Until recently, discussions about plant hardiness revolved around winter temperatures. The USDA’s Hardiness Map divides the United States into zones based on minimum winter temperatures. Our area here in north central Illinois falls into Zone 5 of the map. Plants that can survive occasional winter temperatures of 20 below are considered hardy for us.

However, experience will teach you that some plants deemed hardy to Zone 5 will merely survive and others will actually thrive. That’s because there’s more to hardiness than surviving winter lows. The intensity of the sun’s light and heat, the relative humidity, and the difference between night and day temperatures all play a part in hardiness and performance.

In the past few years, the focus has turned to summer hardiness, or heat tolerance. Dr. Marc Cathy, in association with the American Horticultural Society, has drawn up a map dividing the country into heat zones. If and when this map gains general acceptance, we will see plants rated for not only winter hardiness, but summer hardiness as well.

These zones can be a good starting point when selecting new plants for your garden, but there are some additional points to keep in mind. Here are a few to consider.

It might seem logical to assume that plants capable of surviving Zone 4 winters would easily grow in the warmer climates of Zone 6 or 7. While that may be true for some, it may not hold for others. Some Zone 4 plants can’t stand the hot humid summers of Zone 6 and give up the ghost when the Dog Days of summer arrive. A good example is Artemisia ‘Silver Mound’. It grows well here in Henry County, Illinois, but dries up and blows away in Henry County, Tennessee, Zone 6.

Microclimate also plays a role in plant hardiness. Some Zone 6 plants will survive in Zone 5 if planted in a location sheltered from winter winds and afternoon winter sun. Japanese maples and some azaleas can grow around here in protected locations.

Microclimate can also work against you. There are probably some Zone 4 areas in your landscape that will finish off a number of plants that normally grow here.

The recent spell of unbearably hot dry weather was a good opportunity to observe the summer hardiness of plants in the garden. In my landscape several varieties showed outstanding resilience while others were fried to a crisp. The dark mahogany leaves of Heuchera ‘Palace Purple’ (Coral Bells) changed to dark crispy potato chips in the sweltering heat. Purple cone flowers faded to a pinkish tan but the foliage held up well. The coneflower ‘White Swan’ showed no signs of heat stress and most ornamental grasses came through the ordeal unscathed. Some hostas didn’t handle the stress well, even in full shade. Joe Pye weed showed no stress where it had adequate moisture.

Your powers of observation will serve you well as you select plants for your garden. Visit display gardens at regular intervals throughout the growing season, especially after unusual or extreme weather conditions. You will quickly see a number of plants that will do well in conditions similar to those of your own garden.