While severely cold winters have become less and less common in our region, it is obvious that we still can count on our extremely hot summers. When selecting trees, shrubs or perennial plants for local landscapes, a plant’s ability to withstand the stress of multiple days of high temperatures during the summer should be considered along with a plant’s ability to survive cold winter temperatures.
The late Dr Marc Cathey, American Horticulture Society (AHS) president emeritus, noted that heat damage is not as obvious as severe cold temperature injury that can kill or damage a plant. Heat damage typically is more of a chronic condition with plants failing over time from accumulated stress that leads to poor growth and attack by insects or disease.
That’s why in 1997 the American Horticulture Society under the direction of Dr Cathey developed the AHS Heat Zone Map. Similar to the U.S. Department of Agriculture’s Hardiness Zone Map, the AHS Heat Zone Map is divided into zones. The Heat Zone map has 12 zones based on the average number of days that “zone” experiences with temperatures above 86 degrees. Above the suitable zones, a plant will suffer heat damage.
Most of Benton and Franklin counties is rated as being in AHS Heat Zone 6 with greater than 45 days and less than 60 days above 86 degrees. However, the area immediately outside the Tri-Cities is rated in AHS Heat Zone 7, with greater than 60 days and less than 90 days above 86 degrees. Thank goodness we aren’t in Zone 1, with less than one heat day, or Zone 12, with greater than 210 days!
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It is important to note that the AHS Heat Zone Map assumes “that adequate water is supplied to the roots of the plant at all times. The accuracy of the zone coding can be substantially distorted by a lack of water, even for a brief period in the life of the plant.” Most plants we place in our area home landscapes are not native to our region and require adequate supplemental watering. Indicating a plant is heat tolerant in our “zone” doesn’t mean that it is drought tolerant.
Water isn’t the only factor that could skew a plant’s ability to thrive in a particular heat zone. Soil aeration and drainage; exposure to light; air circulation; exposure to radiant heat from mulches, structures and paving; soil fertility; and soil pH all affect a plant’s ability to thrive in a particular heat zone.
I am seeing more and more trees and shrubs with a USDA Hardiness Zone Map rating and a AHS Heat Zone Map rating in catalogs and on plant labels. When you go plant shopping, look for these ratings to help ensure your plants will have a long and happy life.
Reminder: Our area is in USDA Hardiness Zones 6B to 7A.
-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.