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 a sure bet to die. 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 in 1990 is available at http://www.usna.usda.gov/Hardzone/ushzmap.html. Zones are divided according to the average annual minimum temperature. The lower the zone rating, the lower the temperature the plant will tolerate.
The National Arbor Day Foundation 2004 revised map is available at http://www.arborday.org/media/zones.cfm This map lists several areas in a warmer zone. USDA lists much of our area as zone 5b with an average annual minimum temperature of -10 to -15 degrees F. National Arbor Day lists us as zone 5 at -10 to -20 degrees F or zone 6 at 0 to -10 degrees F. To find your zone just go to the website and type in your zip code.
We can generally 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. How lucky do you feel? Paraphrasing Tony Avent from Plant Delights Nursery, "I consider a plant as not hardy only after I 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. Ideally for better plant survival we would wish for temperatures to lower gradually over time, stay cold at relatively stable temperatures then gradually warm in the spring. Well, Opus the penguin wished for wings that worked. 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 damage such as twig dieback. Some maintenance practices encourage plants to go dormant for example refraining from late summer tree fertilization 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 may 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 period of cold (below 40 degrees F) in order for growth to begin once the weather warms.A plant's winter hardiness is just one aspect to survivability. How well an individual plant will survive depends on additional factors such as the plant's overall health, dormancy, maintenance techniques, proper planting and plant maturity. No surprise here but healthy, well-established plants survive winter better. Stress from drought, flooding, nutrient deficiencies, disease and insect attack may predispose plants to winter injury. One more reason to keep our plants healthy.