Shade and Street Tree Care (B 1031) University of Georgia Extension With proper care, trees can be valuable commodities around our homes, communities and urban landscapes. Providing care requires understanding tree biology, or how and why trees function. Trees constantly interact with the environment, including changes in soil, light, temperature, moisture, competitors and pests. Humans can produce additional stress by altering environments, but with proper care and maintenance trees can survive and thrive in your landscape. 2014-07-25 17:31:52.0 2006-06-02 14:27:02.0 Shade and Street Tree Care | Publications | UGA Extension Skip to content

Shade and Street Tree Care (B 1031)

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Revised by Timothy Daly1
Original manuscript by James T. Midcap2 and
Kim D. Coder3

With proper care, trees can be valuable commodities around our homes, communities and urban landscapes. Providing care requires understanding tree biology, or how and why trees function. Trees constantly interact with the environment, including changes in soil, light, temperature, moisture, competitors and pests. Humans can produce additional stress by altering environments, but with proper care and maintenance trees can survive and thrive in your landscape.

To best care for shade and street trees, it is important to learn to recognize problems and understand how trees react to changes in their environment. Caring for trees when they are young can prevent many major defects as they mature and ensure good growth with long-term structural stability. Trees with structural weaknesses can be potentially dangerous to humans and property. These “hazard” trees need to be identified and immediately removed.

Proper care can also correct life-threatening problems, ensure continued health and protect trees from environmental extremes and construction damage. Correctly diagnosing the real cause of problems rather than simply treating symptoms is important. For example, treating a symptom, such as yellow foliage, by applying fertilizer without first determining the true cause of the yellowing may cause undue stress for the tree. Timely treatments that are properly applied will keep trees in the best health possible.

How Trees Grow

To understand tree life, death and proper care, it is essential to understand how trees relate to different parts of their environment. Trees are complex and highly adaptable organisms. Knowing how they function can help ensure tree health.

Trees shoots grow up into the air. The leaves collect carbon dioxide gas from the air and capture light. Roots grow downward and outward into the soil and survive where there is moisture and oxygen. Roots absorb water and essential elements while providing structural support. Chemicals (hormones) govern the timing of shoot and root growth.

Leaves

Green leaves contain chlorophyll, which captures light. The chlorophyll pigment holds light long enough for other molecules to absorb the light energy. Captured light energy activates carbon dioxide molecules gathered from the air. Carbon atoms in each molecule string together like beads and release oxygen. Strings of carbon form sugar, starch and other compounds trees use as food.

Carbohydrates, which come from sugar and starch, move down the tree from the leaves to the roots through the inner bark of the twigs, branches and trunk -- not from the roots to the treetop. Along the way, living cells use those carbohydrates as food. Unused food is stored as starch. A branch or twig must make and store all its own food. Food for next spring?s growth is stored in the last few annual rings of wood close to the branch tip.

Buds

Leaves develop in the growing tip of the shoot. Hardened bud scales cover and protect each growing tip and the developing leaves inside. Each bud consists of protective covers, new stem pieces, new leaves and the growing tip. Buds form in the axil (stem connection) of every leaf and produce hormone signals to keep the top of the tree in communication with the root system. Some buds hold flowers.

Tree buds contain growing points that produce
leaves and shoots. Figure 1. Tree buds contain growing points that produce leaves and shoots. Buds can be grown over with bark and released to grow at a later time.

Within every bud there are leaves with new buds at their base. This redundancy (nesting one bud inside another) allows trees to survive poor environmental conditions and pest attacks.

Buds failing to expand into shoots may be overgrown by bark. Every year bark-overgrown buds grow only enough to keep pace with the tree expansion. These buds are called latent or dormant buds. Latent buds occur around each branch base since every branch originated from a single bud. Branch pruning can release dormant buds near the pruning wound.

Trees maintain thousands of growing points (buds), but few have the opportunity to expand and grow. The terminal bud on a branch controls all the buds below, and the most active buds control the tree. Removing a dominant terminal bud can release the controlled buds.

Bark

Tree bark insulates the living tree against the environment and keeps water in the tree. Old corky bark consists of wax and oil-impregnated walls of collapsed cells. Specialized air channels called lenticels occur in the bark and allow trees to receive oxygen. The living cells of the inner bark transport carbohydrates.

Trunk

The tree stem, or trunk, supports and elevates the leaves, and includes a main trunk, branches and twigs (branches are large twigs and twigs are small branches) that increase in size annually.

The trunk is the transport system for moving materials from the root to the leaves and back again. Water and essential elements move up the trunk and out the branches to the leaves. These materials move in the outer-most annual rings of wood. Carbohydrates move in the opposite direction ? through the inner bark from the leaves down the branch and trunk to the roots.

Roots collect water and essential elements that
move up the trunk and into the branches. The leaves lose
water, collect carbon dioxide and light, and manufacture
food, which moves down the branches and trunk and back
into the roots. Figure 2. Roots collect water and essential elements that move up the trunk and into the branches. The leaves lose water, collect carbon dioxide and light, and manufacture food, which moves down the branches and trunk and back into the roots.

Tree stems must grow every year. Each spring and summer a new sheath of living wood covers last year?s tree. This year?s tree is simply last year?s tree refurbished with new tissue. If a tree cannot grow every year, it will decline and die.

A cross section of a tree trunk has many layers. The outside of the tree is dead bark, which protects the tree. The inner bark is alive and carries food from the top of the tree to the roots. Food and other materials move downward in the inner bark and then toward the tree center through ray cells.

Cambium

The cambium is a layer of cells between the bark and wood that rapidly divides to produce wood and inner bark. The cambium layer expands the twig, branch, trunk and woody root diameters every year, and is a major reaction site that responds to injury.

Inside the cambium are annual rings of wood. The large pores in each annual ring carry water up to the leaves. Although these large pore cells are dead, some fiber cells surrounding the large pores are living. The outer four to 20 annual rings, called sapwood, are usually alive and light colored. The center of large tree trunks is usually darker colored and is called dead heartwood. Many chemicals are produced or deposited in the heartwood. Heartwood in some trees is resistant to decay.

Tree stem structure showing the position of
major parts. Figure 3. Tree stem structure showing the position of major parts.

Branches

Branches attach to the tree trunk by interlocking branch and trunk tissues. A wood branch collar produced by the trunk holds the branch base. Branch and trunk tissues expand against each other in the branch crotch. Bark pushes up into a ridge called the branch bark ridge. When the bark ridge is unable to push outward, the bark becomes surrounded by woody trunk and branch tissue. Overgrown bark is referred to as “included bark.” Branches with included bark are weak and likely to split apart.

Leaves on every branch or twig must produce enough food to feed that branch or twig. Food does not move from the roots or other branches to supply a starving branch. Branches unable to support themselves are sealed off. Branches that lose their connection to water and elements cannot produce food. Only productive branches survive.

Roots

Roots develop and survive where plenty of oxygen and moisture occur. Root tips grow and colonize soil areas during most of the year and stop only when soil temperatures are cold.

Most tree roots extend out between two and five canopy diameters from the main stem. Most active roots occur in the top 12 inches of soil. The heavier the soil, the closer to the surface roots will be because they need oxygen. Roots cannot grow well below heavy hardpans or in flooded or compacted soils.

Roots occur as both perennial woody roots and annual absorbing roots. Woody roots become thicker every year with wood and bark just like stems and branches. Woody support roots grow downward and outward to anchor the tree in place. Massive numbers of annual absorbing roots develop from woody roots.

Annual absorbing roots form shallow, horizontal fans growing in the soil. These roots are the ones that take up water and essential elements. Thousands of annual root fans develop and then die during the growing season. Root fans occur where there is plenty of water and essential elements, such as beneath decomposing litter on the soil surface.

Tree roots try to avoid each other when they are young, but may be forced together to form a graft union as they grow larger. Root grafting can cause problems under special circumstances. For example, all elms on a street may be connected by root grafts. These grafts can conduct diseases from one tree to the next.

Roots absorb and transport water and essential elements. Elements move slowly downward with water from one soil particle to another. Tree root fans lie horizontally near the soil surface to capture water and dissolved elements as they move past.

Leaves control water absorption in the root. Water in the leaves evaporates (transpiration) through leaf pores (stomates). As the water molecules evaporate, they pull up water molecules behind them. Leaf transpiration pulls long strings of water from the soil into the root, up the stem and to the leaf. During drought conditions, leaves cannot produce enough force to remove water from dry soils and can be damaged.

Defense

Trees defend themselves by sealing off problems from the rest of the tree with barrier walls, which develop when damage occurs or the inside of the tree is exposed to the environment. These barriers form biologically and physically sealed compartments. Decay, disease, insect and mechanical damage are walled-in to seal damaged portions off from the rest of the tree.

Communication

Tree branches must communicate with roots and trunks to ensure proper growth and defense. Trees do not have a nervous system like animals but do have a chemical communication system. Buds produce auxin, a communication substance that passes from the shoot tip and leaves to the root tips through living cells. Root tips produce a second communication substance, cytokinin, which enters the water stream and moves to the top of the tree.

The ratio of these and other growth hormones changes with seasonal activity and the health of the shoots and roots. Each cell in the tree continually reads the combined messages so shoots always know what the roots are doing, and vice-versa. Responses are dictated by the tree?s genetic make-up. Synthetic communication substances, such as herbicides, can disrupt growth.

Growth

Not all parts of a tree grow at the same time. Trees grow in episodes. Roots grow for a while, then the shoots grow. In a large oak, for example, one branch may grow for a couple of weeks, then another branch will grow.

Growth requires plenty of water. Growing points inside buds create new cells, but water must be available to expand these cells. Without water for hydraulic expansion, tissues remain small. Extension growth requires the right chemical signal, cell replication and enough water to enlarge the cells.

Tree growth rates change from day to day, day to night and throughout the seasons. The better the water conditions, the more growth a tree will experience. Since water is more readily available at night, most tree elongation occurs at this time.

Climate and Site Influences

Climate influences tree growth principally through light, temperature, moisture and wind, and affects optimum spacing, irrigation, fertilization, pruning and pest control. Seasonal climate changes determine which trees adapt and thrive. Shade and street trees must be able to endure and respond to climatic changes over their life spans.

Light

The sunny days of summer may have light intensities four times that needed for maximum photosynthesis, but beneath tree leaves light intensity falls off quickly. Light levels within the canopy may be so low that leaves cannot make enough food to survive. Other plants also may not be able to grow beneath a tree because of dense shade.

Many trees are sensitive to photoperiod, the day length that regulates vegetative and reproductive activity. Photoperiod can influence leaf shape, leaf drop, fall color and the onset of dormancy. The long days and short nights of spring and summer promote vegetative growth. The short days of late summer and fall slow elongation and initiate overwintering activities.

The length of the dark period between light periods actually regulates photoperiod response. Streetlights can artificially shorten night length and promote continuous growth. As a result, sensitive trees may begin growth early in the spring or fail to halt growth in the fall and experience cold injury. High-pressure sodium, metal halide and incandescent street lamps are most likely to cause growth problems. Mercury vapor lamps affect trees the least. Table 1 lists light-sensitive trees.

Table 1. Selected tree response to nighttime lighting.

Sensitive
Tolerant
Basswood
Ash
Birch
Bradford Pear
Black Locust
Ginkgo
Catalpa
Hickory
Cottonwood
Holly
Dogwood
Magnolia
Elm
Oak
Goldenrain-tree
Pine
Hemlock
Spruce
Honeylocust
Sweetgum
Maple
Walnut
Redbud
Silverbell
Sycamore
Yellow-poplar
Zelkova

Temperature

Temperature is an uncontrollable environmental factor. It is closely related to the amount of sunlight present, since visible light energy is absorbed and radiated as heat. Sunlight not reflected by or used in the leaf is felt as sensible heat, which can be dissipated by water evaporation from leaves or soil.

Low temperatures can kill or injure trees. Critical periods for cold injury include spring and fall frosts, the coldest portion of mid-winter and cold periods immediately following winter warming periods. Cold injury causes leaf and flower bud death, tree trunk sunscald, immature stem dieback and frost damage to tender shoots and flowers.

Rapid temperature changes can be especially devastating. Sudden temperature drops in late fall before trees harden-off can result in severe injury. Warmer temperatures in late winter or early spring allow trees to lose hardiness. Extreme temperature drops under these conditions can result in tree death.

Tree species and cultivars also influence hardiness. Growing hardy selections minimizes potential injury from low temperatures. To enhance fall shoot maturation, reduce water and fertilization of trees sensitive to fall cold late in the season. Once cold weather begins, moist soils ensure adequate water supplies and improve soil heat absorption. Vigorous trees with large food reserves withstand cold temperatures better.

Tree-dormancy chemicals in vegetative and flower buds prevent growth during winter. The dormant resting condition is the key to winter survival because it prevents growth during periods of warm winter weather. During winter, dormancy chemicals are slowly broken down. Serious injury may occur after dormancy inhibitors are gone and warm temperatures initiate growth. Rapidly dropping temperatures can then kill active buds and tissues. Little can be done to prevent this type of injury. Some trees lose their dormancy inhibitors after only a month or two of cold weather and are repeatedly injured by cold.

Cold temperatures are not the only climatic extreme that causes tree injury. High temperatures can also cause injury by desiccation (drying out) when transpiration greatly exceeds moisture absorption. As the water content in leaves decreases, stomates close, leaves wilt, transpiration drops and leaf temperatures increase due to reduced evaporative cooling. High night temperatures increase respiration rates and food consumption. Trees living in constant high temperatures may exhibit little growth.

Moisture

Natural rainfall provides moisture for new and established trees. Periodic droughts can be devastating. Going weeks without moisture can create problems for trees, especially those with small soil moisture reservoirs (i.e., limited root zones). Many landscaped areas have restricted moisture supplies. Building and paving can increase transpiration and reduce moisture in root zones.

Low soil moisture causes leaves to wilt and young shoots to droop. When low moisture persists, leaf margins and tips begin to brown. Browning spreads between the veins and continues until the leaves are completely dead or drop off. Irrigate before permanent injury occurs to the foliage to keep trees healthy.

Soils

Soil supplies trees with water and essential elements and is frequently responsible for the poor performance of many shade and street trees. Soil is a complex physical, chemical and biological system. All soils are biologically alive with a wide range of organisms, including bacteria and fungi. About half the volume of an ideal soil consists of pore spaces filled with air and water. Soil characteristics influence water infiltration and movement, water and essential element retention, and aeration. The hills, slopes and valleys of the landscape influence trees through water accumulation, runoff, erosion and temperature fluctuation.

Soil pore size determines air movement and root growth. Most soil particles are held together as aggregates. Aggregate formation produces large pores that aid in the infiltration and movement of water and the exchange of air. The water and essential element-holding capacity of individual soil particles still function within each aggregate. Soil aggregates are fragile and easily compressed or destroyed. Excessive traffic or construction, especially when the soil is wet, readily destroys aggregates and reduces water and air movement. Site development and building construction can compact soils, and adding soil fill around existing trees prevents oxygen and water movement to the roots.

Soil depth and texture determine the moisture and essential element reservoir and influence rooting depth. Usually, the deeper the soil, the greater the water and essential element supplies. Distinctly different soil layers can interrupt air and water penetration. Subsoil layers with high clay content cause water to accumulate and form a perched water table. Poor aeration from water driving the oxygen out of the soil pores above the clay layer restricts root growth. Sand or gravel layers can also interrupt the normal penetration of roots and water.

Soil Elements

Soil elements are available to trees in the soil solution. Only a small portion of essential elements in a soil is available for tree use at any one time. Essential elements are absorbed by roots, held by the soil or leached deeper into the soil. As existing elements become depleted, more elements move into the soil solution. Clay particles attract positively charged ions, like potassium, calcium and magnesium, to their negatively charged surface. These ions held near clay particles move into the soil solution and are taken up by trees or move deeper in the soil.

Negatively charged ions, like nitrates, phosphates and sulfates, are held near organic materials with positive charges. These ions are quickly taken up or leached from the soil. Negatively charged ions may combine with other elements, precipitate and become insoluble (such as phosphorus). Soil microorganisms can quickly change nitrogen compounds into inert atmospheric nitrogen.

pH

Soil reaction, expressed as pH, refers to the acidity or alkalinity of a soil. Soil pH influences tree growth by affecting solubility of essential elements and the activity of microorganisms. Soils with high pH, above 7.8, are highly alkaline and have little iron or manganese available. Under acidic conditions (low pH), manganese and aluminum may reach toxic levels. Microbial activity in soils, especially nitrogen fixing bacteria, decreases as soil pH decreases below 5.5. Most trees grow in the pH range of 5.5 to 7.8. Acid-tolerant trees can grow in soils as low as pH 4.0.

Pruning

Pruning determines the future shape, structural design and continued health of a tree. Correct pruning prolongs tree health and reduces future maintenance. Improper pruning may be life threatening. Flush-cut pruning causes severe damage. A proper pruning cut for larger branches has three steps:

  1. Cut on the underside of the branch 8 to 12 inches out from the trunk. Undercut about one-third of the way through the branch to prevent bark tearing.
  2. Move out 2 to 3 inches beyond the first cut and cut down. The second cut completely servers the branch and it falls away without tearing the bark.
  3. Remove the remaining stub. Make the cut outside the branch bark ridge and branch collar.
To remove heavy branches without damaging
    the tree, a three-cut sequence is recommended. Cut
    to the branch collar (swollen area where the branch joins
    the main trunk) and avoid leaving a stub. Figure 4. To remove heavy branches without damaging the tree, a three-cut sequence is recommended. Cut to the branch collar (swollen area where the branch joins the main trunk) and avoid leaving a stub. (This figure was taken from “Pruning Ornamental Plants in the Landscape,” UGA Cooperative Extension Bulletin 961.)

Trunk tissue at the bark ridge and in the branch collar remains uninjured. The branch collar tissue forms a natural protective barrier against pests and decay. A proper pruning cut leaves the smallest possible wound to callus over.

Proper pruning leaves the branch collar intact
    with no branch stub. Figure 5. Proper pruning leaves the branch collar intact with no branch stub. (Photo: Timothy Daly, Gwinnett County Extension.)

Prune twigs, small branches or large limbs to the outside of the branch collar and bark ridge. Flush-cutting a branch even with the stem is unprofessional and abusive to trees because it damages the trunk, inhibits natural barrier formation and provides entry for pest or decay organisms. Proper pruning leaves the branch collar intact with no branch stub. Stubs prevent wound closure and can result in decay entering directly into the main stem of the tree. The remaining branch collar initially bulges out from the trunk but loses its prominence in a few years.

Tree Shaping

Use proper branch pruning techniques to remove unwanted branches and to shape your trees. Never stub back or shear trees. Cutting off the ends of the outside branches leads to dead stubs that become a liability. Dormant buds grow, forming new branches around the wounds. These new branches grow rapidly and densely. Repeated and progressively heavier pruning will be required over time. Pests and decay will enter into the stubbed branch ends. Large numbers of weak branches will continue to grow.

To shape a tree or remove unwanted branches, prune the branches at the trunk or where they are attached to a major branch. Pruning wounds are left inside the tree crown, rather than at branch tips, reducing the number of sprouts because of shading. Follow the three-cut pruning technique outlined above. Proper branch pruning pays off in good tree health.

Wound Paints

Wound paints are cosmetic only and are not required for treating fresh wounds. They do not hasten callus development or protect the tree from decay. In fact, paints can increase pest problems and prevent wound closure. Use tree wound paints only when clients demand disguising bright pruning cuts.

Topping

“Topping” a tree involves cutting the main trunk or major branches off below the top of the tree. Treetop removal should be done only under extreme circumstances. Consider total tree removal before topping. Tree crowns may be lowered when growing into overhead wires, buildings or other trees. Improper crown removal results in decay, heavy resprouting, loss of aesthetic form and severely reduced life span.

Trees that have been topped are weakened and
are more susceptible to damage and pests. Figure 6. Trees that have been topped are weakened and are more susceptible to damage and pests. (Photo: Timothy Daly, Gwinnett County Extension Agent)

Drop-crotch crown removal (cutting back to existing large branches) is the best way to lower a tree?s crown. Always follow proper pruning techniques by cutting parallel with the branch bark ridge. Do not leave a flat, horizontal cut.

Tree topping is not recommended. When necessary,
use a properly applied drop-crotch method to lower
tree canopies. Always remove branches back to large
lateral branches. Figure 7. Tree topping is not recommended. When necessary, use a properly applied drop-crotch method to lower tree canopies. Always remove branches back to large lateral branches.

Girdling

Girdling wounds can kill trees by preventing the transport of food and raw materials. Damage from cuts, lawn mowing, weed removal with a weed eater, or pressure from ties or chains on trunks and branches can damage the inner bark and cambium. Tight guywires, tree wrap f

Status and Revision History
In review Feb 24, 2009
In review Feb 24, 2009
Published with minor revisions on Jul 9, 2011
Reviewed on Jul 25, 2014