Trees for Hot Sites

 

Trees for Problem Landscape Sites — Trees for Hot Sites

Authors: Bonnie Appleton, Extension Specialist, Hampton Roads AREC, Virginia Tech; Eva Lynn Trump Rudiger, Graduate Student, Virginia Tech; Roger Harris, Editorial Contributor, Virginia Tech Department of Horticulture; Kathy Sevebeck, Editorial Contributor, Virginia Tech College of Natural Resources; Dawn Alleman, Editorial Contributor, Norfolk VCE; and Lynnette Swanson, Editorial Contributor, Chesapeake VCE.

Publication Number 430-024, posted January 2000



Hot landscape sites require special consideration before trees are planted. Trees can survive, and even thrive, in hot sites if the site is prepared correctly, if heat-tolerant species are selected, and if the trees are properly maintained. A variety of different locations and situations qualify as hot landscape sites.

Hot site locations

Because of large masses of asphalt and concrete that absorb and reflect heat, urban areas tend to be an average of 9° F to 12° F warmer than surrounding wooded areas. Buildings and roads cool slowly, so heat continues to radiate even after sunset ‹ an effect called the “urban heat island.”

The foremost Norway maple is nearest the paving and as a result has scorched leaves.
 

  • Areas adjacent to buildings also tend to be hot. Buildings reflect heat onto trees, especially along southern and western walls.
    Heat reflected from the building has scorched the leaves of this linden.
  • Sites near roads and parking lots are hot. The temperature of automobile surfaces can exceed 122° F during the summer. When combined with the heat from car fumes and the temperature of asphalt and concrete, these sites can be lethal for trees.
    The foreground tree is dying because it is planted in a small volume of hot, dry soil in a parking lot.
  • Underground utilities can also create hot sites. Soils around certain utilities, such as steam lines, can be significantly hotter than adjacent soils.
  • Containers and raised beds become hotter than in-ground planting areas. Where soil is above ground level and uninsulated by surrounding soil, sites are subject to greater extremes of heat and cold.
  • Open areas, such as fields and parks, are typically hotter than wooded areas where trees provide shade for each other and the surrounding soil. Leaf litter on the ground in wooded areas keeps the soil cooler, as does mulch in open, unshaded landscapes.

    Dogwoods planted in hot, unshaded areas are more likely to suffer heat damage than… …dogwoods planted in their natural habitat – shaded, wooded areas.

    Why heat is a problem for trees

    High temperatures have a detrimental effect on tree leaves and roots. Increased leaf temperatures cause trees to cool themselves through the process of transpiration. As temperatures rise, water vapor is released through small pores (stomata) in the leaf surface, thereby cooling the tree. On a hot day, a large deciduous tree can transpire as much as 100 gallons of water into the surrounding air. This volume of water is not always available to trees on hot sites because of inadequate moisture in he soil. A lack of available water to trees in hot sites often results in scorched (dried) leaf margins or dead leaves.

    Scorched leaves on a flowering dogwood grown in full sun.
    High temperatures can also adversely affect roots. Optimum tree root growth occurs when soil temperatures are between 60° F and 80° F. When soil temperatures exceed 95° F, roots cannot function and begin to die. Loss of vital roots, and their uptake of water and dissolved nutrients, can result in tree death.

    Preparing hot sites for planting

    To help trees adjust to hot sites, preparation prior to tree planting is important. If large planting beds will be used, organic matter such as compost can be incorporated into the soil to improve soil structure, air movement and water retention. Incorporation of organic matter into the backfill soil for individual planting holes is not recommended. Installation of an irrigation system may be beneficial because irrigation can supply water for transpirational cooling.

    Selecting trees for hot sites

    Trees that are genetically capable of tolerating high temperatures should be selected for hot sites. Some species of trees are naturally heat-resistant, and many cultivars are available that have been developed for their ability to withstand high temperatures. Consideration should be given to site moisture levels because some trees can withstand heat only when adequate moisture is available.

    Maintaining trees on hot sites

    After heat-tolerant trees have been planted, additional maintenance is needed to ensure long-term tree health. Irrigation can be critical. Trees should receive 1″ of water per week during the growing season to replace water lost through transpiration. Water deeply and include areas beyond the tree’s dripline (crown spread) where small absorbing roots are located.

    Mulching is also very important. Maintain a 2 – 4″ layer of mulch over as much of the tree’s root zone as possible. Mulching will help keep the soil surrounding the roots cooler, prevent moisture evaporation and water runoff, minimize competing weed growth, and reduce the amount of light and heat that reflects onto leaf and stem surfaces. Remove competing vegetation (weeds, grass) frequently to improve tree survival.

    Fertilize trees in hot sites only as needed. Nitrogen causes trees to grow quickly, and an extra flush of new leaves may wilt and die due to heat stress. Do soil testing and observe plant health to determine if and when supplemental fertilization is needed.

    Prune trees as necessary to remove broken, diseased, damaged or leggy growth. Pruning improves overall tree health, reduces water demand, and decreases the amount of water that is lost due to transpiration.

    In summer, shade newly planted trees with netting or boards until their roots become established and capable of absorbing adequate water for transpiration. Leaves on trees transplanted during summer heat, or into hot sites, are sometimes sprayed with antitranspirants to prevent excessive water loss. In extreme heat, leaves are even stripped from trees to combat desiccation.

    Trees for hot sites

    Common Name Latin Name Cultivars and Comments
    Trident maple Acer buergeranum Tough, pest-resistant
    Hedge maple Acer campestre Tolerates drought
    Norway maple Acer platanoides ‘Summer Shade’
    Red maple Acer rubrum Tolerates urban conditions
    Sugar maple Acer saccharum ‘Green Mountain’ and ‘Legacy’, but not for Southeastern Virginia
    Freeman maple Acer x freemanii ‘Armstrong Two’
    Red horsechestnut Aesculus x carnea ‘Briotii’
    River birch Betula nigra Provide irrigation
    White birch Betula platyphylla ‘Whitespire’ resists borers
    Shagbark hickory Carya ovata Very adaptable
    Common hackberry Celtis occidentalis Tough, wind-tolerant
    Cockspur hawthorn Crataegus crusgalli Tolerates drought
    Washington hawthorn Crataegus phaenopyrum Tough, thorny
    Japanese cryptomeria Cryptomeria japonica Evergreen
    Leyland cypress x Cupressocyparis leylandii Evergreen; avoid wet areas
    Hardy rubber tree Eucommia ulmoides Tolerates urban conditions
    White ash Fraxinus americana Tolerates drought
    Green ash Fraxinus pennsylvanica Tough, wind-tolerant
    Ginkgo Ginkgo biloba Select male trees
    Honey locust Gleditsia triacanthos Tough, tolerates poor soil; ‘Shademaster’ is thornless
    Chinese juniper Juniperus chinensis Evergreen; ‘Spartan’
    Rocky mountain juniper Juniperus scopulorum Evergreen; ‘Skyrocket’
    Eastern redcedar Juniperus virginiana Evergreen; ‘Glauca’
    Goldenraintree Koelreuteria paniculata Very tough summer bloomer
    Crape myrtle Lagerstroemia spp. Tough summer bloomer
    Sweetgum Liquidambar styraciflua Fruitless ‘Rotundiloba’
    Waxmyrtle Myrica cerifera Tolerates sandy soil
    Colorado spruce Picea pungens Monitor for spider mites
    Chinese pistache Pistacia chinensis Adaptable, pest-resistant
    London planetree Platanus x acerifolia Tolerates drought
    Chinese podocarpus Podocarpus macrophyllus Pest-resistant
    Pissard plum Prunus ceracifera ‘Atropurpurea’
    Yoshino cherry Prunus x yedoensis Monitor for borers
    Callery pear (cultivars) Pyrus calleryana Avoid ‘Bradford’
    Laurel oak Quercus hemisphaerica ‘Darlington’ for coastal areas only
    Willow oak Quercus phellos Tolerates urban conditions
    English oak Quercus robur Tolerates drought
    Red oak Quercus rubra Tolerates urban conditions
    Live oak Quercus virginiana Good coastal selection
    Japanese pagodatree Sophora japonica Tolerates urban conditions
    American arborvitae Thuja occidentalis ‘Pyramidalis’ for form
    Oriental arborvitae Thuja orientalis Tolerates poor soil
    Littleleaf linden Tilia cordata ‘Greenspire’
    Silver linden Tilia tomentosa Tolerates drought
    Lacebark elm Ulmus parvifolia Tolerates poor soil, urban conditions
    Chastetree Vitex agnus-castus Tough summer bloomer
    Japanese zelkova Zelkova serrata Tolerates poor soil

    Some trees, such as Box Elder (Acer negundo), Tree-of-Heaven (Ailanthus altissima), and Black Locust (Robinia pseudoacacia) are heat tolerant and, if found growing on a site, should be considered for use, but otherwise are not considered to be desirable landscape trees due to insect problems and/or invasive growth.

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