About Cold Hardiness
Every plant species is adapted to the growing conditions in the region where it is native or has been growing successfully for years, and each grows best elsewhere only wherever climate and temperatures are similar to these familiar conditions. Because they can withstand the coldest winter temperatures in these areas, they are described as “hardy” there. For instance, a camellia may be cold hardy in Georgia where it lives normally, but it not in New York where winters are more severe.
The cold hardiness of each plant is a concern because the typical residential yard is full of plants that are from other regions with varying maximum winter temperatures. While they are probably basically winter hardy for your yard (because they have survived several winters there already) individual ones may be only marginally so, experiencing stress during the cold that reduces their vigor and pest resistance year round. Others, while hardy, may be inherently brittle or shallow rooted and easily damaged by winter weather. In either case, these plants would benefit from some winter protection.
Winter can be tough for outdoor landscape plants in the northern states. This is the time our plants can suffer serious root damage, sometimes because of something we didn’t do last fall. February can be Mother Nature’s payback time.
We may grumble and gripe when we have to shovel snow off the sidewalk and driveway, but our plants just love snow. Snow cover is an amazing insulator for the soil. Snow is 90% air so it works just like the insulation material we use in our homes. Just two to four inches of snow, if it hangs around for a few weeks, will often keep the soil temperature just a bit above freezing even though the air temperatures are staying in the 20’s or lower during the day.
We are taught that trees and shrubs “go dormant” in the winter. That is true, but the internal functions of these plants do not stop, they just slow way down. As long as the soil is not frozen, trees, shrubs, and even some perennials will still be growing roots right through December. While we had all that snow on the ground in December, our trees and shrubs were still taking up nutrients and water and still losing water through transpiration. While the earthworms were definitely down below 12 inches taking a winter nap, many of the beneficial soil microbes, probably wearing mittens, were still plugging along creating nutrients, making space to store water, and protecting plants from disease.
Problems begin when that snow melts leaving the ground bare. Cold winds and low temperatures over bare soil will cause the soil to freeze. If it stayed frozen, things would be less stressful. However, it is the freezing and thawing that occurs in southeastern Michigan in a normal winter that can do bad things to plants. When that occurs, plant roots can be damaged or killed. Perennials and small shrubs can even be popped out of the ground as the soil moves with the freeze-thaw cycle.
When the soil does finally freeze, usually in early January, high winds become more dangerous because they cause trees, especially evergreens, to lose water more quickly at the very time water is no longer available from the frozen soil. Evergreen needles at the ends of branches can look burned and then die. The more shallow roots can dry out and die.
The least number of problems occur if that soil stays above freezing as long into the winter as possible. While snow cover in November, December and early January helps to keep the ground from freezing, the very best way to avoid damage from the freeze-thaw cycle is to have applied a 3 to 4 inch layer of organic mulch around all young trees, shrubs, and perennials some time in October or early November. The best material is chopped leaves, but finely shredded bark will also do the job. If there is no snow cover, that mulch will protect the roots of plants by reducing soil temperature fluctuations when the air temperature is going up and down each day.
Is it too late to mulch your small trees and shrubs? Not at all, but it is hard to find shredded bark in the garden centers; check out the big boxes. It will become available in March as the stores stock up for spring.
If you have evergreen shrubs, there is another step to help them get through the winter with less damage. Once that soil is frozen, there is no water available to those plants. However, since they are evergreens and have leaves or needles that are still transpiring water, all be it slowly, they are vulnerable to damage to those leaves and buds as they are buffeted by late winter winds.
If you didn’t get a chance to put mulch around all your small trees, shrubs, and perennials last fall, they are definitely vulnerable to Mother Nature’s desiccating winds and fluctuating temperatures. Get some mulch down come spring and your plants will be protected. It doesn’t pay to mess with Mother Nature.
We have a very complete range of plant protection products in our Yardener's Tool Shed; click here