Planning Ahead New Heat Zone Map Lets You Pick Plants That Will Thrive When Your Area Warms Up

If you’re a gardener, chances are good that you consider the U.S. Department of Agriculture’s Plant Hardiness Zone Map a horticultural bible.

The map, which divides the United States into 11 zones, lets you know which plants grow best where you live so you don’t plant things that are likely to die when the temperatures dip too low.

But cold is only one of the factors that determine whether plants will thrive or die. During the past decade, changes in weather patterns that some scientists attribute to global warming meant the USDA map was no longer enough.

As a result, the American Horticultural Society has created the AHS Heat Zone Map to help you select the plants that will thrive when temperatures are the hottest.

The new map divides the United States into 12 color-coded zones based on how many heat days (over 86 degrees Farenheit) each region experiences annually. High-temperature data from 4,745 weather stations nationwide was compiled from 1974-1995. The zones range from cool-summer areas with less than one day per year over 86 degrees (such as parts of Alaska in zone 1) to subtropical areas with an average of more than 210 days per year over 86 degrees.

“In all my priorities, it became the top priority to do a map like this,” says H. Marc Cathey, author of the recently released book, “Heat-Zone Gardening” (Time-Life, $24.95). Cathey is traveling to 40 flower shows around the country lecturing on the new map.

Cathey has been working on the heat-zone project for 14 years. A research horticulturist at the USDA for 24 years, he became director of the U.S. National Arboretum in 1981 where he was instrumental in the organization of the hardiness zone map.

“Five of the last 10 years were the hottest on record and the growing season is 7-11 days longer than it was 40 years ago,” Cathey says, explaining why the new map was needed.

In addition, he says gardeners who called to talk to him on talk radio began asking for names of plants that could last better through the hot summer.

“Heat doesn’t kill the plants, but it slows them down,” says Cathey, president emertius of the American Horticultural Society, a non-profit organization known for its gardening education programs.

“At about 86 degrees, the plant’s message system begins to shut down, affecting branching and leaf growth.”

Cold can kill a plant right away, but heat damage starts to show up more gradually. Flower buds start to wither. Leaves droop and become better targets for insect damage. As chlorophyll disappears, the leaves appear white or brown. The plant may live in this state for a few years, Cathey says, but when the dessication is high enough, the enzymes that control growth are deactivated and the plant dies.

Unlike the more general hardiness zone map, this new heat-zone map is so precise that it even divides zip codes.

You won’t be able to look up your zip code, however, in Cathey’s book. It’s what he calls the “first step,” a guide that allows the gardener to get a general idea of the heat zone for the region and match it with profiles on 500 plants. He says it will take several years to refine all the data by zip code.

But until then, this new information serves as an introduction to plants that gardeners might not have thought they could grow.

Cathey says in about three years most of the plant and seed packet labels will contain both the hardiness zone and heat zone numbers. The first catalog with the new designations should appear this fall, he says. Both Monrovia and Goldsmith Seeds catalogs are working on the new labeling.

Cathey says the goal is to broaden the variety of plants available to gardeners and to make sure they can be grown successfully. Gardeners, he says, were growing the same 30-50 plants whether they live in Ohio or Florida. This tool will help them branch out.