Summertime Heat Tolerance

Plant Hardiness and Heat Tolerance

It’s summertime and the livin’ is easy; fish are jumpin’ and although the cotton is not yet high, the weeks of 90-degree temperatures are upon us. This got me to thinking about how plants get heat-stressed and the entire concept of heat-tolerance ratings. However, since the gardening public is more familiar with the idea of cold hardiness, I’m going to start with that.

Cold Hardiness
Plant hardiness is a complex phenomenon and depends on many variable qualities of individual plants (not of their projected sites). The Royal Horticulture Society (RHS) Shorter Dictionary of Gardening points out some natural adaptations to cold temperatures include deciduous or herbaceous habit, procumbent growth, thicker bark, seed dormancy or accelerated life cycle, among others.

Cold hardiness is a characteristic attributed to plants that are capable of withstanding the rigors of winter without greenhouse protection. Often abbreviated as “hardiness”, the general population sometimes uses the term loosely to mean a general toughness or ability to survive, but that’s not strictly the case. The actual story is really interesting!

Hardiness Zones

Back in 1960 the U.S. Department of Agriculture (USDA), in cooperation with the American Horticulture Society (AHS), produced an ingenious map based on annual minimum temperatures. Isotherms, contour lines connecting points of equal temperature on a map, were developed using data from 124,500 weather stations and analyzed by the Meteorological Evaluation Services, Inc., in Amityville, NY and labeled Hardiness Zones. Brilliant! They extrapolated variable characteristics of hardy plants to Zones on a map, thus producing an approximation; a guide or rule of thumb; shorthand, as it were, to enable people to get an idea of where plants will make it through the winter.

I am too young (!) to remember a time before Hardiness Zones were in common usage in the horticultural world, but this was a real breakthrough. This precious map was widely publicized, often printed on the inside covers of gardening reference books and was/is used extensively. In 1990 the map was revised to include Canada, Mexico, and Hawaii, and later zone maps were also produced by others for Europe, South Africa, Australia, New Zealand, Japan and China. But keep in mind, theses zones are simply an approximate indication of the coldest temperature band in which the plant will survive. Thus, most gardeners are familiar with the concept of plant hardiness.

Heat Tolerance
However, cold isn't the only factor determining whether plants will survive in the garden. At 86 degrees F. (30 degrees C.) plants begin to experience physiological damage, and with the advent of weeks of 90 degree Farenheit temperatures in Atlanta I really wanted to look at the concept of heat tolerance in plants.

As part of the 75th anniversary celebration of the AHS (1997), a Heat-Zone Map was published, based on the number of days each year that the daily high temperatures reach or exceed 86 degrees. The data used to create the map was gathered and again analyzed by the Meteorological Evaluation Services, Inc. This map has 12 zones which overlap but unfortunately do not follow the hardiness zones exactly, so when using both maps to make selections, two zones are indicated for plant. Each of the 12 zones of the Heat Tolerance Map indicate the average number of days each year that a given region experiences "heat days"-- temperatures over 86 degrees (30 degrees Celsius)-- at the point when plants begin to decline due to the heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days). The use of Hardy or Half-Hardy Tropicals (what we used to think of as “houseplants”) in the garden provides an entire category of plants that laugh at heat waves, and it seems to me that this trend became popular after the advent of the Heat Tolerance data.

Plants vary in their ability to withstand heat, not only from species to species but even among individual plants of the same species! Unusual seasons-fewer or more hot days than normal-will invariably affect results in the garden. And even more than with the hardiness zones, gardeners find that many plants will survive outside their designated heat zone because of complications by many factors such as microclimates, rainfall, hot winds or cloud cover.

On the AHS website, H. Marc Cathey, AHS President Emeritus, points out, “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.”

So the AHS Plant Heat-Zone Map is used the same way that the Hardiness Map is used, but unlike the Hardiness Zone Map, this map is highly proprietary. “The AHS Heat-Zone map is a copyrighted document that is wholly owned by the American Horticultural Society. Any reference to, reproduction of, or attempt to code plants using the map's information without written consent by AHS is a violation of the copyright. Durable full-color posters of the AHS Heat-Zone Map are available for $9.95 each. To order check the website http://www.ahs.org/publications/heat_zone_map.htm or call (800) 777-7931 ext. 119.”

I think because the Heat Tolerance Map is not open source, this resource is less widely used, which I think is shortsighted; too bad. What do you think? Do you bother with the heat tolerance zones when choosing plants for your gardens?

Photo credits: Thermometer-Ohio State Engineering, Hardiness Zone Map-Missouri Extension, Tropical containers-author photo.