The Homeowners Column

The Homeowners Column

Just like us plants vary in how well they tolerate heat

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

Are you an English wallflower ready for zones 7-1 or maybe a perky petunia at zones 12-3? Gardeners already know and "newbies" will find out that there are many variables to gardening success and heat is one of those variables.

Typically when we select a perennial plant we look at its winter hardiness zone rating. The zone ratings tell us how well a plant should survive our winters ("should" being the operative word.) According to the United States Department of Agriculture (USDA) Plant Hardiness Zone Map first developed in 1960 and revised in 1990 central Illinois is zone 5b. However in 2004 The Arbor Day Foundation revised the USDA map and most areas were given a warmer zone rating with central Illinois warming to a zone 6. Check in books and catalogs to determine which version is referenced.

But as we all know cold is not the only environmental extreme. This summer has reinforced that heat can be just as devastating as cold. New plantings with reduced root systems are particularly prone to heat damage with obvious symptoms of wilting, tip dieback, brown leaves or brown leaf margins, and leaf drop. However heat damage may not always be obvious and can be characterized by 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.

In 1997 The American Horticultural Society 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. Most of us can relate to that.

Plant heat zone ratings list the most extreme heat zone first. 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.

Heat zone ratings 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 www.ahs.org for information about heat zones and winter hardiness zones. A durable full-color poster of the AHS Heat Zone Map is available for $9.95 by ordering online or calling (800) 777-7931 ext. 119.

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. 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.

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