Edited by Len Phillips

Drought damaged trees points to a serious flaw in the tree management ordinances (also called community tree laws, landscape codes, or tree bylaws). These codes need to be revised in cities and towns across the country to avoid the selection of shade trees that do poorly in local natural disasters. This article will discuss drought survival. Hurricane and flood survival was discussed in previous Seminars. Survival in other natural disasters will be discussed in future Seminars.

Effects of Low Moisture
Some decrease in growth or flowering can be expected during a period of limited moisture. Severe drought can result in decreased resistance to insect and disease pressure, a decrease in leaf size and number, and a overall decline in growth rate and tree vigor. High temperatures and wind, heat and light reflection from nearby hard surfaces, and high fertilization rates can increase the potentially damaging effects of low moisture on tree growth and survival.

Tree Care
Trees are one of the best investments in areas which experience low water availability. They need to be regularly watered in a limited area for the first three years of growth until they become established and the root system develops to where it can obtain water from the environment. Most trees require very little irrigation water after establishment. Root growth and decomposition provide organic matter to the soil, which will increase soil infiltration and capacity rates of water, and reduce water run-off. Trees shade the ground and reduce the heat and evaporation caused by direct sunlight, which in turn, allows better water infiltration into the soil. Trees (after establishment) do not require their entire root system to receive water in order to survive. Comparatively, trees require less water per canopy area than landscape shrubs, flowers, and turf.

Best Management Practices
The following are some best management practices for responding to water resource challenges during reduced water availability:

• Deeply soak trees and shrubs only after they show initial signs of water stress, and apply water in the morning or evening during periods of low sun and heat, (between 9 pm and 9 am) to prevent excessive evaporation.
• Use a drip emitter, soaker hose, targeted bubbler, or other low-flow garden hose to direct water to the trees roots (not the trunk), and allow the water to slowly seep deep into the soil. A slow trickle is the most effective method for absorption. Watering bags provide an excellent mechanism for ensuring a slow drip which infiltrates into the soil. Watering with a hose at high speed usually results in run-off and rapid evaporation, and encourages root growth near the surface, increasing the tree's susceptibility to water stress.
• Cover bare soil with mulch to retain more soil moisture. Irrigation should be set so it wets the soil under the mulch, or irrigates long enough to thoroughly saturate the mulch area.
• Fall planted trees and shrubs have demonstrated an increased ability to survive moderate moisture levels compared to those transplanted in the spring or summer.

Planting Trees during a Drought
Planting smaller trees [2 in. (5 cm) or less caliper for deciduous trees and 6 feet (1.8 m) or less height for evergreen trees] reduces the monetary investment and risk of planting during drought periods. Planting smaller trees allows the tree to establish more quickly than planting a larger tree and will require less maintenance over time.

• Planting trees during times of drought and water restrictions should only be done with caution and if there is a persuasive need. An understanding of the risks and consideration of proper maintenance activities necessary to establish trees during these periods is crucial.
• Plant trees to replace drought stressed and dead trees, which can help reduce the negative effects of drought on the landscape by lowering the heat and solar radiation reaching the soil.
• Keeping trees in the landscape helps reduce soil erosion, by stabilizing soils and intercepting rainfall, significantly reduces storm water runoff, and shades landscapes and structures to help minimize water and energy use.
• Factors to consider when planting trees and shrubs include soil conditions, available space above and below ground, exposure, moisture, and light requirements.
• Select trees from species that are hardy to the region and fit well with the xeriscape principles of maintaining an attractive landscape with minimal water use.
• Proper mulching and adherence to watering guidelines for trees and shrubs will help establish newly planted trees in times of drought.

Why Plant Trees During a Drought?
Planting trees and shrubs during drought can be risky. If watering restrictions are in place, establishing trees in a semi-arid region may be difficult enough without an extended drought to contend with. However, by eliminating all tree planting we are missing many opportunities to sustain our urban forest. It is critical to not only have a diverse set of species in the landscape but also a diversity of age among those species in the landscape. This means planting new and replacement trees each year, even during times of drought, to replace trees that will be lost to age, injury, and other causes.

Carefully planting trees continues to maintain soil stability, reduce soil erosion, control and utilize storm water runoff, shade moisture-starved lawns and reduce energy usage by shading homes in summer and blocking winds in winter. A well-stocked urban forest also acts as an air filter and purifier, absorbing carbon dioxide and emitting oxygen to help provide cleaner air.

LARGE DROUGHT TOLERANT TREES
Scientific Name Common Name
Acer leucoderme Chalkbark maple
Acer platanoides Norway maple
Acer pseudoplatanus Sycamore maple
Acer rubrum Red maple
Acer x freemanii Freeman maple
Alnus glutinosa Common alder
Betula lenta Sweet birch
Calocedrus decurrens Incense cedar
Carpinus betulus European hornbeam
Carya illinoensis Pecan
Castanea sativa Spanish chestnut
Catalpa speciosa Northern catalpa
Cedrus atlantica 'Glauca' Blue atlas cedar
Cedrus deodara Deodar cedar
Cedrus libani Cedar of Lebanon
Celtis laevigata Hackberry
Celtis occidentalis Hackberry
Cladrastis kentuckea Yellowwood
Cryptomeria japonica Cryptomeria
Cunninghamia lanceolata China fir
Cupressus arizonica Arizona cypress
Eucommia ulmoides Hardy rubber tree
Fraxinus americana White ash (in non-EAB infested areas)
Fraxinus pennsylvanica Green ash (in non-EAB infested areas)
Ginkgo biloba (male) Ginkgo
Gleditsia triacanthos Honeylocust
Gleditsia triacanthos inermis Thornless honeylocust
Gymnocladus dioicus Kentucky coffeetree
Ilex opaca American holly
Juniperus saliciola Southern red cedar
Juniperus virginiana Eastern red cedar
Kalopanax pictus Castor-aralia
Liquidambar styraciflua Sweetgum
Maclura pomifera Osage-orange
Magnolia grandiflora Southern magnolia
Metasequoia stroboscopes Dawn redwood
Nyssa ogeche Ogeche gum
Oxydendrum arboreum Sourwood
Phellodendron amurense Amur cork tree
Picea glauca Alberta spruce
Picea pungens Colorado blue spruce
Pinus bungeana Lacebark pine
Pinus cembra Swiss stone pine
Pinus echinata Shortleaf pine
Pinus elliottii Slash pine
Pinus flexilis Limber pine
Pinus heldreichii Bosnian pine
Pinus koraiensis Korean pine
Pinus mugo Mugo pine
Pinus nigra Austrian pine
Pinus rigida Pitch pine
Pinus sylvestris Scotch pine
Pinus taeda Loblolly pine
Pinus thunbergiana Japanese black pine
Pinus virginiana Virginia pine
Pinus wallichiana Himalayan pine
Platanus acerifolia London plane tree
Pyrus calleryana cvs. Cleveland, Aristocrat, Capital, New Bradford, Redspire,
Quercus acutissima Sawtooth oak
Quercus alba White oak
Quercus bicolor Swamp white oak
Quercus coccinea Scarlet oak
Quercus falcata Southern red oak
Quercus hemisphaerica Darlington oak
Quercus imbricaria Shingle oak
Quercus lyrata Overcup oak
Quercus macrocarpa Bur oak
Quercus nigra Water oak
Quercus nuttalii Nuttall oak
Quercus palustris Pin oak
Quercus phellos Willow oak
Quercus prinus Chestnut oak
Quercus rubra Red oak
Quercus shumardii Shumard oak
Quercus stellata Post oak
Quercus virginiana Live oak
Robinia pseudoacacia Black locust
Sassafras albidum Sassafras
Taxodium ascendens Pond cypress
Taxodium distichum Baldcypress
Tilia americana American linden
Tilia cordata Littleleaf linden
Tilia tomentosa Silver linden
Ulmus alata Winged elm
Ulmus americana cvs. Liberty, Valley Forge
Ulmus parvifolia Lacebark elm
Zelkova serrata Zelkova 

SMALL DROUGHT TOLERANT TREES
Scientific Name Common Name
Acer barbatum Southern sugar maple
(A. saccharum ssp. floridanum)
Acer buergeranum Trident maple
Acer campestre Hedge maple
Acer ginnala Amur maple
Acer leucoderme Whitebark maple, calk maple
Acer negundo Boxelder
Acer truncatum Purple blow maple
Aesculus californica California buckeye
Aesculus pavia Red buckeye
Albizia julibrissin Mimosa
Alnus japonica Japanese alder
Alnus serrulata Tag alder
Carpinus betulus European Hornbeam
Carpinus betulus fastigiata Upright European hornbeam
Carpinus caroliniana American hornbeam, Ironwood
Carpinus japonica Japanese hornbeam
Carpinus orientalis Oriental hornbeam
Cercis canadensis Eastern redbud
Cercis canadensis ssp retisus Oklahoma redbud
Cercis canadensis ssp. texensis Texas redbud
Cercis chinensis Chinese redbud
Chionanthus retusus Chinese fringe tree
Cornus mas Cornelian cherry dogwood
Cotinus coggygria Smoketree
Cotinus hybrids GraceCotinus obovatus American smoke tree
Crataegus crus-galli Cockspur hawthorn
Crataegus laevigata Scarlet hawthorn, English hawthorn
Crataegus phaenopyrum Washington hawthorn
Crataegus x lavallei Lavalle hawthorn
Crataegus viridis Green hawthorn
Cupressus sempervirens Italian cypress
Cydonia sinensis Chinese quince
Cupressus arizonica cvs. Arizona cypress
Elaeagnus angustifolia Russian olive
Euscaphis japonica Korean sweetheart tree
Heptacodium miconiodes Seven-son flower
Hovenia dulcis Japanese raisin tree
Ilex 'China Boy', 'China Girl' China holly
Ilex cassine Dahoon holly
Ilex cornuta cvs. Burfordii, D'Or, O' Spring
Ilex decidua Possumhaw
Ilex latifolia Lusterleaf holly
Ilex pedunculosa Long stalk holly
Ilex vomitoria Yaupon holly
Ilex x 'Emily Bruner' Emily Bruner holly
Ilex x 'Mary Nell' Mary Nell holly
Ilex x 'Nellie R. Stevens' Nellie R. Stevens holly
Ilex x attenuata Savannah, Foster, Sunny Foster, East Palatka
Ilex x koehneana Koehne holly
Juniperus scopulorum Rocky Mountain juniper: Blue Haven, Skyrocket, Wichita Blue,
Juniperus virginiana cvs. Blue Mountain, Hillii, Canaertii
Juniperus chinensis Chinese juniper: Wintergreen, Spartan, Hooks
Koelreuteria bipinnata Goldenraintree
Koelreuteria paniculata Goldenraintree
Lagerstroemia fauriei Japanese crapemyrtle
Lagerstroemia indica Crapemyrtle
Lagerstroemia indica x fauriei Choctaw, Muskogee, Natchez, Tuscarora
Lithocarpus henryi Henry tanbark oak
Ilex vomitoria 'Pendula' Weeping yaupon holly
Maackia amurensis Amur Maackia
Magnolia grandiflora 'Little Gem' Little Gem magnolia
Magnolia hybrids Ann, Betty, Judy, Mary Nell, Galaxy
Malus hybrids David, Harvest Gold, Indian Summer, CallawayMalus spp. Crabapple
Morus australis 'Unryo' Contorted mulberry
Ostrya virginiana American hophornbeam or ironwood
Oxydendrum arboreum Sourwood
Parrotia persica Persian ironwood
Persea borbonia Redbay
Photinia serrulata Chinese photinia
Picea glauca (dwarf cultivars) Conica
Picea pungens (dwarf cultivars) Bakeri, Fat Albert, Foxtail
Pinus mugo Mugo pine
Pinus nigra cvs. Arnold Sentinel, Monstrosa
Pinus rigida Sherman Eddy
Pinus strobus cvs.
Pinus thunbergiana cvs.
Pistacia chinensis Chinese pistache
Poncirus trifoliata Hardy orangePtelea trifoliata Hop tree
Rhamnus caroliniana Carolina buckthorn
Rhus typhina Staghorn sumac
Trachycarpus fortunei Windmill palm
Ulmus glabra 'Horizontalis' Tabletop Scotch elm
Vitex agnus-castus Chastetree; vitex
Ziziphus jujuba Common jujuba

Sources

• Evans, Erv, "Drought Tolerant Trees", North Carolina State University, 2000.
• Mann, Gordon, "Trees during a Drought", Archive #39 from Online Seminars for Municipal Arborists, July/August 2011.

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