Extension Educator, Horticulture
While hovering over the warmth of my cup of tea, I decided a good topic of discussion would be cold tolerance. If we grew only locally native plants, we wouldn't have to be concerned with cold tolerance.
But we grow peonies from China, dahlias from Mexico, tulips from Turkey and rhubarb from Manchuria. While we appreciate the beauty and durability of native plants, the quest for growing exotic plants is an interest and even a passion for many gardeners. One of the first questions to ask when making a perennial plant selection is how well will it survive a central Illinois winter. Horticulturists have helped in the decision making process by rating plants according to hardiness zone maps. The maps list the average annual minimum temperature for regions of the country.
The United States Department of Agriculture map has 11 zones with zone 1 being the colder regions of Canada and zone 11 being southern Mexico and Hawaii.
Our area of central Illinois is listed as zone 5 or more precisely zone 5b with an annual minimum temperature of -10 to -15 degrees F. We can grow plants rated to zone 5, 4 or 3 without much worry concerning cold tolerance.
However, other criteria besides cold tolerance will also determine whether a plant survives. A plant's summer heat tolerance (a critical factor around here), soil acidity needs, tolerance to flood or drought and sun exposure needs can also be critical factors in determining a plant's survivability.
How well an individual plant will survive depends on additional factors such as the overall health of the plant, plant maturity and what stage of growth its in when cold hits. Healthy plants will survive cold temperature better than plants entering winter in poor health. Generally more mature plants survive cold stress better than newly established plants. A deep snow cover helps.
Gradually colder temperatures are less stressful on plants than a sharp drop in temperature in early fall. If severe cold temperatures hit when a plant is not quite dormant in the fall or if its coming out of dormancy in the spring, the plant may not survive down to the -10 degrees its rating declares.
Just like my feet and hands are consistently icebergs, not all plant parts are equally winter hardy. Cold temperature which destroys peach blossoms may do very little, if any harm to the peach tree. Our late spring frosts often damage the magnolia and peach blossoms but the plants continue to thrive. Also flower buds are more cold hardy than the developed flowers. Buddleia, butterfly bush, is a woody shrub in southern regions, but here it must form new shoots each year.
Hardiness zones are strictly a guide and not the last word in whether a plant will survive in an area. Your backyard may have a slightly different microclimate. Areas with south facing brick walls, courtyards, areas near concrete parking lots, or near bodies of water may be warmer.
These warmer microclimates can be utilized to grow plants listed as zone 6 with some risk and zone 7 with a great deal of risk and certainly some added protection of straw or leaves is needed. In some years of consecutively mild winters the plants may do fine but then a winter of the century comes along and it's time to get out the catalogs to look for a plant replacement.
Much of gardening is experimental. I agree with Tony Avent of Plant Delights Nursery. I have to kill a plant at least three times before I consider it not hardy.
I'm sure Roger Swain will share his ideas on plant hardiness at the Armchair Gardening program on February 21. Call 333-7672 for more info.