|American Horticultural Society Heat Zone Map|
Although nearly all American reference books, gardening magazines, nursery catalogs, and seed packets describe plants using the U.S. Department of Agriculture's Plant Hardiness Zone Map, the impact of heat on plants should also be considered.
The American Horticultural Society publishes a Heat Zone Map. There are 12 zones on the map that indicate the average number of days each year a zone experiences "heat days" or temperatures over 86 F (30 C). At 86 F plants begin to suffer damage. Roots may stop growing, flower buds may wither, leaves may droop or appear white or brown because chlorophyll may disappear.
The Meteorological Evaluation Services company (which was also involved in the creation of the USDA Hardiness Zone Map) analyzed daily high temperatures recorded between 1974 and 1995. Only National Weather service stations that recorded maximum temperatures for at least 12 years were included to create the map. Weather stations at or near mountain peaks in sparsely populated areas were not included.
I found the map inaccurate for my area. I typed in my zip code in the heat zone finder at ahs.org. It put my area in heat zone 8, which means 90 to 120 days over 86 F (30 C). On average here in the desert we experience 186 days over 86 F, which would put my area in zone 11. The map was published in 1997, so it probably needs updating.