Pensacola News Journal

For: Saturday, June 9, 2001

By: Daniel E. Mullins

Extension Horticulture Agent

Santa Rosa County

The Heat Zone Map: A Valuable Resource for Gardeners

Can high summer temperatures cause landscape plant damage? You bet! We are well aware of cold injury, but are less familiar with the effects of heat on the different plant species.

The effects of heat damage are usually subtle: flower buds wither, intense green leaf color declines, leaves wilt easily, there is increased vulnerability to pests and roots stop growing. These symptoms can continue, reappearing for several summers until the enzymes that control growth are shut down.

Death from heat is due to a slow, lingering decline. Many plants begin sustaining physiological damage at 86 degrees F., with the number of days above this threshold having a cumulative effect.

Obviously, plant species and varieties vary in their ability to tolerate extended periods of both high and low temperatures. Wouldn’t it be great if we could choose plants, knowing the exact zone where they perform best?

The American Horticultural Society has produced a Plant Heat-Zone Map that will help in choosing just the right plant for a specific location. This map was developed so that gardeners can identify the heat zone for their region. The country is divided into 12 zones, based on the average number of days each year that the temperature reached or exceeded 86 degrees F.

Data for the map was collected from 4,745 weather stations for many years. This data was analyzed by the same company that produced the USDA Cold Hardiness Map. The two maps, and the resulting plant codes, are used in exactly the same way.

Using the AHS Plant Heat Zone Map and the USDA Cold Hardiness Map we can more accurately choose plants that have a better chance of thriving. The next step is providing information about which plants are appropriate for a specific heat and hardiness zone. Plants will hopefully one day be sold with tags containing two numbers – the range of heat and cold that they will tolerate.

Gardeners who make plant choices outside of their particular zone might be able to grow them, but not without extra effort. The idea when using the heat zone map is to make gardening easier and more fun by choosing plants best adapted to our area. It’s like swimming with the current rather than against it.

The American Horticultural Society is located at George Washington’s River Farm. The Heat-Zone Map can be ordered by contacting the Society at 7931 East Boulevard Drive, Alexandria, Virginia 22308-1300, or call 1-800-777-7931.

Question of the Week: I recently sprayed my azaleas and the leaves suddenly turned brown. Could I have damaged them by spraying.

Answer: This question was accompanied by a branch sample that clearly revealed spray burn. In this case a labeled product was used, but some damage was sustained anyway. Plants can be damaged even when sprayed with an approved product if they are drought stressed at the time of application, if the product is mixed improperly or if it is applied during a hot, bright time of day.