The Homeowners Column

The Homeowners Column

Heat Zones

Photo of Sandra Mason

Sandra Mason
State Master Gardener Coordinator

Are you an English wallflower zones 7-1, or maybe a perky petunia at zones 12-3? Gardeners know and "newbies" will find out that there are many variables to gardening success. When we select perennial plants one important guideline is its hardiness zone rating. The zone ratings tell us how well a plant should survive our winters (should being the operative word here.) Central Illinois is zone 5b according to the United States Department of Agriculture's Plant Hardiness Zone Map first developed in 1960 and revised in 1990.

But as we all know cold is not the only environmental extreme. Our heat can be just as devastating as cold. However heat damage may not be as obvious. It is usually characterized by a slow decline. Plants may appear stunted and not thrive due to poor root growth or flower buds may die or leaves may appear yellow or whitish.

The American Horticultural Society has developed a heat zone map to help take some of the guess work out of knowing which plants will thrive or at least tolerate the heat in your area.

The map is divided into 12 zones with zone 1 being areas with less than one heat day to Zone 12 with more than 210 heat days. Heat day totals are the average number of days each year that an area has temperatures over 86 degrees (30 degrees Celsius). That is the temperature where plants begin suffering physiological damage from heat. (People too, I think.)

As an example, Black-eyed Susans, which do well here, are listed as heat zones 9-1. Delphiniums, which often suffer in our heat, are rated 6-1. Much of central Illinois is zone 6 so as you can see we are on the top range of heat that a delphinium will tolerate.

Since the heat zone ratings are fairly new they are not listed as regularly as hardiness zones. Eventually plants will be listed with two ratings - the heat zone and hardiness zone in books and garden centers. In the mean time you can refer to the AHS website for information and a copy of the map. You can also get a durable full-color poster of the AHS Heat Map for $9.95 by calling (800) 777-7931 ext. 110.

The American Horticultural Society has published a reference book, AHS Great Plant Guide, which lists over 2,000 plants and their heat zone and hardiness zone ratings. It's also just a great book with lovely colored photos of what AHS categorizes as great plants. Their criterion includes: excellent for ornamental use; neither frail, weak nor weedy; available in horticultural trade; and not particularly susceptible to insects or diseases.

Remember heat zones and hardiness zones are just guides. Many factors affect a plant's survival. For instance, when a plant experiences cold can mean life or death of the plant. A plant normally listed as hardy in your area may still die if the cold occurs early in the season when the plant is not thoroughly dormant.

Also the availability of water can effect how well plants live in heat. Mulching will help to conserve water. Providing water to the plant roots instead of overhead will reduce moisture lost through evaporation and runoff from over head watering. Also areas of your landscape may have microclimates of added heat such as next to brick walls or asphalt. Hey, if gardening were straight forward, I'd be out of a job.

Not sure what to do with those extra zucchinis? Than donate them to the Plant-a-Row for the Hungry Program by bringing them and any other produce to the Urbana Schnucks supermarket at 200 Vine Street from 10am 1pm each Saturday through the summer. Master Gardeners will be there to help load. All produce goes to the Eastern Illinois Foodbank.

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