ground for re-planting without soil around the roots.

bark [bahrk]
the external covering of woody stems.

bark tracing [bahrk trey-sing]
a technique that promotes tree healing by cutting away torn or injured bark to leave a smooth edge.

barrier zone [bar-ee-er zohn]
new wood created by trees in response to injury containing natural fungicides. The barrier zone restricts the spread of decay.

biennial [bahy-en-ee-uhl]
a plant completing its life in two years. It usually grows vegetatively the first year, then flowers and fruits the second year. Compare to annual and perennial.

biodegradable [bahy-oh-di-grey-duh-buhl]
capable of being broken down through the action of living organisms.

biological control [bahy-uh-loj-i-kuhl  kuhn-trohl]
managing pests, diseases or weeds through the use of other living organisms. Also called biocontrol.

biorational [bahy-oh rash-uh-nl ]
any type of insecticide active against pest populations, but relatively innocuous to non-target organisms, and, therefore, non-disruptive to biological control. Some, but not all, biorational products are organic.

biotic [bahy-ot-ik]
pertaining to living organisms

bipinnate [bahy-pin-eyt]
pinnately compound leaves in which the leaflets are themselves pinnately compound; also called "twice-pinnate."

bleeding [blee-ding]
in a tree, the flow of sap from a wound.   

blight [blahyt]
the rapid discoloration, wilting and death of plant tissue. Can also refer to a disease with these characteristics.

bole [bohl]
the main trunk of a tree below the branches.

botanicals [buh-tan-i-kuhls]
compounds made from plants. In arboriculture, botanicals are plant-based substances used for the treatment or prevention of diseases or pests.

bracing [brey-sing]
installing metal rods in trees for additional support.

branch bark ridge [branch bahrk rij]
raised strip of bark that forms in a branch crotch.

branch collar [branch kol-er]
a "shoulder" or bulge formed at the base of a branch by the annual production of overlapping layers of branch and stem tissue

branch union [branch yoon-yuhn]
point where a branch attaches to a trunk or another branch.

bridge graft [brij graft]
a method of repairing a damaged trunk in which tree cuttings are grafted above and below the trunk injury to reconnect the tree’s flow of nutrients to the roots.

broadcast fertilization [brawd-kast fur-tl-ahy- zey-shuhn]
the application of fertilizer over the soil surface. Contrast with drill-hole fertilization and liquid fertilization.

brown rot [broun rot]
a fungus that breaks down the cellulose and hemicellulose in wood,  causing shrinking cracked, wood with a brown discoloration. Often referred to as dry rot. Contrast with soft rot and white rot.

bud [buhd]
small axillary or terminal protuberance on the stem of a plant that may develop into a flower or shoot.

buffering capacity [buhf-er –ring   kuh-pas-i-tee]
ability of a soil to maintain (i.e., resist change in) its pH.

burl  [burl]
a tree growth in which the grain has grown in a deformed manner.

buttress roots [buh-tris roots]
roots at the trunk base, extending above the ground.

butt rot [buht rot ]
fungal rot in the lowest portion of a tree trunk, where it meets the ground.

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C's

cabling [key-bling]
stabilizing a tree by inserting a cable through branches high in the canopy.

callus [kal-uhs]
the tissue that forms over the wounds of plants, protecting the inner tissue and causing healing.

cambium [kam-bee-uhm]
a thin layer of meristematic tissue between the inner bark (phloem) and wood (xylem), where annual rings are formed.

canker [kang-ker]
localized diseased area on stems, roots, and branches.

canopy [kan-uh-pee]
the branches and foliage of a tree or group of trees’ crowns.

capillary water [kap-uh-ler-ee waw-ter] 
see available water.

carotinoid [kuh-rot-n-oid]
any of a group of red and yellow pigments contained in some plants. See xanthophyll and anthocyanin.

causal agent [kaw-zuhey-juhnt]
organism or other  factor that induces a disease or disorder.

cavity [kav-i-tee]
a hollow within a tree stem, usually caused by decay.

cell [sel ]
the basic structural and functional unit of living organisms.

cellulose
[sel-yuh-lohs]
a complex carbohydrate that is the chief component of plant cell walls.

central leader [sen-truhl lee-der]
main stem of a tree.

chelates [kee-leyts]
in plants, organic matter that keep nutrients (usually iron) soluble and available for plant absorption.

chipping [chip-ping]
turning wood into wood chips, usually with the use of a chipping machine.

chlorophyll [klawr-uh-fil]
the green pigment of plants, essential for photosynthesis.

chloroplast  [klawr-uh-plast]
a specialized organelle that capture light energy for photosynthesis.

chlorosi [klaw-roh-sis]
an abnormal yellow plant discoloration caused by a nutrient deficiency that makes the plant unable to produce chlorophyll.

class [klas]
taxonomic group below the division level but above the order level.

clay [kley]
soil particles with a typical grain size less than 0.002 millimeter or a soil predominantly comprised of these particles. Grains are smaller than sand or silt.

CODIT [koh-det]
acronym for Compartmentalization of Decay in Trees. See compartmentalization.

codominant branches/codominant stems [koh-dom-uh-nuhnt]
two or more main stems, nearly the same size in diameter, that emerge from the same location on the main trunk.

colloid [kol-oid]
in soil, tiny particles that behave as a repository of nutrients and moisture and buffer the variations of soil solution ions.

companion cells [kuhm-pan-yuhn sels]
a phloem cell containing a nucleus, adjacent to a sieve tube.

compartmentalization [kuhm-pahrt-men-tl-luh-zey-shuhn]
a natural defense process in trees that enables them to seal off a wounded area to limit the spread of disease and decay. Often referred to as CODIT: Compartmentalization of Decay in Trees.

compost [kom-pohst]
(noun) a mixture of various decaying organic substances, such as dead leaves or manure, used for fertilizing soil. (verb) to create compost.

compost tea [kom-pohst tee]
a liquid soil nutrient containing live beneficial organisms.

compound leaf [kom-pound leef ]
a leaf with two or more leaflets. Contrast with simple leaf.

compression wood [kuhm-presh-uhn wood]
reaction wood in conifers that develops on the underside of branches or leaning trunks.

conduction [kuhn-duhk-shuhn]
 the movement of water or nutrients within a plant. 

conifer [koh-nuh-fer]
cone-bearing trees or shrubs, typically evergreen.

conk [kongk]
the fruiting body or nonfruiting body (sterile conk) of a fungus, often associated with decay.

controlled-release fertilizer [kuhn-trohld ree-lees fur-tl-ahy-zer]
gradual-release or slowly soluble form of fertilizer.

cordate [kawr-deyt]
heart-shaped. Used to describe a leaf, such as those of the Eastern Redbud.

cork cambium [kawrk kam-bee-uhm ]
meristematic tissue from which forms the outer layer of bark.

cotyledon [kot-l-eed-n]
A leaf of the embryo of a seed-bearing plant. 

crenate [kree-neyt]
having a notched or scalloped margin, as a leaf.

critical root zone [krit-i-kuhl  root zohn] the soil area around a tree where the roots are located. Often estimated in inches as 12 x the tree diameter.

crook [krook]
abrupt bend in a branch or trunk.

cross section [kros sek-shuhn]
section perpendicular to the axis of longitudinal growth.

crotch [kroch]
branch union; fork.

crown [kroun ]
upper part of a tree, including all the branches and foliage.

crown cleaning [kroun klee-ning]
selectively removing dead, dying, diseased, and broken branches from the tree.

crown raising: [kroun rey-zing ]
selectively pruning lower limbs from a tree crown in order to provide clearance.

crown reduction [kroun ri-duhk-shuhn]
selectively pruning to reduce the height and/or spread of a tree.

crown restoration [kroun  res-tuh-rey-shuhn]
pruning to restore the natural growth habit of a tree that has been damaged by tree topping or other problems. Restoration pruning.

crown rot [kroun  rot ]
disease or decay at the base of a tree (root crown).

crown thinning [kroun  thin-ing]
selectively pruning live branches to reduce crown density.

cultivar [kuhl-tuh-vahr ]
“cultivated variety” of a plant. Reproduced from cuttings rather than seeds. See variety.

cultural control [kuhl-cher-uhl kuhn-trohl ]
modifiying a growing environment to reduce the prevalence of unwanted pests, e.g. changing the soil pH, amount of sunlight, temperature etc.

cuticle [kyoo-ti-kuhl]:
a waxy layer covering a leaf or other part that retards water loss and insect damage, e.g. the natural waxy covering of apples.

cytokinins [sahy-tuh-kahy-nin]
plant hormones, produced by the roots, that travel upward through the xylem and promote tissue growth and budding.

cytoplasm [sahy-tuh-plaz-uhm]
the cell substance between the cell membrane and the nucleus, containing the cytosol, organelles, cytoskeleton and various particles.

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D's

DBH: acronym for tree Diameter at Breast Height. Measured at 1.4 meters (4.5 feet) above ground in the United States, 

decay [dih-key]
(noun) an area of wood that is undergoing decomposition; (verb) rot.

deciduous [dih-sij-oo-uhs]:
trees that shed their leaves annually.  Contrast with evergreen.

decline [dih-klahyn]
the gradually diminishing health or condition of a tree.

decurrent [dih-kur-uhnt]
rounded or spreading growth habit of the tree crown. Contrast with excurrent.

deficiency [dih-fish-uhn-see]
regarding plants, an insufficient quantity of a required nutrient.

defoliation [dee-foh-lee-ey-shuhn]
leaf loss or removal.

dehiscence [dih-his-uhns]
In plants, the natural bursting of capsules (e.g. fruit) to release their contents.

denitrification [dee-nahy-truh-fah-kay-shuhn]
process of reducing nitrates to nitrogen gases.

dentate [den-teyt]
having serrations (teeth) perpendicular to the leaf margin. Contrast with serrate.

desiccation [des-i-key-shuhn]
severe dehydration.

determinate growth  [di-tur-muh-nit grohth] 
in botany, growth that stops once a genetically pre-determined structure has formed. Contrast with indeterminate growth.

dicot [dahy-kot]
See dicotyledon.

dicotyledon (dicot) [dahy-kot-l-eed-n; dahy-kot] 
An angiosperm that is not a monocotyledon, having two cotyledons in the seed.

dieback [dahy-bak]
condition in which the branches in the tree crown die from the tips inward.

diffuse porous [dih-fyoos-pawr-uhs]
pattern of wood development in which the pore size is the same throughout the annual ring. See ring porous.

dioecious [dahy-ee-shuhs]
species of plants in which male and female flowers are on separate plants. See monoecious.

disease [dih-zeez]
condition that impairs the performance of one or more vital functions. Usually associated with infectious agents. Different from disorder.

disorder [dis-awr-der] 
abnormal condition that impairs the performance of one or more vital functions. Usually associated with noninfectious agents. Different from disease.

division [dih-vizh-uhn]
for plants, the primary subdivision of a taxonomic kingdom, grouping together all organisms that have the same body plan.  Syn. phylum.

dormancy [dawr-muhn-see]
a state of minimal metabolic activity and cessation of growth, typically part of an annual cycle.

dormant [dawr-muhnt]
in a period of dormancy.

dormant bud [dawr-muhnt buhd]
 a bud that has not been stimulated to grow.

dormant oil [dawr-muhnt oil]
botanic or petroleum-based oil sprayed on plants during the season of dormancy to suppress insect pests. See horticultural oil.

double serrate [duhb-uhl ser-eyt]
the toothed margin of a leaf, with smaller teeth between the teeth.

downy mildew [dou-nee mil-doo]
fungi  that develop during wet periods, usually on the underside of a leaf. See powdery mildew.

drainage  [drey-nij]
the removal of surface and sub-surface water. Drainage is impacted by the particle size of the soil components, compaction, and other factors.

drill-hole fertilization [dril-hohl fur-tl-ahy- zey-shuhn]
applying fertilizer by drilling holes in the soil within the root zone. Contrast with broadcast fertilization and liquid fertilization.

drip irrigation  [drip ir-i-gey-shuhn]
a system of plant irrigation involving the controlled delivery ofwater directly to individual plants through a network of tubes or pipes.

drip line [drip lahyn]
the area defined by the outermost circumference of a tree canopy.

dry rot [drahy rot]
see brown rot.

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E's

earlywood [ur-lee-wood]
the portion of an annual ring formed during the spring, characterized by large cells and thin walls, often lighter in color. Contrast with latewood.

ecosystem [ek-oh-sis-tuhm]
a system consisting of living organisms and their environment.

ectomycorrhizae [ek-tuh-mahy-kuh-rahy-zee]
type of mycorrhizae that form a non-penetrating an outer sheath around the root. See mycorrhizae. Contast with endomycorrhizae.

edema [ih-dee-muh]
watery swelling in plant tissue.

endomychorrhizae [en-doh-mahy-kuh-rahy-zee]
type of mycorrhizae that grow within root cells. See mycorrhizae. Contrast with ectomycorrihizae.

entire [en-tahyuhr]
a leaf with a smooth margin, without serration or lobes.

epicormic shoot [ep-i-kawrm-mik shoot]
a shoot arising from a latent or adventitious bud.

epidermis [ep-i-dur-mis] 
the thin outer layer of cells of seed plants and ferns.

epinasty [ep-uh-nas-tee]
increased growth on the upper side of a leaf, causing it to bend downward. Often a symptom of plant disease, or herbicide damage.

epiphyte [ep-uh-fahyt]
nonparasitic plant that grows upon another plant, taking nutrition and moisture from surrounding air, rain and debris. See parasite, saprophyte.

eradication [ih-rad-i-key-shun]
completely removing an organism from particular area.

espalier [ih-spal-yer]
(1) (noun)a trellis or framework used to grow a tree or shrub within a plane; (2) (noun) a plant trained in that manner; (3) (verb) to train plants in that manner.

essential elements [uh-sen-shuhl el-uh-muhnts]
17 elements essential to the growth and development of trees:  carbon (C), hydrogen (H), oxygen (O), phosphorus (P), potassium (K), nitrogen (N), sulfur (S), calcium (Ca), iron (Fe), magnesium (Mg), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), chlorine (Cl), and nickel (Ni). The group contains both macronutrients and micronutrients.

evapotranspiration [ih-vap-oh-tran-spuh-rey-shuhn]
return of water to the atmosphere by evaporation from the soil surface and transpiration by plants.

evergreen [ev-er-green]
tree or other plant that does not shed all of its foliage annually. Contrast with deciduous.

exfoliating [eks-foh-lee-ey-ting]
in a tree, bark that peels off in shreds or layers.

exogenous [ek-soj-uh-nuhs]
having stems that grow by the addition of wood to the outside, beneath the bark.

exotic species [ig-zot-ik spee-sheez]
a species not native to a region.  See native species, invasive species, naturalized species, introduced species.

exotropy [ek-soh-truh-pee]
the ability of a root to resume its original direction of growth after being deflected by an object.

exudation [eks-yoo-dey-shuhn]
oozing from pores or breaks in cell membranes.

 

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F's

family [fam-uh-lee]
taxonomic group under the order level and above the genus level.

fascicle [fas-i-kuhl]
a small cluster of leaves, flowers or needles.

fertilization [fur-tl-uh-zey-shuhn]
(1) the application of essential minerals to a plant, usually through the soil. (2) pollination.

fertilizer [fur-tl-ahy-zer]
substance added to a plant or the surrounding soil to supplement the supply of essential elements.

fertilizer analysis [fur-tl-ahy-zer uh-nal-uh-sis]
percentage of nitrogen (N), phosphorus (P), and potassium (K) in a fertilizer.

field capacity [feeld kuh-pas-i-tee]
the maximum amount of water that soil can hold

flagging [flag-ing]
in trees, wilting branches and browning foliage; also refers to trees exposed to harsh winds that only retain foliage on one side.

flush cut [fluhsh kuht]
pruning cut through and/or removing the branch collar, causing unnecessary injury to the trunk or parent stem.

foliage [foh-lee-ij]
the leaves of a plant or tree; leaves in general.

foliar analysis [foh-lee-er uh-nal-uh-sis]
laboratory analysis of the mineral content of foliage.

foliar application [foh-lee-er ap-li-key-shuhn]
applying a fertilizer, pesticide, or other substance directly to the leaves.

forestry [fawr-uh-stree]
the science of planting and taking care of trees and forests.

frass [fras]
fecal material and/or wood shavings produced by insects.

fronds [fronds]
large, divided leaf structures found in palms and ferns.

frost crack [frawst krak]
vertical split in the bark of a tree caused by the rapid contraction and expansion of water inside the tree combined with a vulnerability in the bark.

fruiting body [froo-ting bod-ee]
the spore-containing part of a fungus. When visible on a tree, usually an indication of fungal invasion beneath.

fungicides [fuhn-juh-sahyds]
substances toxic to fungi.

fungus (pl. fungi) [fuhng-guhs] [fuhn-jahy]
group of organisms from the kingdom Fungi, including yeasts, molds, mushrooms, rusts and smuts.

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G's

gall [gawl]
abnormal swelling of plant tissues caused by insects, fungi or bacteria. Contrast with burl.

genus [jee -nuhs] 
Botanical classification under the family level and above species.

geotropism [jee-uh-troh-piz-uhm] 
plant growth produced as a response to the force of gravity, either positive as in the direction of gravity (roots) or negative as in opposite the direction of gravity (shoots).

gibberellins [jib-bur-rehl-lens] 
group of plant hormones involved in cell elongation and other physiological processes.

girdling [guhrd-ling]
in tree, restriction or destruction of the vascular system within a root, stem, or branch that causes an inhibition of the flow of water in the phloem.

girdling root [guhrd-ling root]
a root that encircles all or part of the trunk of a tree or other roots, constricting the vascular tissue and inhibiting the movement of water and photosynthates.

grade [greyd]
(noun) the slope, or percentage of change, in the surface level of the ground (3) (verb) to change or groom the surface level or contours of the ground.

grading [greyd-ing]
in a landscape, the contour, elevation and slope.

graft [grahft]
(verb) to join together tissues from the same or different plants ; (noun) a piece of plant tissue (the scion), normally a stem, that is made to unite with an established plant (the stock), which supports and nourishes it.

graft union [grahft  yoon-yuhn]
junction between root stock and scion wood.

gravitational water [grav-i-tey-shuhn-l waw-ter]
water in excess of field capacity, that occupies larger soil pores and reduces soil aeration.

green mulch [muhlch] 
mulch composed of green (rather than woody) organic matter and not fully composted or decomposed. Also cut or mowed herbaceous material (grasses, etc.) that are left to decompose naturally and improve the underlying soil quality.

grinding [grahynd-ing]
for trees, a method of removing stumps by pulverizing them with a machine.

groundwater [ground-woh-ter]
water naturally stored beneath the ground consisting mainly of water that has seeped down.

ground rod [ground rod]
metal rod used in grounding a tree lightning protection system.

growth rate [grohth reyt]
speed at which something grows.

growth rings [grohth rings]
rings of xylem that are visible in a cross section of the stem, branches, and roots of some trees. In temperate zones, the rings typically represent one year of growth and are sometimes referred to as annual rings. Annual ring.

guard cells [ghard sels]
a pair of specialized cells that regulate the opening and closing of a stomatal pores (see stomata) to balance water loss with the intake of carbon dioxide.

gummosis [guh-moh-sis]
the exudation of sap or gum from the bark, often in response to disease or insect damage.

guy [guhy]
a cable between a tree or branch and an external anchor  to provide supplemental support.

guying [gahy-ing]
installing a stabilizing cable.

gymnosperm [jim-noh-spurm]
plants with exposed seeds, usually within cones. Conifers and cycads. Contrast with angiosperm.

gypsum [jip-suhm]
hydrated calcium sulfate. Sometimes used as calcium additive for soil.

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H's

hanger [hang-er]
broken or cut branch that is caught in a tree.

hardened off [hahr-dnd awf]
a plant that is acclimated to the cold or a new environment.

hardiness [hahr-dee-nis]
in horticulture, the genetically determined ability of a plant to survive low temperatures.

hardpan [hahrd-pan]
compacted soil layer nearly impervious to water, air, and roots.

heart rot [hahrt rot]
fungal disease that causes decay at the center of the trunk or branches.

heartwood [hahrt-wood]
the hard central core of an exogenous tree.  Contrast with sapwood.

heat tolerance [heet tol-er-uhns]
the ability of a plant to endure high temperatures.

hedge [hej]
a row of bushes or small trees planted close together, especially when forming a boundary.

hemicellulose [hem-i-sel-yuh-lohs]
complex carbohydrates that surround cellulose fibers in plants.

herbicides [hur-buh-sahyds]
chemical compounds that kill vegetation.

honeydew [huhn-ee-doo]
a sugary substance secreted by certain insects, including aphids and some scale insects, when feeding on plants.

horizon  [huh-rahy-zuhn]
within soil, the horizon is a layer parallel to the surface whose characteristics differ from the layers above and below. See soil profile.

horizontal boring [hawr-uh-zon-tl bawr-ing]
using a specialized machine to bore below the surface of the ground without an open trench. Alternative for installation of underground utilities that avoids cutting of tree roots or damage to hardscape or existing utilities. Contrast with radial trenching, trenching, and tunneling.

hormone [hawr-mohn]
see plant hormone.

horticultural oils [hawr-ti-kuhl-cher-uhl oils]
highly refined petroleum oils or botanicals (such as cottonseed or neem oil) that may be applied to plants to control certain insects and pests.

horticulture [hawr-ti-kuhl-cher]
the cultivation of flowers, fruits, vegetables, and ornamental plants.

host [hohst]
a living organism from which a parasite obtains nutrition.

humus [hyoo-muhs]
the dark, stable form of organic matter produced by the decomposition of plants and animals.

hypha (pl. hyphae) [hahy-fuh; hahy-fey]
in a fungus, one of the threadlike elements of the mycelium.

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I's

implant [ im-plant]
a device, capsule, or pellet inserted into the tree’s xylem system to treat or prevent diseases, disorders, or pest problems. Contrast with macroinfusion, macroinjection, and microinjection.

included bark [in-kloo-did bahrk]
bark that becomes embedded in a crotch (union) between branch and trunk or between codominant stems.  Usually indicates a weak union.

indeterminate growth [in-di-tur-muh-nit grohth] 
continued growth due to continuing activity of the apical and lateral meristems. Contrast with determinate growth.

infectious [in-fek-shuhs]
capable of being spread to plants from other plants or organisms.

infiltration [in-fil-trey-shuhn]
the downward entry of water into the soil. Contrast with percolation.

infiltration rate [in-fil-trey-shuhn reyt]
the speed at which water penetrates the soil.

inoculum [ih-nok-yuh-luhm]
part of a pathogen that enters the host and can cause disease.

inorganic fertilizer [in-awr-gan-ik fur-tl-ahy-zer]
mineral fertilizer, not coming from plant or animal sources. Contrast to organic fertilizer.

insect growth regulators [in-sekt grohth reg-yuh-ley-ters]
substances, man-made or naturally occurring, that affect the growth and development of insects.

instar [in-stahr]
The stage of growth between molts in the development of an insect.

Integrated Pest Management (IPM) [in-ti-grey-tid pest man-ij-muhnt]
method of controlling plant pests through a process of examination, identification and treatment. IPM focuses on employing a broad range of techniques to effectively control damaging insects while minimizing environmental impact.

Integrated Vegetation Management (IVM) [in-ti-grey-tid  vej-i-tey-shuhn man-ij-muhnt]
method of managing plants in which compatible and incompatible vegetation are identified, action thresholds are considered, control methods are evaluated, and selected control(s) are implemented to achieve a specific objective, based on effectiveness, environmental impact, site characteristics, safety, security, and economics.

International Society of Arboriculture (ISA)
[in-ter-nash-uh-nl suh-sahy-i-tee uhv ahr-ber-i-kuhl-cher]
the leading research, education and certification organization in arboriculture.

internode [in-ter-nohd]
the area between the nodes on a stem.

interveinal tissue [in-ter- veyn-l tish-oo]
leaf tissue between the veins or vascular bundles.

introduced species [in-truh-doost spee-sheez]
organisms not native to a region. Contrast with exotic species, invasive species, native species, and naturalized species.

invasive species [in-vey-siv spee-sheez]
non-native organisms likely to spread, disrupting the natural balance of an ecosystem. See exotic species, native species, naturalized species, introduced species.

IPM
see Integrated Pest Management.

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J's

.

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K's

kingdom [king-duhm]
primary taxonomic group, separating plants from animals, fungi, protozoa, algae and bacteria.

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L's

larva (pl. larvae) [lahr-vuh; lahr-vay]
in insects, an immature life stage between the egg and adult.

latent bud [leyt-nt buhd
a bud held in dormancy for more than one year by hormones originating from the terminal bud. See dormant bud.

lateral [lat-er-uhl]          
in botany, a branch, leaf, or bud that grows out from the side of a stem or trunk.

lateral branch [lat-er-uhl branch]
a branch growing from the side of a stem or trunk.

lateral bud [lat-er-uhl buhd]
a vegetative bud on the side of a stem. Contrast terminal bud.

lateral root [lat-er-uhl root]
a root branch arising from the main axis.

latewood [leyt-wood]
the portion of a tree’s annual ring (growth ring) that forms during summer, characterized by small-diameter cells with thick walls. Contrast with earlywood.

leaching [leech-ing]
the tendency for elements or compounds to wash down through the soil.

leader [lee-der]
the primary terminal shoot or trunk of a tree.

leaf [leef]
the primary, photosynthetic organ of a plant, connected to a stem by a petiole.

leaf apex [leef ey-peks]
the tip of the leaf blade.

leaf base [leef beys]
the bottom part of the leaf blade.

leaf blotch [leef bloch]
irregularly shaped areas of disease on plant foliage.

leaflet [leef-lit]
a separate part of a compound leaf.

leaf margin [leef mahr-jin]
outer edge of the leaf blade.

leaf scar [leef skahr]
scar left on the twig after a leaf is shed.

leaf spot [leef spot]
patches of disease or other damage on plant foliage.

lenticel [len-tuh-sel]
small opening in the bark that permits the exchange of gases; a pore.

lesion [lee-zhuhn]
local area of diseased or damaged tissue.

lifting [lift-ing]
pruning lower limbs from a tree to provide clearance. Syn: crown raising.

lignification [lig-nuh-fi-key-shuhn]
process in which cell walls become thicker and stronger by impregnation with lignin.

lignin [lig-nin]
an organic substance that, with cellulose, forms the chief part of woody tissue.

limb [lim] 
a large, primary branch of a tree.

lion tailing [lahy-uhn tey-ling]
a poor pruning practice in which so many branches are removed from the lower limbs that the clump of foliage left at the top resembles a lion’s tail.

liquid fertilization [lik-wid fur-tl-ahy- zey-shuhn]
applying liquid fertilizer by injection into the root zone of a tree or by application to soil surface or foliage. Contrast with broadcast fertilization and drill-hole fertilization.

liquid injection [lik-wid in-jek-shuhn] 
method of injecting liquid fertilizer into the root zone of a tree.

litter [lit-er]
a layer of partially decomposed leaves, twigs and other plant material on the ground. Also called leaf litter.

loam [lohm]
a rich, friable soil containing a relatively equal mixture of sand and sail and a somewhat smaller proportion of clay.

lobe [lohb]
a leaf segment that projects outward. Contrast with sinus.

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M's

macroinfusion [mak-roh-in-fyoo-zhuhn]
technique to introduce pesticides, antibiotics, nutrients or fungicides directly into the xylem of a tree to treat or prevent diseases, disorders, or pest problems through a relatively large-diameter hole in the trunk or a root. Contrast with implant, macroinjection, and microinjection.

macroinjection [mak-roh-in-jek-shuhn]
a trunk injection technique that injects antibiotics, nutrients, fungicides or pesticides under pressure, requiringa relatively large-diameter hole in the trunk. Contrast with implant, macroinfusion, and microinjection.

macronutrient [mak-roh- noo-tree-uhnt]
essential element  that is required by plants in relatively large quantities, consisting of primary nutrients, i.e. nitrogen (N), phosphorus (P), potassium (K) and secondary nutrients, i.e. calcium (Ca), magnesium (Mg), and sulfur (S) (Silicon is sometimes included in this group as a tertiary nutrient or as a beneficial, rather than essential, nutrient.) Contrast with micronutrient.

macropore [mak-roh-pohr]
relatively large space between soil particles that is usually air filled and allows for water movement and root penetration. Contrast with micropore.

meristem [mer-uh-stem]
the undifferentiated tissue in which active cell division takes place. Found in the root tips, buds, cambium, cork cambium, and latent buds.

mesophyll [mez-uh-fil]
The middle layers of a leaf where most photosynthesis takes place. Composed primarily of palisade and spongy parenchyma cells.

microbial extracts  [mahy-kroh-bee-l ik-strakts]
substances derived from microorganisms. May be used to control certain pests and to help trees better absorb nutrients.

microinjection [mahy-kroh-in-jek-shuhn]
trunk injection technique using a small-diameter trunk penetration to introduce pesticides, antibiotics, nutrients or fungicides directly into the xylem. Contrast with implant, macroinfusion, and macroinjection.

micronutrient [mahy-kroh-noo-tree-uhnt]
essential element that is required by plants in small quantities, including iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), chlorine (Cl), molybdenum (Mo)and nickel (Ni). Contrast with macronutrient.

micropore [mahy-kroh-pohr]
the space between soil particles, important to the activity of microbes, that is relatively small and likely to be water filled. Contrast with macropore.

mineral [min-er-uhl]
naturally occurring, inorganic solid that has a definite chemical composition and possesses characteristic physical properties. Certain minerals are considered essential elements for tree growth and development.

minimum irrigation [min-uh-muhm ir-i-gey-shuhn]]
practice of minimizing irrigation needs through the use of drought-tolerant plants and watering only when necessary. See xeriscape.

mites [mahyts]
small, often minute, arthropods in the order Acarina of the class Arachnida that may feed on plants, other mites, or small insects.

monocot
see monocotyledon.

monocotyledon (monocot) [mon-uh-kot-l-eed-n; mon-uh-kot]
Any of a class of angiosperm plants having a single cotyledon inthe seed. Monocotyledons have leaves with parallel veins, flower parts in multiples of three, and fibrous root systems. Grasses, lilies, irises and orchids are monocotyledons. See dicotyledon.

monoecious [muh-nee-shuhs] 
species with male and female flowers borne on the same plant. Contrast with dioecious.

morphology [mawr-fol-uh-jee]
the study of the form and structure of plants and other living organisms. Contrast with anatomy, physiology, and taxonomy.

mortality spiral [mawr-tal-i-tee spahy-ruhl]
a sequence of stressful events or conditions causing the decline and eventual death of a tree.

mulch [muhlch]
a covering, as of straw, compost, or plastic sheeting, spread on the ground around plants to prevent excessive evaporation or erosion, enrich the soil, suppress weeds, etc.

mycelium [mahy-see-lee-uhm]
the vegetative body of a fungus.

mycorrhiza (pl. mycorrhizae) [mahy-kuh-rahy-zuh; mahy-kuh-rahy-zey]
a symbiotic association of the mycelium of a fungus  with the roots of certain plants, in which the hyphae form a woven mass around the rootlets or penetrate the cells of the root.

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N's

native species [ney-tiv spee-sheez]
the plants indigenous to a region. Naturally occurring and not introduced by man. Contrast with exotic species, introduced species, and naturalized species.

natural pruning [nach-er-uhl proon-ing]
a process of branch removal in which the pruning cuts are made at nodes and in relation to the positions of the branch collar and branch bark ridge. See Target pruning.

naturalized species [nach-er-uh-lahyzd spee-sheez]
non-native species that has become established in a region and propagates without human assistance. Contrast with exotic species, introduced species, invasive species and native species.

necrosis [nuh-kroh-sis]
the localized death of tissue in a living organism.

needle [need-l]
the slender leaf of a conifer.

nematode [nem-uh-tohd]
a microscopic roundworm. Many are beneficial organisms, but some feed on plant tissues and may cause disease or damage.

node [nohd]
slightly enlarged portion of a stem where leaves and buds arise. Contrast with internode.

nomenclature [noh-muhn-kley-cher]
scientific naming system for living organisms.

nut [nuht]
a dry, hard, one-seeded fruit

nutrient cycling [noo-tree-uhnt  sahy-kling]
within an ecosystem, the movement of nutrients back into living organisms as organic matter decomposes.

nutrient [noo-tree-uhnt ]
a mineral substance that is absorbed by  the roots of plants for nourishment.

nymph [nimf] 
immature form of an insect with incomplete development, resembling a smaller version of the adult without wings.

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O's

oblique [uh-bleek]
leaves with an asymmetrical shape, with one side larger than the other.

obtuse [uhb-toos]
leaves that have rounded ends.

opposite [op-uh-zit]
pertaining to leaf or branch arrangement, leaves or branches situated two at each node, directly across from each other on the stem. Contrast with alternate and whorled.

order [awr-der]
taxonomic group below the class level but above the family level.

organic fertilizer [awr-gan-ik fur-tl-ahy-zer]
fertilizer derived from plants or animals; compare to inorganic fertilizer.

organic layer [awr-gan-ik ley-er]
layer of organic matter at the soil surface.

organic matter [awr-gan-ik mat-er]
decaying material from living organisms. See compost, green mulch, humus, mulch.

osmosis [oz-moh-sis]
diffusion of water through a semi-permeable membrane from a region of higher water potential (lower salt concentration) to a region of lower water potential (higher salt concentration).

oviposit [oh-vuh-poz-it]
to deposit or lay eggs, especially by insects.

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P's

palmate [pal-meyt]
type of compound leaf with four or more lobes or leaflets radiating in a fanlike pattern. Contrast with bipinnate and pinnate.

parasite [par-uh-sahyt]
organism living in or on another living organism (host) from which it derives nourishment to the detriment of the host, sometimes killing the host. Contrast with epiphyte and saprophyte.

parenchyma cells [puh-reng-kuh-muh sels]
the fundamental tissue of plants, composed of thin-walled cells able to divide.

parent material [par-uhnt  muh-teer-ee-uhl]
the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form.

pathogen [path-uh-juhn]
causal agent of disease. Usually refers to microorganisms.

perched water table [purcht waw-ter tey-buhl]
the accumulation of water in an upper soil layer above the actual water table.

percolation [pur-kuh-ley-shuhn]
the movement of water through the soil. Contrast with infiltration.

perennial [puh-ren-ee-uhl]
a plant with a life cycle of at least 3 years.

permanent wilting point [pur-muh-nuhnt wilt-ing  point]
the point at which a plant cannot pull any more water from the soil and suffers permanent damage. Compare to available water, field capacity, gravitational water, and saturation point.

petiole [pet-ee-ohl]
the stalk or support axis of a leaf.

petiolule [pet-ee-uhl-yool]
the stalk of a leaflet.

PHC
see Plant Health Care.

phellogen [fel-uh-juhn]
layer of dividing and expanding cells outside the phloem that generates the outer bark. See cork cambium.

phenology [fi-nol-uh-jee]
the science dealing with the influene of climate on the recurrence of such annual phenomena as budding.

phenols [fee-nawl]
naturally produced organic alcohols with acidic properties. One of several chemical defense compounds in trees.

pheromone [fer-uh-mohn]
in insects, a chemical substance that serves as a method of communication with other individuals of the same species.

phloem [floh-em]
the part of a vascular bundle consisting of sieve tubes, companion cells, parenchyma cells, and fibers and forming the food-conducting tissue of a plant. Contrast with xylem.

photoperiod [foh-tuh-peer-ee-uhd]
the length of daylight and/or darkness required for certain developmental processes and growth in a plant.

photosynthate [foh-tuh-sin-theyt]
general term for the sugars and other carbohydrates produced during photosynthesis.

photosynthesis [foh-tuh-sin-thuh-sis]
process in green plants (and in algae and some bacteria) by which light energy is used to form glucose (chemical energy) from water and carbon dioxide.

phototropism [foh-to-truh-piz-uhm]
the influence of light on the direction of plant growth; the tendency of plants to grow toward light.

phylum (pl. phyla) [fahy-luhm; fahy-luh]]
the primary subdivision of a taxonomic kingdom, grouping together all classes of organisms that have the same body plan. In plants, the term division is often used instead of phylum.

physiological disorder [fiz-ee-uh-loj-i-kuhl dis-awr-der] 
in plants, a disorder not directly caused by an insect, pathogen, or injury, such as poor light, lack of nutrients or overwatering.

physiology [fiz-ee-ol-uh-jee] 
in arboriculture, the study of the life function of a tree. Contrast with anatomy, morphology, and taxonomy.

phytochrome [fahy-tuh-krohm]
a plant pigment that is associated with the absorption of light in the photoperiodic response and that may regulate various types of growth and development.

phytohormones [fahy-tuh-hawr-mohn]
see plant hormone.

phytotoxic [fahy-tuh-tok-sik]
inhibiting the growth of or poisonous to plants.

pinnate [pin-yet]
a type of compound leaf, with leaflets along each side of a common axis. Contrast with bipinnate and palmate.

plant growth regulator [plant grohth reg-yuh-ley-ter]
substance that affects the growth of a plant or a plant part. May be naturally produced (hormone) or synthetic. See plant hormone.

Plant Health Care (PHC)
comprehensive program to manage the health, structure, and appearance of plants in the landscape.

plant hormone [plant hawr-mohn]
any of the various plant compounds that control growth and differentiation of plant tissue. Also called phytohormones.

planting specifications [plant-ing spes-uh-fi-key-shuhns]
detailed plans and statements of particular procedures, requirements, and standards for planting.

plasmodesma (pl. plasmodesmata)[plaz-muh-des-muh; plaz-muh-des-muh-tuh]
any of many minute strands of cytoplasm that extend through plant cell walls and connect adjoining cells.

pleaching [pleech-ing]
specialty technique of pruning and training branches or vines in which they are interwoven, often to form an arbor, wall or arching tunnel.

pollarding [pol-erd-ing]
specialty pruning technique in which a tree with a large-maturing form is kept relatively short.

powdery mildew [pou-duh-ree mil-doo]
a plant disease caused by parasitic fungi which produce a powderlike film of mycelium on the surface of host plants, characterized by yellowing and death of the foliage. See downy mildew.

primary disorder [prahy-mer-ee  dis-awr-der]
initial source of disease or damage in a tree. Damage can be increased by a secondary disorder.

primary nutrients [prahy-mer-ee noo-tree-uhnts]
most important mineral macronutrients for plants, i.e. nitrogen, phosphorus and potassium. See micronutrients, macronutrients, secondary nutrients.

protoplast [proh-tuh-plast]
the contents of a cell within the cell membrane.

pupa (pl. pupae) [pyoo-puh; pyoo-pee]
the resting stage of an insect just prior to transforming to an adult.

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Q's

quick-release fertilizer [kwik ree-lees ]
a fertilizer with less than 50 percent water-insoluble nitrogen. compare to slow-release fertilizer.

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R's

radial aeration [rey-dee-uhl air-ey-shun]
see radial trenching.

radial transport [rey-dee-uhl trans-pawrt]
the movement of fluid across the rings of a tree.

radial trenching [rey-dee-uhl trench-ing]
a technique for aerating the soil around a tree by digging trenches in a spoke-like pattern, removing and replacing the soil.  

raising [rey-zing]
selective pruning to provide vertical clearance. See crown raising.

ray [rey]
parenchyma tissue that extends radially across the xylem and phloem of a tree.

reaction wood [ree-ak-shuhn wood]]
wood formed in leaning or crooked stems or on lower or upper sides of branches as a means of counteracting the effects of gravity. See compression wood and tension wood.

reduction [ri-duhk-shuhn]
in trees, pruning to decrease the height or spread of a branch or crown.

removal cut [ri-moo-vuhl kuht]
a cut that removes a branch at its point of origin. See branch collar.

resistance [ri-zis-tuhns]
the capacity of an organism to withstand the effects of a harmful physical or environmental agent, such as a microorganism or pollutant.

resistant varieties [ri-zis-tuhnt vuh-rahy-i-teez] 
plant varieties that are tolerant of, or not susceptible to, certain diseases or pests.

Resistograph [ri-zist-uh-graf]
the brand name of a device consisting of a specialized micro-drill bit that drills into trees and graphs density differences that are used to detect decay

resource allocation [ree-sawrs al-uh-key-shuhn]
in plant physiology, the distribution and use of photosynthates for various plant functions and processes.

respiration  [res-puh-rey-shuhn]
in plants, the process by which carbohydrates are converted into energy by using oxygen.

restoration pruning [res-tuh-rey-shuhn proon-ing]
pruning to improve the structure, form, and appearance of trees that have been improperly trimmed, vandalized, or damaged.

rhizodermis [rahy-zuh-dur-mis]
primary surface layer of the root, similar to epidermis but of different origin and function.

rhizosphere [rahy-zuh-sfeer]
the area of soil that surrounds the roots of a plant and is altered by the plant’s root growth, nutrients, respiration, etc.

ring porous [ring pawr-uhs]
a pattern of wood development in which the large-diameter vessels are concentrated in the earlywood. Contrast with diffuse porous.

ring shake
see shake.

root [root]
a part of a plant that grows downward into the soil, anchoring the plant and absorbing nutrients and moisture.

root ball [root bawl]
soil containing all or a portion of the roots that are moved with a plant when it is planted or transplanted.

root barrier [root bar-ee-er]
membranes or sheets installed vertically in the soil to limit or direct the growth of tree roots.

root bound [root bound]
condition in which plant roots are overcrowded in a container or site and root growth is restricted.

root cap [root-kap]
the loose mass of epidermal cells covering the apex of most roots, that protect the meristematic cells behind them.
.
root collar [root kol-er]
in trees, the transition zone between the roots and the trunk, usually at or near ground level.

root crown [root kroun]
same as root collar.

root flare [root flair]
see trunk flare.

root graft [root graft]
the natural union of two roots, either from the same plant or from two different plants. Can result in disease transmission.

root hairs [root hairz]
an elongated tubular extension of an epidermal cell of a root, used to absorb water and minerals from the soil.

root pruning [root proon-ing]
severing roots to aid in transplanting, excavation or disease management.

root:shoot ratio [root shoot rey-shoh]
relative proportion of root mass to crown mass.

root stock [root stok]
root or part of tree used for plant propagation by grafting to scion wood.

root zone [root zohn]
horizon or layer within the soil, typically extending beyond the drip line, where roots exist.

rust [ruhst]
disease caused by a certain group of fungi and characterized by reddish brown spots on the foliage and/or the formation of stem galls.

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S's

saline [sey-leen]
water solution containing dissolved salt.

saline soils [sey-leen soilz]
soils with a high concentration of soluble salts, often causing poor plant growth.

saline–sodic soils [sey-leen-soh-dik soilz]
soils with high concentrations of both soluble salts (saline) and sodium ions (sodic). See saline soils and sodic soils.

salt index [sawlt in-deks]
osmotic potential ratio of a fertilizer compared to sodium nitrate, based on the relative value of 100. The higher the salt index, the more likely plant damage will occur.

sand [sand]
soil particles with a size between 0.05 and 2.0 millimeters in diameter. Contrast with clay and silt.

sanitation [san-i-tey-shuhn]
horticultural practice of removing dead, infested, or diseased plant parts to reduce the spread of insects or disease.

saturation point [san-i-tey-shuhn point]
point at which a soil will no longer absorb any amount of water.

saprophyte [sap-ruh-fahyt]
an organism that lives on and may act to decay dead organic matter. Contrast with epiphyte and parasite.

sapwood [sap-wood] 
the softer part of the wood between the inner bark and the heartwood.

scaffold branches [skaf-uhld bran-chiz]
permanent or structural branches that form the scaffold architecture or structure of a tree.

scale [skeyl]
one of a group of insects that attach themselves to plant parts and suck the sap.

scion [sahy-uhn]
top part of a graft, with leaves and buds, that is grafted to the root stock.

scorch [skawrch]
browning and shriveling of foliage, especially at the leaf margin.

secondary disorder [sek-uhn-der-ee dis-awr-der]]
disorder that develops after a plant is stressed by a primary one.

secondary nutrients [sek-uhn-der-ee noo-tree-uhnts]
macronutrients  required in moderate amounts by plants, i.e. calcium, magnesium and sulphur. See also essential elements, macronutrient, micronutrient, and primary nutrients.

secondary pest [sek-uhn-der-ee pest]
insect or other pest problem that develops on a plant stressed and weakened by another factor. See secondary disorder.

selective herbicide [si-lek-tiv hur-buh-sahyd]
a herbicide that is effective only on specific plant types (e.g., broadleaves, grasses) or species.

senescence [si-nes-uhns]
process of aging. Process leading to leaf drop in deciduous plants.

serrate [ser-yet]
sawtooth margin of a leaf, with the teeth pointed forward. Compare to dentate.

shake(s) [sheyk]
separation of wood at the growth rings or rays, usually along the barrier zone that forms in the compartmentalization process (CODIT). See ring shake(s), radial shake(s).

shoot [shoot]
stem or branch and its leaves, especially when young.

shrub [shruhb]
a woody plant, smaller than a tree usually having multiple stems branching from or near the ground.

sieve cells [siv sels]
long, slender phloem cells in gymnosperms.

sieve tube elements [siv toob el-uh-muhnts]
specialized phloem cells involved in photosynthate transport. Exist only in angiosperms.

sign [sahyn]
physical evidence of a causal agent (e.g., insect eggs, borer hole, frass). Contrast with symptom.

silt [silt]
soil particles with a grain size between 0.002 and 0.05 millimeter (coarser than clay particles but finer than sand). Contrast with clay and sand.

silviculture [sil-vi-kuhl-cher]
the study and practice of the maintenance and growth of forests. Forestry.

simple leaf [sim-puhl leef]
a single-bladed leaf, not composed of leaflets. Contrast with compound leaf.

sink [singk]
in plants, a part that uses or stores more matter or energy than it produces. For example, a forest acts as a sink for carbon dioxide, absorbing more than they produce.

sinker roots [sing-ker roots]
downward-growing roots that provide anchorage and take up water and minerals. Especially useful during periods of drought.

sinus [sahy-nuhs] 
space between two lobes of a leaf. Contrast with lobe.

skeletonized [skel-i-tn-ahyzd]
leaf-feeding damage caused by insects, characterized by the loss of tissue between the leaf veins.

slowly soluble fertilizer  [sloh-lee sol-yuh-buhl fur-tl-ahy-zer]
a fertilizer formulation that is slowly hydrolyzed in the soil.

slow-release fertilizer [sloh ree-lees fur-tl-ahy-zer]
pelletized fertilizer whose rate of release varies depending upon soil moisture and temperature; compare to quick-release fertilizer.

sodic soils [soh-dik soilz]
soils with relatively low levels of soluble salts and a concentration of sodium high enough to adversely affect soil structure.

soft rot [sawft rawht]
the decay of plant tissues characterized by the breakdown of tissues within the cell walls. Contrast with brown rot and white rot.

soil [soil]
surface layers of sand, clay, silt and organic material that support plants.

soil amendment [soil uh-mend-muhnt]
material added to soil to improve its physical, chemical, and/or biological properties.

soil analysis [soil uh-nal-uh-sis]
the analysis of soil to determine pH, mineral composition, structure, salinity, and other characteristics.

soil compaction [soil kuhm-pak-shuhn]
the compression of the soil, often as a result of vehicle or heavy-equipment traffic, that pushes out air between soil particles, making it denser.

soil-drench application [soil-drench ap-li-key-shuhn]
a method of applying chemicals (usually herbicides or tree growth regulators) to trees through the soil, where it can be taken up by roots.

soil horizon [soil huh-rahy-zuhn]
a layer or zone of the soil profile with physical, chemical, and biological characteristics that differ from adjacent layers.

soil particle [soil pahr-ti-kuhl]
the smallest grains of soil categorized by size into clay, silt, or sand.

soil profile [soil proh-fahyl]
a vertical section through the soil showing all of the soil horizons.

soil structure [soil struhk-cher]
the arrangement of solid particles in soil and the air between them.

soil texture [soil teks-cher]
the relative fineness or coarseness of a soil due to particle size (sand, silt, and clay).

species [spee-sheez]
the major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but are not able to breed with members of another species.

splits [splitz]
open cracks or fissures in tree trunks or branches.

staking [steyk-ing]
supporting a tree with stakes and ties. Usually used in reference to newly planted trees.

stele [stee-lee]
the central cylinder or cylinders of vascular and related tissue in the stem, root, petiole, leaf, etc. of the higher plants.

stem [stem]
a woody structure bearing foliage and buds that gives rise to other stems (branches).

stippling [stip-pling]
speckled or dotted areas on foliage or bark.

stoma (pl. stomata)  [stoh-muh; stoh-muh-tuh]
a small pore on leaves or stems, through which gases are exchanged and water loss is regulated.

stress [stress]
in plants, a factor that negatively affects the health of a plant.

structural defects [struhk-cher-uhdee-fekts]
any naturally occurring or secondary conditions such as cavities, poor branch attachments, cracks, or decayed wood in the trunk, crown, or roots of a tree that may contribute to structural failure.

structural pruning [struhk-cher-uhl  proon-ing]
pruning to establish a strong arrangement or system of scaffold branches.

structural roots [struhk-cher-uhl  roots]
large, woody, tree roots that anchor and support the trunk and crown. Roots characterized by secondary thickening and relatively large diameter, giving form to the root system and functioning in anchorage and support.

stub [stuhb]
portion of a branch or stem remaining after a stub cut, branch breakage, or branch death.

stump [stuhmp]
the base part of a tree that remains standing after the tree has been felled.

stump grinder [stuhmp grahyn-der]
piece of equipment used to grind tree stumps in the ground.

stunting [stuhnt-ing]
preventing from growing or developing to the usual extent.

sub-opposite [suhb-op-uh-zit ]
the leaf arrangement in which leaves are nearly opposite, but are slightly alternate, with one bud slightly below the one opposite.

subordination pruning [suh-bawr-dn-ey-shuhn proon-ing] 
improving the structure of a tree by selectively shortening branches in order to promote the growth of other branches.

subsurface application [suhb-sur-fuhs ap-li-key-shuhn] 
the placement of fertilizer or other material below the soil surface.

sucker [suhk-er]
a shoot arising from the roots. Contrast with watersprout.

summer wood [suhm-er-wood]
see latewood.

sunburn  [suhn-burn]
injury to bark and cambium caused by a combination of excessive light and heat and insufficient soil moisture.

sunscald [suhn-skawld]
injury to bark tissues on the trunk and branches caused by rapid changes in temperature, especially on warm days and cool nights in winter.

surface application [sur-fuhs ap-li-key-shuhn] 
placement of fertilizer or other material on the ground surface. Broadcast.

symbiosis  [sim-bee-oh-sis]
association of two different types of living organisms that is often, but not always, beneficial to each.

symplast [sim-plast]
the continuous system of protoplasts, linked by plasmodesmata and bounded by the cell wall.

symptom [simp-tuhm]
in plants, a reaction to a disease or disorder (e.g., wilting, dieback). Contrast with sign.

systemic [si-stem-ik]
(1) substance, like a pesticide, that moves throughout an organism after it is absorbed. (2) any condition, disease, disorder, or pest that affects the entire organism.

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T's

tannins [tan-inz]
organic substances produced by trees. Believed to be involved in a tree’s chemical defense processes.

tap root [tap-root]
central, vertical root growing directly below the main stem or trunk that may or may not persist into plant maturity.

taper [tey-per]
the change in diameter over the length of trunks, branches or roots.

target canker [tahr-git kang-ker]
type of perennial canker that gains its name from the appearance caused by concentric rings, each of which represents a year’s growth of wound tissue around the infected area.

target pruning [tahr-git proon-ing]
see natural pruning.

taxonomic group [tak-so-naw-mek groop]
any of several hierarchic levels in the classification of living organisms.

taxonomy [tak-son-uh-mee]
science that studies the description, denomination, and classification of living organisms based on their similarities and differences. Contrast with anatomy, morphology, and physiology.

tension wood [ten-shuhn wood]
in broadleaved trees, reaction wood that forms on the upper side of branches or the trunks of leaning trees. Contrast with compression wood.

terminal bud [tur-muh-nl buhd]
a bud at the tip of a twig or shoot. Apical bud. Contrast with lateral bud.

terracing [ter-uh-sing]
method used to lower the soil grade in steps or stages.

thinning [thin-ing]
in pruning, selectively removing live branches to provide light or air penetration through the tree or lighten the weight of the remaining branches.

topping [top-ing]
inappropriate pruning technique to reduce tree size. Cutting back a tree to a predetermined crown limit, often at internodes.

topsoil [top-soil]
the fertile, upper layer of the soil.

tracheid  [trey-kee-id]
an elongated, tapering xylem cell adapted for the support and transport of water and elements. Compare with  vessel.

translocation [trans-loh-key-shuhn]
movement of sugars in the phloem.

transpiration [tran-spuh-rey-shuhn]
the passage of water through a plant from the roots through the vascular system to the atmosphere.

transplanting  [trans-plant-ing]
moving a plant to a new location.

transplant shock [trans-plant shok]
plant stress following transplant; characterized by reduced growth, wilting, dropping foliage,or death.

tree [tree]
a woody perennial usually having one dominant trunk and a mature height greater than 5 meters (16 feet). Contrast with shrub.

tree available water [tree uh-vey-luh-buhl waw-ter]
see available water.

tree growth regulator (TGR) [tree grohth reg-yuh-ley-ter]
a chemical that can be applied to trees to slow terminal growth by reducing cell elongation.

tree island [tree ahy-luhnd]
an enclosed planting bed surrounding a tree, often within a paved area or adjacent to a street.

tree shelter [tree shel-ter]
a tube placed around a tree seedling for protection and growth enhancement.

tree spade [tree speyd]
mechanical equipment to dig, transport, and replant trees with a sufficiently large volume of roots and soil.

tree well [tree wel]
in arboriculture, a wall constructed around a tree to protect the trunk by maintaining the original grade between the trunk and the wall when the grade is raised by filling outside and behind the wall.

tree wrap [tree rap]
material used to wrap the trunks of newly planted or transplanted trees or to protect thin-barked mature trees when they are newly exposed to the sun. See sunburn.

trenching [trench-ing]
a linear, open excavation, often used to install utilities or structural footings. Can cause tree root damage. Contrast with horizontal boring, tunneling, and radial trenching.

tropism [troh-piz-uhm]
the tendency of a plant to grow in response to an external stimulus such as gravity (geotropism) or light (phototropism).

trunk flare [truhngk flair]
the transition zone from trunk to roots where the trunk expands into the buttress or structural roots. See root flare.

trunk injection [truhngk [in-jek-shuhn]
technique to introduce substances directly into the xylem of a tree to treat or prevent diseases, disorders, or pest problems.

tunneling [tuhn-l-ing]
digging, often with special machinery and shoring or other supports, below the surface of the ground without an open trench. Contrast with horizontal boring and radial trenching.

turgid [tur-jid]
fully hydrated to a normal state of distension. The opposite of wilted.

turgor [tur-ger]
the normal distention or rigidity of plant cells, resulting from the pressure exerted by the cell contents on the cell walls.

twig [twig]
a small, woody branch, stem, or shoot.

tyloses [tahy-loh-sis]
protrusions from parenchyma cells that enter and block xylem vascular cells when those cells become inactive or injured.

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U's

urban forestry [ur-buhn fawr-uh-stree]
the management of naturally occurring and planted trees and associated plants in urban areas.

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V's

variety [vuh-rahy-i-tee]
a naturally occurring subdivision of a species having a distinct difference and breeding true to that difference. Compare to cultivar.

vascular discoloration [vas-kyuh-ler dis-kuhl-uh-rey-shuhn]
a darkening of the vascular tissues of woody plants in response to disease.

vascular system [vas-kyuh-ler sis-tuhm] ]  
in a plant, the vessels and tissue that carry or circulate fluids such as sap.

vascular tissue [vas-kyuh-ler tish-oo]
tissue that conducts water or nutrients.

vector [vek-ter]
in pathology, biotic or abiotic agent that transmits a pathogen.

venation [vee-ney-shuhn]
the arrangement of veins in a leaf.

vertical mulching [vur-ti-kuhl muhlch-ing]
an aeration or fertilization technique. Drilling vertical holes in the soil and filling them with materials to improve aeration.

vessels [ves-uhls]
end-to-end, tubelike, water-conducting cells in the xylem of angiosperms.

vigor [vig-er]
overall health; capacity to grow and resist stress.

virus [vahy-ruhs]
ultramicroscopic, metabolically inert, infectious agent that can reproduce only in living cells of other organisms. Can cause disease.

vista pruning [vis-tuh proon-ing]
selective pruning to enable a view from a predetermined point.

vitality [vahy-tal-i-tee]
overall health. Ability of a plant to deal effectively with stress.

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W's

water-holding capacity [waw-ter-hohl-ding kuh-pas-i-tee]
the ability of a soil to hold moisture.

water-insoluble nitrogen (WIN) [waw-ter-in-sol-yuh-buhnahy-truh-juhn]]
nitrogen fertilizer in a form that is not readily soluble in water.

watersprout [waw-ter-sprout]
upright, epicormic shoot arising from the trunk or branches of a plant above the root graft or soil line. Contrast with sucker.

water table [waw-ter tey-buhl]
upper level of groundwater in the soil.

white rot [hwahyt rot]
fungal decay of wood in which both cellulose and lignin are broken down. Contrast with brown rot and soft rot.

whorled [hwurld]
leaves, twigs, or branches arranged in a circle around a point on the stem. Contrast with alternate and opposite.

wilt [wilt]
(1) (noun) the drying out, drooping and withering of the leaves of a plant due to inadequate water supply, excessive transpiration or vascular disease. (2) (noun) one of several diseases characterized by wilting; (3) (verb) to lose turgor or to wilt.

wilting point
see permanent wilting point.

windthrow [hwind-throh]
tree failure due to uprooting caused by wind.

winter kill [win-ter kil]]
injury from cold winter temperatures.

witches’ broom [wich-ez broom]
an abnormal brushlike growth of weak, closely clustered shoots or branches on a tree caused by fungi or viruses.

woundwood [woond-wood]
lignified, differentiated tissues produced on woody plants as a response to wounding. Contrast with callus.

wood [wood]
hard, fibrous inner part of tree trunks, branches, and stems. The xylem.

wound dressing [woond dres-ing]
compound applied to tree wounds or pruning cuts.

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X's

xanthophyll [zan-thuh-fil]
yellow pigment found in plants. Compare to anthocyanin and carotenoid.

xylem [zahy-luhm]
a compound tissue in vascular plants that helps provide support and that conducts water and nutrients upward from the roots, consisting of tracheids, vessels, parenchyma cells and woody fibers.Contrast with phloem.

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Y's

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Z's

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