Mulch matters

Applying winter mulch is usually our final gardening chore of the season, but skipping this chore can be risky.

All of our plants are more likely to survive the winter in good condition if they're mulched in fall. That's especially true for any perennials, bulbs, shrubs and trees that have been recently planted, are marginally hardy here or aren't well-established in the garden.

When to mulch

In most cases, it's best to wait until the ground begins to freeze to mulch most flowering bulbs and perennials. But if the temperatures don't drop by early December, go ahead and mulch even if the soil isn't frozen.

Plants that are not fully hardy here, such as strawberries and many of our roses, should have a cover of protective mulch before temperatures drop below 20 degrees. (Without protection, strawberry plants may survive, but their minuscule flower buds will not. And roses may die back to their roots, which renders them worthless if they are grafted.)

Some years, we get several inches of snow before we can mulch. Don't let that stop you. Go ahead and apply your mulch right on top of the snow.

Snow does provide some insulation, but it's not a good idea to rely on snow alone to protect your plants. Snow can melt during a midwinter mild spell (the infamous "January thaw"), leaving plants vulnerable when temperatures plunge again.

Winter vs. summer mulch

Whether it's applied in summer or fall, mulch acts as insulation, moderating conditions in the soil where roots grow.

In early summer, mulch helps keep the soil temperature steady so it doesn't get too warm for good root growth. Summer mulch also helps the soil retain moisture by cutting down on surface evaporation and prevents the soil from becoming hard and compacted. And, of course, mulch helps suppress weeds by cutting off the sunlight that weed seeds need for germination.

Winter mulch behaves similarly to a thick down comforter: It not only helps keep cold air from penetrating the soil (potentially damaging plant roots), it also caps the soil, trapping its warmth and preventing it from escaping into the atmosphere.

As spring approaches, mulch helps keep the soil cool so roots don't start growing too early, which would allow plants to come out of dormancy before the weather settles into a milder pattern.

Best mulches

Straw or marsh hay are the most effective winter mulches because their hollow stems trap lots of air. If you use one of these for mulch, a 4-inch layer should be sufficient.

Straw mulch should be gradually removed as the ground thaws in spring.

Fallen leaves are nature's mulch. They're popular with gardeners, too, because they're plentiful and free. But because rain and snow can pack leaves tightly together, driving out much of the air space, you need lots of leaves to insulate plants well. Aim for a depth of 8 to 10 inches. To prevent them from blowing around the yard, chop them into small pieces by running the lawn mower over them.

You also can mulch flower beds with bagged leaves, which makes the spring cleanup easier. Just be sure to fill the plastic bags only partially with leaves, so the bags make good contact with the ground.

Like straw, leaf mulch should be gradually removed in spring. Add leaves to a compost pile or save them to mulch vegetables and annuals in early summer.

Wood chips or shredded bark are commonly used to mulch around the base of young trees and shrubs. Instead of waiting until the ground starts to freeze, mulch trees and shrubs early in autumn to allow more time for root growth before winter sets in.

Chips and bark also can be used to mulch perennial gardens. Add a few inches of mulch over bulb beds and around perennials as the soil begins to freeze.

In spring, there's no need to remove chips. Instead, add an inch or so of new wood chip mulch as the old mulch breaks down.

Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.