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Digging In: Much ado about mulch

  • Article by: Nancy Rose
  • August 16, 2009 - 10:28 AM

This year's drought was a good reminder that just a few inches of mulch can help keep soil moist and cool in summer. But why do we need to apply extra mulch for winter, since the soil is frozen?

Well, it's really for the same reasons we use summer mulch: to retain soil moisture and moderate soil temperatures.

What winter mulch does

Maintaining even soil moisture through winter helps keep shallow plant roots from drying out and dying. And by holding soil moisture in, winter mulch also helps prevent cold, dry air from penetrating as deeply as it would in dry, bare soil.

Winter mulch isn't meant to keep soil as toasty warm as it was back in August. That's why it isn't applied until after the soil starts freezing. Winter mulch may keep the soil a few degrees warmer, but its real job is to keep the soil evenly cold through winter, avoiding some of the freeze-thaw cycles. Frequent freezing and thawing can damage roots and push the crowns of shallow-rooted plants (think mums and coral bells) out into subzero air, where they can be damaged or killed.

Mulch in warmer winters

In the old days -- when we had real winters and everyone walked 14 miles to school -- we could rely on a good insulating snow cover every winter. Here in Minnesota, snow is a gardener's best friend because it helps moderate soil temperatures and prevent those freeze-thaw cycles.

While our string of warmer-than-average winters has been enjoyable for some folks, the accompanying lack of snow cover has not been good for gardens. In the last couple of years, many gardeners have unexpectedly lost perennials and bulbs they assumed were hardy. It was most likely early winter cold snaps when there was no snow cover that did them in.

That's all the more reason to apply winter mulch this year, especially if you planted bulbs or perennials this fall.

In addition to offering early winter protection, winter mulch keeps the ground cold in early spring, which helps keep bulbs and flowers from emerging too early, only to get zapped by a late cold spell. Recently planted trees and shrubs, especially any that are marginally hardy, will benefit from winter mulch around the root area as well.

How to apply winter mulch

Wait until the ground starts to freeze. (This varies from year to year, but usually ranges from mid-November in northern Minnesota to mid-December in southern Minnesota.)

Once it's mulched, evenly frozen soil resists heaving caused by freeze-thaw cycles. Waiting until the soil freezes to mulch also helps cut the number of burrowing rodents such as mice and voles digging around mulched plants.

• Use a 4- to 10-inch layer of mulch for most sites.

Clean straw or hay makes great winter mulch because the hollow stems (from wheat, barley, oats or grass) create an insulating air layer.

Dry leaves work, too, although rain and snow tend to make them mat together, which causes them to lose some insulating properties.

• To keep loose leaves from blowing away, build corrals of short wire fencing around plants or garden sections, then fill the corral with leaves.

• If you don't mind the appearance, plastic trash bags half-filled with dry leaves work well as winter mulch. The bags, which keep the leaves dry, can be placed on beds, even if some plants are still standing. (You may need to weigh the bags down with branches, boards or bricks.)

Remove mulch gradually in the spring, but keep some mulch handy in case emerging plants need to be covered again for a frosty night or two.

Reuse winter mulch as mulch in the summer vegetable garden or add it to your compost pile.

Nancy Rose is a horticulturist with the University of Minnesota Extension. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.

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