Extension Educator, Horticulture
Roll the dice. It's time to play "will my plant live outdoors through the winter?" Some plants, palm trees for example, are sure to be compost fodder after winter weather. Plants are also better able to survive winter planted in the ground as opposed to being left in pots. What are the odds for other plants?
To take some of the guess work out of the plant survival lottery, plants are rated according to hardiness zones. The United States Department of Agriculture plant hardiness zone map revised this year is available at http://planthardiness.ars.usda.gov/PHZMWeb/ Zones are divided according to the average annual minimum temperature. The lower the zone rating, the lower the temperature the plant will tolerate. Zones range from frigid 1 to tropical 13. On the 2012 map some parts of Illinois were changed to a warmer zone. Central Illinois ranges from 6a (-10 to -5) and 5b (-15 to -10 degrees F). To find your zone just go to the website and type in your zip code.
Central Illinois gardeners can generally grow plants rated to zone 6 or less. Sometimes we can stretch a zone 7 plant into our area under ideal growing conditions or in a protected area. We gardeners suffer from zonal envy. Our motto: A plant is considered not hardy only after we have killed it at least three times.
We are better able to predict winter survival once we understand how plants are affected by cold. Plant Health Care for Woody Ornamentals, a great reference by University of Illinois and International Society of Arboriculture, states "it's not how cold it gets, but how it gets cold." Rapid temperature fluctuations can be deadly for plants usually considered cold hardy.
Nature reminded us of this lesson this spring when we had an unusually warm March followed by a typically cold April. Plants and flowers that normally would sail through April were severely damaged. Ideally for better plant survival we wish for temperatures to lower gradually over time in the fall, stay cold at relatively stable temperatures through the winter then gradually warm in the spring. Well, I wish to grow two inches taller and Opus the penguin wishes for wings that work. We don't always get what we wish for.
In our climate most woody plants spend the winter blissfully dormant. You know - dormant. That stage men enter into as they sit in the mall waiting for their wives. By late autumn plants are physiologically dormant and can withstand much lower temperatures than if they weren't dormant. For instance a Norway maple can withstand a temperature of -30 degrees F when it's dormant. During the growing season, however, temperatures just below freezing can cause extensive twig dieback. Some maintenance practices encourage plants to go dormant for example refrain from late summer tree fertilization when combined with pruning.
Some plant parts are more cold hardy than others. Cold temperatures which destroy peach blossoms may do very little, if any, harm to the peach tree. The above ground parts of plants can withstand much lower temperatures than the roots. For instance boxwood stems tolerate -10 degrees F where as the roots can be killed at 20 degrees F. Hardy woody plants and perennial flowers may be killed in containers left outdoors all winter. So does that mean you can tuck your baby maple tree next to your recliner during the winter? Not a good idea. Plants in our climatic region generally need a cold period (below 40 degrees F) for growth to resume in spring.A plant's winter hardiness is just one aspect to survivability. An individual plant's life or death depends on additional factors such as the plant's overall health, maintenance techniques, proper planting and plant maturity. No surprise here but healthy, well-established plants survive winter better.