How to Use the AHS Heat Zone Map

Most growers’s have some notion of the cold hardiness zones and what they mean. Thanks to cold hardiness zone map, for example, we know that a tropical variety that might survive winter Tampa won’t stand a chance in West Virginia.  But have you ever considered how much heat a plant variety can tolerate before deciding to grow it? 

While extreme cold can kill a plant overnight, death from extreme heat is gradual and can take much longer - sometimes years. Signs of heat damage can appear in various parts of the plant ; buds wither up, leaves fade to brown or white as their chlorophyll drains away, and leaves shrivel and become the target of insect attacks. A plant stunted from heat damage may even survive in a chlorotic state for several years, but when atrophy reaches a certain point, growth enzymes are deactivated and the plant dies.

Use the Heat Zone Map the same way you use the Cold Hardiness Map : locate the zone you live in and use that number code when selecting plants. The the map’s twelve zones show the average number of days each year that a region experiences temperatures over 86 degrees - called ‘Heat Days’. This is the temperature at which plants begin to suffer damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).

Many plant varieties have now been coded for heat tolerance - the same way they’ve been assigned a cold hardiness zone that indicates tolerance for cold. Eventually the heat zone designations will be paired with cold hardiness zone designations in garden catalogs, nurseries, and reference books. The first two of the four numbers assigned to each plant variety indicate the cold hardiness zone, while the last two indicate the heat zone. So for example, a plant listed as 3-9, 8-2 could be safely planted outdoors year round in zone 7. 

Just remember that the heat zone ratings assume that a plant’s roots have received an adequate supply of water, and even a short period of drought can distort the accuracy of the coding system. Certainly some varieties are more drought resistant than others, no plant can survive becoming completely dehydrated. Herbaceous plants are about 85% water and woody plants are about 50% water, and it’s essential that plant tissues contain enough water to carry out photosynthesis.