Plant Palette

Plant Palette

Hardiness and Heat Zones

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture

Sometimes looks can be deceiving, especially when it comes to mail-order plant catalogs. It's hard not to be seduced by the glossy colorful pages, each one filled with plants that will surely look stunning in your garden. Or will they?

I'm sure many of us have planted a plant that we know will look "perfect" in a particular spot, only to have it die–maybe not at first, but after the first season or first year. One of the mantras that I find myself repeating time and time again to frustrated homeowners is to plant "the right plant in the right place". And believe me, sometimes I don't heed my own advice! There are a lot of factors that contribute to a plant performing well in your garden. But where do you start?

Prior to the 1960's, gardeners had little to rely upon when choosing reliable plants other than word-of-mouth and personal experience. Henry T. Skinner, the second director of the U.S. National Arboretum, released the first "Plant Hardiness Zone Map" in 1960. He worked closely with horticulturalists to utilize meteorological data as an element in predicting plant hardiness.

The basic division on the map is the Hardiness Zone. The U.S. and Southern Canada was divided into ten zones, with each Zone defined by a difference of ten degrees Fahrenheit between average annual minimum temperatures. Each Zone is divided into 'a' and 'b' based on a five degrees Fahrenheit difference in average annual minimum temperature. Based on the latest "official" update of the map released in 1990, Decatur, Illinois and the surrounding area is in Zone 5b.

When the Hardiness Zone Map was initially developed, winter hardiness was singled out as the most important factor in determining plant adaptation and survival.. But survival does not necessarily mean the plant performs well in the garden. Classification of a given plant using the Hardiness Zone Map is intended to indicate regions of the country where the plant performs well. Keep in mind that many plants will survive outside of their given Hardiness Zone but may not look their best.

While Hardiness Zones are a good place to start in classifying plant performance, there are limitations. The biggest limitation appears to be that it doesn't take summer heat into account in determining Zones. While two given locations in the same Zones may have the same average annual minimum temperature, they may differ greatly in their summer temperatures. Differences in summer heat can translate to differences in how well a plant performs.

We all know what happens when a plant suffers from exposure to temperatures too cold for survival: the plant dies. Plants exposed to too much summer heat don't necessarily die right away. Leaves and flower buds may whither and die, roots may not grow, and the plant may look stunted, but remain alive. Potentially, the plant can linger in this sub-par state for years before it is weakened to a point that the next heat wave deals a death blow.

In 1997 the American Horticultural Society (AHS) released their "Plant Heat Zone Map", which like the USDA's map divided the U.S. into zones. The map defines twelve zones, with divisions based on differences in annual average number of "heat" days–number of days with temperatures above 86 degrees Fahrenheit. Based on the map, Decatur, Illinois and surrounding areas are in Heat Zone 7. Some nurseries and mail-order companies are beginning to incorporate Heat Zones into plant descriptions along with the traditional Hardiness Zones.

If not otherwise specified, the tip-off as to whether you're looking at a Hardiness Zone rating or Heat Zone rating is the order of the numbers. Each rating begins with the most extreme condition the plant is rated for. In the case of Hardiness Zones, the first number is the coldest Zone the plant is rated for, so the lower number comes first, such as 5-8.. For Heat Zones, the first number is the hottest Zone the plant is rated for, so the highest number comes first, such as 6-1.

In 2003, AHS attempted an update of the 1990 USDA Hardiness Map using weather data from July 1986 through March 2002. This new data set shifted many of the Zone divisions northward and seemed to indicate the environment was becoming warmer. Many critics cited the data used as skewed, since the years used included "unusually" warm winters. Since there is still much debate about the validity of the "new" map, the 1990 map is still considered to be the official Hardiness Zone Map.

In 2004, the Arbor Day Foundation completed their own revision of the USDA's Hardiness Map. They used fifteen year's worth of data from the same meteorological stations that the USDA records data from. The jury is still out, but the revised map appears to validate and support the data used in the AHS revision, again showing shifts in where the Zone boundaries lie. According to this revised map, Decatur, Illinois and the surrounding area is a solid Hardiness Zone 6. To view an animated map of how this revision altered the previous map, visit .

Of course, there are factors contributing to a plant surviving and thriving that no map addresses, but sometimes we can. Factors within our control include soil quality and pH, air movement, placement to influence heat exposure, light exposure, wind exposure, and proper watering. Unfortunately, some factors like day length are totally out of our control. If you need some help in optimizing these factors for your plants, give the Master Gardeners a call at 217-877-6872. They would be happy to assist you in creating a beautiful garden this year!

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