David Robson: Protect plants from winter winds

Plants are like people. Most plants in our gardens don’t appreciate the cold. They may not mind it, but none are thriving.

Granted, there are some green plants out there.

You can look at the evergreens, from the needled spruces, pines, firs, junipers and yews, to the broadleaf types, including boxwood, rhododendrons, some azaleas and hollies. Some low-lying plants (some ferns, aroids and hellebores) remain green.

These evergreens have built-in genetics that allow them to keep their foliage throughout the winter, though the needles or leaves are not bright green and perky.

Some broadleaf evergreens are more “ever-yellowish-greens.” The cold and wind have destroyed most of the chlorophyll in the plant’s cells but not the underlying pigments or the leaves themselves. Come spring and warmer weather, we’ll see a return of green pigments.

That’s, of course, under ideal situations.

If the weather remains blisteringly cold and winds keep on howling from the north, plant cells will start dying. Once that happens, no amount of warm weather will restore them to their glory.

Years ago, we noted that “it’s not how cold it gets, but how it gets cold that really matters.”
Gradual decreases in temperatures are much better than rapid fluctuations.

But in central Illinois, if you are growing a plant that is adapted to Illinois cold, all you have to worry about is the winter sun and wind.

Plants don’t experience wind chill. They aren’t a warm body generating heat that can be sucked out by wind blowing across its surface. About the only time a plant generates heat is when it’s burned.

But wind still dries out plant tissue much like it can dry out your skin.

Plants counter the drying a couple of ways. First, they drop most of their leaves in the fall and die back to the ground. It’s hard to experience the wind when there is nothing there.

Thick bark also protects against some cold and wind. The thicker the bark, the more insulation a plant has. Even thin-barked trees may have enough codes in their DNA not to experience the cold.

Sitting plants in the yard so they’re protected from wind helps push the winter boundaries on marginally hardy plants. Because most troublesome winter winds are from the north and northwest, develop wind breaks or screens on that side of susceptible plants.

Buildings also block the winter drying winds. Plant cold-susceptible plants on the south sides of garages, houses, sheds and barns. The cold is still there, but at least the winds are funneled around and over.

If the plant is stuck in the middle of the yard, create your own screens with leftover Christmas trees or fencing covered with plastic or burlap — anything that breaks or channels the wind around the specimen.

Granted, it would have been better to put the screening in place during November when the ground was workable. Driving stakes into frozen soil is not easy.

That’s where Christmas trees come in. First, there’s an ample supply right now. Next, they can be stacked or leaned against each other until spring. Some needles will fall off, but they contribute to the soil’s organic matter.

The winter sun is also as bad as the wind at drying.

When the sun shines on a leaf, stem or trunk, it starts warming it up, no matter the temperature outside. We’ve all felt the sun’s warmth even on a sub-freezing day. Plants do, too.

As the plant tissue warms, it gets fooled into thinking it should start drawing moisture from the roots. Then, when the sun goes down, the tissue gets hit with the freezing temperatures. If it’s filled with water, the water expands and destroys the cell walls much like putting a soda bottle in the freezer and forgetting about it.

For limbs and trunks, if this continual freezing and thawing keeps up, you end up with cracks in the bark. Sometimes, these cracks close tightly in spring, such as in a sycamore tree. Other times, like in maples and cherries, the plant may suffer irreversible bark damage.

If needled or broadleaf leaves are affected, they can turn brown and die. This is more of a concern now than in the future. Those leaves should fall off so new leaves can replace them in the spring.

The downside is when buds and stems are killed. The plant may be able to produce new buds below what’s been killed. But no matter — it means you’ll have lots of brown stubs that will need to be pruned come April.

David Robson is a horticulture educator for the University of Illinois Extension. For more gardening information or for your local extension unit office, go to www.extension.uiuc.edu/mg.