OLATHE, Kan. – Many gardeners find the concept hard to accept: You shouldn’t protect most perennial plants from each year’s late-season decline into freezing weather.

“With few exceptions, plants need to go through the transition on their own, so they enter winter dormancy on time. You only apply winter mulch after that, because its sole purpose is to hold in the soil’s cold. That way, it can buffer any air temperature changes that come later on, during winter’s freeze-thaw cycles,” said Dennis Patton, horticulturist with Kansas State University Research and Extension.

In general, plants become fully dormant after two to three hard freezes in the mid to low twenties, he said.

Winter mulch protection is particularly important for any limited- or shallow-rooted plants, Patton said. In Kansas, they include such “semi-hardy” plants as mums and strawberries, as well as all trees, shrubs, perennials and bulbs that have been in the landscape for less than a year.

Winter mulch is vital to the survival of grafted roses, too, he added, which typically mean all hybrid teas.

“Winter’s repeated pattern of freezing and thawing kills more plants in the Midwest than sub-zero temperatures do,” Patton warned. “In unmulched landscapes, the shifts can heave some plants from the ground, exposing roots to air. Plus, they can expose plant grafts and crowns – the places where stems and roots meet -- to hard, killing freezes.”

In late winter or early spring, the mulch’s role can gain even more importance, he said. The insulating cover can hold in soil’s cold so that unseasonably warm weather doesn’t cue plants to break dormancy.

“If these periods are long enough for unmulched perennials to lose some winter hardiness, the plants are open for major, even fatal damage when the weather turns cold again,” Patton said.

Because winter mulch also can moderate soil moisture loss, however, it can benefit any landscape plant. Freeze-thaw cycles tend to dry out the soil, creating plant stress, the horticulturist said. Mulching also protects the soil from the compaction and erosion that winter rains and snowmelt can cause.

For perennial gardens, Patton recommends a 2- to 3-inch-deep layer of a light, airy mulch material, such as straw, shredded leaves or pine needles. (One bale of straw can cover about 100 square feet 3 inches deep.) Deeper layers can suffocate plants over winter.

Plants with woody stems or trunks will need an inch or two of “air space” between mulch and wood – “as if their trunk is inside the hole of a mulch doughnut,” he said. “As they decompose, mulch ‘volcanoes’ that touch the main stem can cause the same damage as placing a woody plant too deep in the ground.”

Grafted roses are the exception to the doughnut rule, Patton warned. The best protection for their crown and graft is a mound of garden soil, gathered from elsewhere in the landscape. The soil cone should be about 6 inches deep. A top layer of straw or leaves will help prevent erosion.

“You should periodically inspect roses and other low-branching shrubs until spring growth begins and you remove winter’s mulch,” he said. “You may have to institute other protective measures if varmints are using the mulch as cover or climbing on top of snow-covered mulch to gnaw on branches.”

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by: Kathleen Ward
kward@ksu.edu
K-State Research & Extension News

Dennis Patton is at 913-715-7000 or dpatton@ksu.edu.