In his book “Following the Equator,” Mark Twain says: “The captain had been telling how, in one of his Arctic voyages, it was so cold that the mate’s shadow froze fast to the deck and had to be ripped loose by main strength. And even then he got only about two-thirds of it back.”
Twain may have died many years ago, but he could very well have been talking about the weather we are having this week across much of North America. We have had polar vortexes many times before (2019 and 2014 come to mind) and will have them many times in the future.
Mark Twain may have never heard of a polar vortex, but one exists year-round at each pole on Earth and many other planets. When the arctic polar vortex is strong it stays constrained close to the North Pole. When it weakens, it forms lobes that slide down the face of the Earth, pushing cold weather to the south.
So what can we expect in our gardens from such a cold spell? Be thankful if your landscape is covered in snow. Much of the northern United States had several inches of snow this past weekend. Snow is an excellent insulator, and it protects a lot of plants. Lawns, groundcovers, bulbs, and perennials under snow will have little problem with the bitter cold just inches above them.
The flower buds on many shrubs are not as hardy as the leaf buds. This is evident every spring when we see shrubs that have flowers blooming only on the branches that were protected under snow drifts. This phenomenon will be more evident this spring. The flowers won’t be there, but the plants will survive.
Tree trunks and branches last longer than branches on shrubs that are often pruned out after a few years. If your trees have been in the ground for several years, they have experienced cold like this before. The cold weather last week also helped to acclimate the plants to the colder air this week. If it had been warm last week, some plant tissue may have started breaking dormancy and that tissue would have been more likely to have freeze damage this week.
There is little that can be done to protect trees and shrubs from this cold spell. Small shrubs could be covered in snow. The biggest concern on trees and shrubs is for the ones that are grafted. The desirable above-ground portion of the plant may die and, because it is protected by the snow, the rootstock may survive. If it sends up new growth in the spring it won’t be the desirable plant. For instance, if your rose bush flowers change color this spring, the top died.
Wind chill is not a factor for plants. They do not feel colder when the wind is blowing. But the wind can be a factor in plant survival. Winter air is relatively dry. Winds can dry out the stems, buds, and, on evergreens, the leaves. In the spring, it won’t be possible to tell if dead branches died because of the cold or from drying out too much. In either case, they will need to be pruned off.
The cold will reach the southern United States and Gulf Coast, but these areas receive frosts and freezes nearly every winter. Gardeners in these areas know to take in tender plants and to cover ones that can’t be moved indoors.
Overall, the vast majority of plants won’t be harmed by this cold spell. One benefit that may occur is that many insect pests spend the winter tucked into cracks and crevices on tree trunks and branches. They may succumb to the cold, so it will be interesting to see if that makes a difference this year.
