Tree Care Glossary & Encyclopedia

A small, aphid-like insect that feeds on the sap of conifers, often producing white woolly wax coverings on branches or needles.

Roots that develop from non-root tissue, such as stems or branches, typically in response to environmental stress, buried stems, or high moisture conditions.

A trained professional who specializes in the care and maintenance of trees, including planting, pruning, diagnosis, and removal.

A plant disease caused by pathogenic bacteria that typically enter through wounds or natural openings and cause symptoms like oozing, wilting, or galls.

Small beetles that bore through tree bark to lay eggs in the inner bark, creating characteristic gallery patterns and often introducing fungal pathogens.

The use of living organisms such as predators, parasites, or pathogens to suppress pest populations to acceptable levels.

A rapid and severe plant disease that causes sudden browning, wilting, and death of leaves, flowers, branches, or entire plants.

An insect whose larval stage feeds by tunneling into the wood, bark, or cambium of trees, causing internal damage that can weaken or kill the host.

Structural support systems installed in trees to reduce the risk of failure in branches or stems with weak attachments.

A thin layer of actively dividing cells between the bark and wood of a tree that produces new growth each year.

The thin cylinder of actively dividing meristematic cells between bark and wood that is responsible for producing all new wood and inner bark growth.

A localized dead area on the bark or cambium of a tree trunk or branch, caused by fungal or bacterial infection, that often appears as sunken, discolored, or cracked bark.

The uppermost layer of branches and leaves of a tree, forming the crown that intercepts sunlight and rainfall.

A condition in which leaves produce insufficient chlorophyll, resulting in yellowing of leaf tissue while veins may remain green, often caused by nutrient deficiency or high soil pH.

A pruning technique that reduces the overall size of a tree's canopy by cutting branches back to suitable lateral branches.

A standard measurement of tree trunk diameter taken at 4.5 feet (1.37 meters) above ground level, used for inventory, assessment, and regulatory purposes.

The removal of dead, dying, or diseased branches from a tree's canopy to reduce hazard risk and improve appearance.

An insect or pathogen that destroys or consumes the leaves of a tree, reducing its ability to photosynthesize and produce energy.

The progressive death of branches starting from the tips and advancing toward the trunk, often indicating underlying disease, root damage, or environmental stress.

A highly refined petroleum or plant-based oil sprayed on trees during dormancy to smother overwintering insects and mite eggs on bark surfaces.

The measurable benefits that trees and forests provide to humans, including air purification, stormwater management, carbon sequestration, and urban heat reduction.

A systemic insecticide from the avermectin class, delivered via trunk injection, that is the gold standard treatment for emerald ash borer and other wood-boring insects.

A plant disease caused by pathogenic fungi, which represent the largest and most diverse group of tree pathogens.

A chemical substance used to prevent, inhibit, or kill fungal pathogens that cause plant diseases.

An abnormal swelling or growth on a plant caused by insects, mites, fungi, or bacteria that manipulate plant tissue development.

A geographically defined area classified by its average annual minimum temperature, used to determine which plant species can survive winter in a given location.

The dense, inner core of wood in a tree trunk that no longer conducts water but provides structural support.

A plant species that provides nutrition or habitat for a specific pest or pathogen, supporting its survival and reproduction.

A systemic neonicotinoid insecticide widely used in tree care to control sucking and boring insects, typically applied as a soil drench or trunk injection.

A chemical substance used to kill or repel insects that damage trees, applied through various methods including spraying, injection, or soil application.

A science-based decision-making process that combines biological, cultural, physical, and chemical tools to manage pests with minimal environmental risk.

A non-native organism whose introduction causes or is likely to cause ecological or economic harm, outcompeting native species and disrupting ecosystems.

A professional credential awarded by the International Society of Arboriculture that verifies an arborist's knowledge and competence in tree care.

The immature, often worm-like feeding stage of an insect that undergoes complete metamorphosis, typically the stage that causes the most damage to trees.

A common group of foliar diseases that produce discrete spots or lesions on leaves, caused by various fungi or bacteria.

A porous area in the bark of woody stems and roots that allows gas exchange between internal tissues and the external atmosphere.

A region of undifferentiated plant cells capable of active division, responsible for all new growth in length and girth.

The practice of applying a layer of organic or inorganic material around the base of a tree to conserve moisture, moderate soil temperature, and suppress weeds.

A plant species that occurs naturally in a particular region, having evolved there over thousands of years without human introduction.

A botanical insecticide and fungicide derived from the seeds of the neem tree (Azadirachta indica) that disrupts insect feeding and development.

A microorganism such as a fungus, bacterium, virus, or phytoplasma that causes disease in plants.

A monitoring device baited with synthetic insect pheromones to attract and capture specific pest species for population assessment.

The living vascular tissue in plants that transports sugars and other organic compounds produced by photosynthesis from the leaves to the rest of the tree.

A systemic triazole fungicide widely used in tree care to prevent and treat vascular and foliar fungal diseases, commonly applied via trunk injection.

The selective removal of plant parts such as branches, buds, or roots to improve tree structure, health, or appearance.

A geographically defined area where the movement of regulated articles such as wood, plants, or soil is restricted to prevent the spread of a specific pest or disease.

The transitional zone at the base of a tree where the trunk flares out and meets the major structural roots, also called the root flare or trunk flare.

A disease in which fungal or oomycete pathogens decay the root system of a tree, causing decline, instability, and potentially death.

The area of soil occupied by a tree's root system, typically extending well beyond the drip line of the canopy.

The outer, lighter-colored layers of wood in a tree trunk that actively conduct water and dissolved minerals from roots to leaves.

A group of small sap-feeding insects that attach to bark, leaves, or fruit and often appear as immobile bumps or waxy coatings on plant surfaces.

A treatment method in which diluted pesticide or nutrient solution is poured onto the soil around a tree's base for uptake through the root system.

Microscopic pores on leaf surfaces, flanked by guard cells, that regulate the exchange of gases (CO2 and oxygen) and water vapor between the leaf interior and the atmosphere.

The percentage of ground area covered by the vertical projection of tree crowns, a key metric for measuring urban forest health and planning.

A designated area around a tree, typically defined by the drip line, that is protected from construction activity to prevent root damage.

A systematic process of evaluating a tree's structural condition and site factors to determine the likelihood and consequences of failure.

A method of delivering pesticides, nutrients, or growth regulators directly into a tree's vascular system through small holes drilled in the lower trunk.

The collective population of trees within a city or developed area, including street trees, park trees, residential yard trees, and remnant natural areas.

A plant disease in which the vascular system becomes blocked by a pathogen, preventing water transport and causing leaves and branches to wilt and die.

The woody vascular tissue in trees that transports water and dissolved minerals upward from the roots to the leaves.