The old gold standard for determining if a plant will live or die in your climate has always been the U.S. Department of Agriculture's cold-hardiness map, which I wrote about last week. While it's a great reference, it's not the only tool in the shed for gardeners.
"Cold isn't the only factor determining whether our plants will survive and thrive. Particularly during seasons of drought, we are all aware of the impact that heat has on our plants," according to American Horticultural Society President Emeritus H. Marc Cathey.
"The effects of heat damage are more subtle than those of extreme cold, which will kill a plant instantly. Heat damage can first appear in many different parts of the plant: Flower buds may wither, leaves may droop or become more attractive to insects, chlorophyll may disappear so that leaves appear white or brown, or roots may cease growing. Plant death from heat is slow and lingering. The plant may survive in a stunted or chlorotic state for several years. When desiccation reaches a high enough level, the enzymes that control growth are deactivated and the plant dies."
Because of these concerns, the Horticultural Society came up with a new way to classify different planting zones. The AHS heat-zone map is used in much the same way as the USDA cold-zone map, but with different criteria for determining the zones.
The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days" - temperatures higher than 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).
Thousands of garden plants have now been coded for heat tolerance, with more to come in the near future. You will see the heat-zone designations joining cold-hardiness-zone designations in garden centers, references books and catalogs.
For local gardeners, the Kanawha Valley is in the range of zones 6 and 7. Individual gardeners can find their zones by typing in their ZIP code in the zone finder at www.ahs.org/publications/heat_zone_finder.htm.
Looking at the map, West Virginia is all over the zones - we have zones 2 through 7 within our borders.
How the map was created
The data used to create the map were obtained from the archives of the National Climatic Data Center. From these archives, Meteorological Evaluation Services Co. Inc. in Amityville, N.Y. - which was also involved in the creation of the hardiness map - compiled and analyzed National Weather Service daily high temperatures recorded between 1974 and 1995. Within the contiguous 48 states, only NWS stations that recorded maximum daily temperatures for at least 12 years were included. (Because of the amount of missing data in Alaska and Hawaii, the 12-year requirement was reduced to seven years at several stations.)
Reach Sara Busse