As I write this, the sky is a brilliant blue, the sun is shining, and the thermometer in my garden reads a pleasant 55 degrees. However, only two weeks ago my plants were subjected to a frigid minus 17, and tomorrow’s high is supposed to barely pass freezing. It’s only February, with plenty of winter yet to come. Sometimes I wonder, how do my shrubs and perennials manage to survive such extremes?
In most years, the parts of the country that experience arctic temperatures also have a significant amount of snow. While we think of snow as very cold, it actually acts as an insulating blanket in our gardens, keeping the soil temperature relatively stable—often not much lower than 32. Then, during warm spells, such as we’re experiencing this week, that snow keeps the ground frozen. Plants stay dormant, and the roots stay buried.
While we have had some snow here in Colorado (finally!), we still don’t have the thick, insulating blanket our plants would like. When the air temperature descends below zero, so does the ground temperature. As a result, plants that may be hardy in Maine and Minnesota are sometimes killed by a relatively warm Colorado winter.
Then along comes our mid-winter thaw. Most of the snow has melted by now, leaving the ground exposed to the intense, high altitude sun. Plants that should be sleeping are beginning to wake up. Even plants that are normally winter-hardy here can lose that hardiness for the season if the weather stays too warm for too long. Then, when the inevitable April snows arrive, the shock is lethal.
As gardeners, we can’t make it snow, but we can help our plants to survive even erratic winters like this one. The solution is simple: mulch. A thick layer of mulch, applied after the ground freezes, will substitute for the missing snow pack, keeping the ground from getting too cold—or too warm too early.
Isn’t it too late? Perhaps. But if your plants are still alive, go ahead and mulch them. Done properly, it surely won’t hurt, and maybe it will save a favorite specimen.
So, how does one mulch properly? There are a few pointers to remember.
- What can be used for mulch? Different mulches are used in different parts of the country. The best option is whatever is available and inexpensive or free. I use straw in my vegetable garden and wood chips elsewhere, because they’re cheap (or free) and plentiful. Some other options might include shredded leaves, leaf mold, compost, wood chips, and shredded bark.
- A layer of mulch (like a layer of snow) insulates the ground as far down as the mulch is high. For example, a one-foot layer of mulch insulates the ground to a depth of one foot. The average frost depth in Colorado is three feet, but you don’t have to use three feet of mulch in your landscape! Since most roots are found in the top 18 inches of soil, an 18-inch layer of mulch is ideal. Just remember to rake most of it off when spring finally arrives. (Leave about four inches for the growing season.)
- Keep damp mulch away from a plant’s crown or main stem. As the mulch rots, so could the plant it’s supposed to protect. The one exception is when you’re protecting a tender plant or graft union (such as on grafted roses) from the cold. In that case, pile the mulch against the plant. Making sure the soil is damp, I then cover the mulch with plastic to keep it from getting waterlogged. Just remember to remove this protection as soon as growth resumes in the spring.
Winter mulch can save your garden, but once the weather warms, some adjustments might be needed. For example, if you have xeric plants, make sure to remove enough mulch to allow the ground to dry out as the snow melts. Soggy soil can be deadly to plants accustomed to perfect drainage. If you’re planning a garden, temporarily clearing off the mulch will allow the soil to warm for planting. (Once the soil is warm, replace the mulch to suppress weeds, encourage earthworms, discourage cutworms, and mitigate extremes in soil temperature and moisture levels.)
If all this seems like a lot or work, just consider all the heavy, persistent snow we don’t have to deal with. I’d much rather scatter straw then shovel several feet of snow!