In areas that experience freezing temperatures, winter mulching differs from mulching during the growing season. We mulch our gardens in the spring to suppress weeds, retain moisture, and feed and warm the soil. While we may spread a layer of soil conditioning compost or manure in the fall, the primary reason for winter mulching is to protect our plants from the harsh conditions of winter freezes, thaws, and winds.
Why Mulch the Garden in Winter?
The main idea behind winter mulching is to keep the ground frozen by shielding it from the warmth of the sun. A steady temperature will keep the plant in dormancy and prevent it from triggering new growth during a brief warm spell. Tender, new growth too soon will just result in more winter dieback. Mulching now will also help conserve whatever water is in the soil, so hopefully, you’ve been keeping your garden beds watered right up until the hard frost.
What Can You Use to Mulch the Garden in Winter?
Any loose, insulating material will do. Keep in mind that you’ll need to remove the mulch in the spring, or at least rake it aside. So choose a material that’s easy to handle. Shredded mulch, straw, pine needles, or shredded leaves are all easy to remove or easy to work into the soil. If your ground doesn’t freeze until after Christmas, you can use the cut boughs of your Christmas tree as a mulch covering. These are nice because they’re so easy to remove in the spring.
For the ultimate low-effort mulching, snow cover can function as a mulch. Snow is a great insulator and protector of plants. Also, some plants will simply collapse onto themselves and act as self-mulches. Chrysanthemums survive best if allowed to do this.
When Should You Apply Winter Mulch?
When you apply winter mulching will largely depend on how severe winter is in your zone. Whether your aim is to protect the soil from eroding or to protect your plants from drying out over the less-humid winter, most plants will benefit from a little extra protection from winter's harsh elements.
In the Fall
Mulching unplanted garden beds can be done at any time in the fall. Ideally, you would plant a winter cover crop and let it sit until you till it under in the spring. If you choose not to plant a cover crop, it would still be beneficial to spread a layer of compost, manure, or shredded leaves to prevent soil erosion. This is especially important for gardens that go fallow, such as vegetable gardens in the winter.
After the First Hard Frost
Mulching to protect most perennial plants—especially newly planted plants—is done when the soil has started to harden, which is generally after the first hard or killing frost. A hard frost is usually defined as when temperatures drop to below 25 degrees Fahrenheit, but you’ll know it when you see the last of the hardy annuals crumbled and brown in the morning. At this point, your perennials should be well into dormancy, and mulching around them won’t encourage tender new growth. The ground has had time to chill and absorb fall moisture. Go ahead and spread a 2- to 4-inch layer of mulch around the base of the plants to protect crowns and surface roots. Grafted plants, like hybrid tea roses, benefit from being mulched more heavily. These are usually mulched with compost or soil and are actually buried to just over the graft union. You can pile the soil up around the stems or you can use some wire fencing and fill the area with compost.
Some shrubs that are evergreen or somewhat evergreen, like rhododendrons and viburnums, can become desiccated by harsh winds. You can protect the branches and buds by wrapping them with burlap or by spraying on an anti-desiccant, like Wilt-Pruf. If you choose to wrap your shrubs, leave space between the branches and the burlap so the burlap doesn't freeze onto the branches and cause its own problem. You can also fill the space between the shrub and burlap with leaves for additional insulation.
Woody plants don’t require as much protection as herbaceous perennials. However, a 2- to 4-inch layer of shredded bark mulch or compost does help conserve the ground moisture. Just be sure not to pile it around the base of the plants. Keep it several inches from the stems or you’ll invite rodents, like voles and mice, who like the cover of mulch while munching on the bark. Mulching up against the stems also holds too much moisture against the plant, providing ideal conditions for diseases to take hold.
After the Plant Has Died Back
When the ground repeatedly freezes and thaws, it expands and contracts (this is called heaving). When a plant is sitting in expanding and contracting soil, its roots get loosened and the plant eventually gets pushed up through the surface of the soil, exposing its crown and roots to freezing temperatures and drying winds. This is why it's good to lay down mulch after the first winter frost, but before the ground has a chance to unfreeze again. If your zone doesn't get a true frost, another indication is to wait until the top of the plant has died back, then apply a layer of mulch.
Removing Winter Mulch
The rule of thumb is to remove winter mulch in the spring when all danger of a hard frost is past. That’s sometimes very hard to judge, as anyone who’s experienced an Easter snowstorm can attest. However, when the ground starts to thaw and the smell of mud is in the air, it’s time to start raking and removing the mulch so that the ground can warm and new growth won’t be inhibited.