What This Year's Polar Vortex Means For Your Winter Garden

Brutally cold temperatures, icy winds, and heavy snow accompany what we commonly call a polar vortex in the winter, but the polar vortex is a dynamic system of interacting forces that exists year-round. When the winds that rotate around the North Pole weaken, cold air escapes to the south and temperatures plummet. Many of the fruit trees, flowering bushes, perennials, and hardy vegetables in your winter garden will survive when temperatures drop into negative numbers, but a few will need protection, and some won't survive. Even heavy snow on your lawn has downsides. 

In 2019's polar vortex, wind chills in Chicago were 50 degrees below zero and the air temperature dropped to minus 23 degrees Fahrenheit. But wind chill, by itself, usually doesn't affect plants; it's the actual air temperature that can cause harm. The leaves and stems of your garden plants are at the same temperature as the air-year round. Wind chill, in its basic definition, indicates how cold it feels, not how cold it actually is. This is why covering your plants when the temperature drops into dangerous territory is helpful. Snow can be useful in the winter garden, providing moisture and protection. In tests of how snow cover affects the temperature around plants, scientists have repeatedly found that the snow-covered soil was significantly warmer than the air above it.

Cold-hardy vegetables that will thrive in your winter garden can tolerate frosts, depending on the varieties you've chosen, but the plants' growth will be slowed and some that form heads like cabbage may lose surface leaves. Leeks, cabbage, and broccoli can withstand temperatures below 28 degrees Fahrenheit, while less cold-hardy vegetables like beets, endive, and carrots tolerate temperatures as low as 28 degrees Fahrenheit without needing extra attention. Tender flower buds on fruit trees suffer, but it's likely the tree will escape harm and enough buds will survive to produce fruit. Flowering bushes like hydrangeas and crepe myrtle that are not cold-tolerant may die back after a prolonged polar vortex, but their hardy root systems produce new growth over time. Your perennials have systems like dormancy to protect them over the winter, but the foliage may die back.

If an abnormally cold winter is in the offing, take steps to protect your winter vegetation from a polar vortex in particular or very low temperatures in general. High winds or heavy snow damage trees with weak branches. Protect your trees from snow and ice damage by pruning in the autumn according to the guidelines for the species. You can brush snow off bushes, but if the branches are encased in ice, let it melt naturally. Covering shrubs or garden plants before temperatures drop is beneficial if it won't be colder than 28 degrees Fahrenheit. Below that, covering isn't effective. Use sheets, tarps, or row cover materials. Elevate the covering so that it doesn't touch the foliage, and remove it in the daytime so plants can warm up in the sun.