Protecting Plants During a Winter Warm Spell

Q: What can I do in the winter to protect my plants during a warm snap?

A: My best advice is don’t wait until a warm snap to care for your plants. Beyond that, there isn’t much you can do. Once a plant breaks dormancy, it will not go back to dormancy, even if the temperatures return to normal. It also loses its ability to adjust to cold temperatures.

Two types of dormancy regulate plants. There’s endo-dormancy, wherein a plant’s internal clock dictates its state—not the weather. A plant in endo-dormancy is tracking chilling units, or hours of time when the temperature is above freezing (that’s right, not below freezing, but above). Unseasonably warm weather will not jolt the plant out of dormancy, unless it has already met its chilling requirements. Eco-dormancy, meanwhile, is the state a plant enters after it counts enough chilling units. A plant in eco-dormancy can be affected by warmer temperatures; it will begin to grow when temps reach the mid-40s (F).

Ask your nursery for trees and shrubs that break dormancy later in the season, regardless of temperatures. Some are that way naturally while others have been bred for the trait. Plants that are native to the middle of the continent (as opposed to the far north or south) tend to have large chilling requirements to meet before they exit endo-dormancy. They’ve evolved to withstand cycles of chilling and thawing, unlike far north plants, which are most often subject to long winters consistently below freezing, and southern plants, which are used to short, mild winters. Therefore you might choose plants native to that middle area.