During the winter months the changes that occur to the top of plants are obvious. Perennials die back to the ground, deciduous trees lose their leaves and ornamental grasses take on wintry hues. But what happens to plants below the ground might surprise you.
Winter growth Only the tops of plants undergo drastic changes in winter. Roots do not go dormant. In fact, they continue to grow and take up water and nutrients, at least until the ground around them freezes. Then they stop growing (a process call quiescence). As soon as the ground thaws, however, they start actively growing again.
Limited tolerance Minnesota-hardy trees and shrubs can endure extremely cold conditions. Some, such as certain junipers, can withstand temperatures as low as minus-40 or minus-50 F. Roots, however, are a different story.
Roots vary in their cold tolerance. Some can handle temperatures down to about 10 degrees, while others are in trouble if temperatures drop much below freezing. Fortunately, soil is a great insulator. Just a few inches underground, temperatures rarely get below 28 degrees or so.
Many perennial plants have roots that can't handle temperatures much below freezing. When the temp dips into single digits for any length of time, the temperatures at the surface of the soil -- where the roots of many perennials are concentrated -- can drop to levels the plants can't handle.
That's why so many gardeners mulch their perennials at the end of fall. Mulch helps insulate the ground to prevent soil temperatures from getting dangerously low.
Good insulators Like soil, snow is a good insulator. But there's no guarantee we'll have enough -- or any -- insulating snow to protect perennials all winter.
The good news is that you can still get out there and protect your plants with mulch. Because the temperatures have been relatively mild, it's unlikely that roots have been damaged yet.
