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.

Choose an organic mulch, such as leaves, straw or wood chips. Spread a thick layer of mulch (at least 4 inches) around the base of perennials and over bulb beds.

When mulching trees, it's important to pull the mulch a few inches back from the trunk. A thick layer of mulch too close to the trunk can cause the roots to girdle the tree.

If you don't mulch at all, Minnesota-hardy plants may do just fine, but Zone 5 plants and tender perennials will be at risk.

The container effect

Plants that are growing in containers outside have little or no winter protection. If you want to overwinter perennials in containers outside, try pushing mulch around the container.

You also can try storing containers in a shed or garage. That offers some protection from extreme cold. Because there's no rain or snow inside a structure, remember to apply a little bit of water to the containers once a month for the duration of the winter -- however long that may be.

Jeff Gillman, an associate professor of horticulture at the University of Minnesota, is the author of several gardening books.