Seasonal mulching 101

Most gardeners know that putting a layer of mulch around their plants in summer is a good idea, but many get confused about what winter mulch does, how it should be applied and when it should be removed.

Summer mulches can be applied any time, from early spring throughout the growing season. Organic materials that easily break down, such as compost and dried grass clippings, make ideal summer mulches. Bark, woodchips and cocoa bean hulls also perform well in most gardens.

We mulch in summer to:

• Conserve soil moisture.

• Cover the soil so weeds don't germinate.

• Protect plant roots from tilling.

• Even out fluctuating soil temperatures.

The enduring myth about winter mulch is that its purpose is to prevent plants from freezing. But winter mulch is supposed to be applied after the ground freezes. That's because its job isn't to keep the cold out, but to hold it in. Here's why:

Freezing is not as harmful to plants as freezing and thawing, especially if we have a series of freezes and thaws throughout the winter. Repeated freeze/thaw cycles can force perennials to heave up out of the soil, damaging or killing the growing crowns of the plants.

Applying winter mulch when the ground freezes keeps the ground more evenly frozen, which helps protect plants.

Typically, the ground freezes in early to mid-November in the Twin Cities. But in some recent years, the ground hasn't frozen until December. That has encouraged some gardeners to rely on snow, nature's mulch, to protect their plants.

Snow can work well as a mulch if it falls just as the ground is freezing, accumulates to a depth of 10 inches or more and stays all winter. But if the snow melts, it leaves unmulched plants exposed and vulnerable.

Good winter mulches hold air spaces, which prevents the mulch from compacting, which in turn allows the plants to breathe. Chopped leaves, straw, evergreen branches and pine needles all make for good mulches. And you can harvest some of them from your own back yard and stockpile for later use.

Some gardeners simply rake the leaves from their lawns onto their perennials. That can work OK if you have trees with small leaves, such as locust, or lots of evergreen needles. But oaks, maple and linden leaves should be chopped (a lawn mower works well) before they can be used as a mulch.

When you mulch perennial and bulb beds and around newly planted shrubs and trees, use a thick layer of mulch, about 8 to 10 inches.

You can start removing mulch gradually in spring, usually in early to mid-April when severe weather is past. Removing the mulch slowly will allow the ground to thaw slowly, which helps protect plants from the often erratic high and low temperatures we can experience at that time of year.