The American Horticultural Society coordinated with the same people who helped develop the USDA Hardiness Zone map to develop a Heat Zone map based on temperature data from the National Climactic Data Center. Daily high temperatures from National Weather Service stations throughout the United States were compiled for the years 1974 through 1995. In 1997 they produced a national map representing their findings.
The result is a map that color codes the country into 12 zones that indicate the average number of days when the temperature is above 86F degrees (30C). These are "heat days". Zone 1 has an average of less than one day per year above 86 degrees while Zone 12 has an average of more than 210 days above 86 degrees.
Why 86F (30C) degrees? That is the point that many plants begin to experience distress and potential damage from sustained heat. Above this point plants can drop blossoms, drop leaves, fade in color, reduce fruit development, and possibly die. Some plants won't die right away but will be stressed for so long that each year they perform less productively than the year before.
Many plants will wilt in heat, but will recover once temperatures fall. Sustained heat can have a serious physiological impact on some plants and triggers a lingering decline to ultimate death. Knowing how a plant will handle hot days is the reason for the AHS Heat Zone Map.
Most gardeners are familiar with the USDA Hardiness Zone system and look for the number on a plant tag when selecting new plantings. I live in Zone 5 and always make sure new perennial plants are at least hardy down to -20F degrees that the zone represents. I prefer plants hardy to Zone 4 for the occasional extremely low temperatures we get that approach -30F in winter.
I'm in AHS Plant Heat Zone 5. That represents 30 to 45 days above 86F degrees. I prefer to select plants for at least Zone 6 for the recent hot summers we've had; Zone 6 allows for 45 to 60 days above 86F. This year we're definitely encroaching on Zone 6 heat days.
Plant growers and distributors that include AHS Plant Heat Zones on tags will list both zone ranges. You'll now find a listing like "3-9, 6-1". That means the plant is suitable for USDA Hardiness Zones 3 through 9 and is suitable for AHS Heat Zones 6 through 1. Many plant catalogs are also including this information in their plant descriptions.
For me, an ideal plant would be something like "4-9, 7-1". That means it can handle the cold of the Hardiness Zone 4 and the heat of Heat Zone 7. My garden is well within both ranges and the plant should do well.
There are some limitations with the AHS Heat Zone map. Because it is relatively new and unknown, there aren't many resources available to make it easy for you to identify your zone. You have to try and determine exactly where your city falls within the zones on the national map. Apparently the AHS had a tool for determining exact locations, but the zone finder application is nowhere to be found now. I haven't been able to find any other source for finding Heat Zones by zip code like the USDA map has.
You can look at the map at: http://www.ahs.org/gardening-resources/gardening-maps/heat-zone-map
At gardening resources.com I found a breakdown of the Heat Zones by state, which was a little easier to read. You can find it here: http://www.gardeningplaces.com/heatzonemap/
Over 15,000 plants have been coded for heat tolerance. As more plants are coded and more companies begin listing both USDA and AHS zones on plant information, you can expect more gardeners to become familiar and comfortable with the conversion to a two-zone system.
For many of us we choose our plants, put them in our gardens, and then see how they do. For various reasons some plants do well while others struggle. Using both zone maps for selecting plants can help us put in plants that will not only grow well, but will thrive.
If some of your plants didn't do well in summer it may be because they weren't able to tolerate your garden's hot days. That may be an indication that they're inappropriate for your region. Understanding and using the AHS Heat Zones can help prevent similar problems in the future.