Extension Educator, Horticulture
While waiting for the snow plow to appear on the horizon, I decided cold tolerance would be a good subject of discussion.
If we grew only native plants, cold tolerance wouldn't be an issue. However, we grow peonies from China, dahlias from Mexico, and tulips from Turkey. The quest for growing exotic plants is an interest and even a passion for many gardeners.
Plant catalogs are brimming with picture-perfect perennial plants. The catalog says it's a perennial, but will it live through one of our winters?
To take some of the guess work out of the perennial plant lottery, the United States Department of Agriculture published its Plant hardiness zone map in 1960 and updated it in 1990. It divides North America into eleven zones of average annual minimum temperature. The lower the zone rating, the lower the temperature the plant will tolerate. Most garden reference books, and catalogs use USDA plant hardiness zone ratings.
Check out the plant hardiness zone map at http://www.ars-grin.gov/ars/Beltsville/na/hardzone/ushzmap.html?
Central Illinois is listed as zone 5 or more precisely zone 5b with an average annual minimum temperature of -10 to -15. We managed to beat the average this last week. We can grow plants rated to zone 5 or less. Sometimes we can stretch a zone 6 plant into our area under ideal growing conditions or in a protected area. Paraphrasing Tony Avent from Plant Delights Nursery: I consider a plant as not hardy only after I have killed it at least three times.
Winter cold is not the only criteria for plant survival. Summer heat is also critical. The American Horticultural Society has recently devised a heat zone map. It sections the US into 12 zones. The zones indicate the average number of days each year that a given region experiences temperatures over 86 degrees F. If you want to be the first on your block to have an American Horticultural Society Heat Map, it's available for $14.95 by calling 1-800-777-7931 ext 45.
Both the heat and cold hardiness maps are only indicators. How well an individual plant will survive depends on additional factors such as the overall health of the plant, plant maintenance techniques, planting site and plant maturity. Some diseases may actually predispose plants to winter injury. Plants weakened by drought or root rots are more prone to cold injury. Generally more mature plants will survive cold stress better than newly established plants.
A deep snow cover helps plants to survive severely cold temperatures. (See there is something good about snow.) Gradually colder temperatures are less stressful on plants than a sharp drop in temperature.
Hardiness is also affected by the return of warm temperature. A few days of warm weather in mid to late winter can reduce plant cold hardiness significantly especially in trees and shrubs. Once cold hardiness is lost from late winter warming, the plant cannot return to the same level of hardiness. If mild winter temperatures continue, then damage is not likely. However should severe cold temperature occur the plant may be damaged.
Not all plant parts are equally winter hardy. Cold temperatures which destroy peach blossoms may do very little, if any, harm to the peach tree.