Extension Educator, Horticulture
Once we settle into our winter survivability our thoughts pass to whether our landscape plants will survive. One of the first questions to ask in winter plant survival is what is the plant's hardiness zone rating. The National Arbor Day Foundation in 2004 revised the1999 United States Department of Agriculture plant hardiness map. It may seem counter to our winter but The Arbor Day 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. The smaller the number the more cold hardy a plant's rating. To find your zone just go to the website and type in your zip code. http://www.arborday.org/media/zones.cfm
However 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, plant maturity and what stage of growth it's in when cold hits. Healthy plants will survive cold temperature better than plants entering winter in poor health. 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 and may help to keep them dormant if we get a winter warming. Snow is one of the best plant insulators. (See there is something good about snow.)
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 temperatures return the plant may be damaged. Gradually colder temperatures are less stressful on plants than a sharp drop in temperature.
If severe cold temperatures hit when a plant is not quite dormant in the fall or if it's 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 open flowers. For example dormant blueberry flower buds can tolerate minus 20 degrees F whereas the open flower buds can be damaged at 28 degrees F.
Hardiness zone ratings don't take into account microclimates. Your backyard may have a slightly different microclimate than your neighbor's. Areas with south facing brick walls, courtyards, areas near concrete parking lots, or near bodies of water may be warmer.
We won't know for sure the extent of plant damage until spring. Don't be too anxious to pronounce a plant dead. Many times twigs may die but enough of the plant remains to resprout.
Once the snow melts be on the look out for plant roots that have been frost heaved out of the soil. Rather than squishing roots back down, just place some mulch or soil over the roots as soon as possible.
Much of gardening is experimental. An old garden adage is "You have to kill a plant at least three times before you can consider it not hardy." And a dead plant is a shopping opportunity.