Winter Mulching

Phillip Peters
Adams County Master Gardener

It seems like only a few months ago we were putting down mulch on our gardens and shrubs to help get our plants through the summer's hot dry season. Mulching in the spring or summer seems a natural part of building the gardens we want to look at and enjoy in the warm months. So the last logical step after tilling, raking, planting and fertilizing the beds is to top them off with a couple inches of mulch to even up the appearance of the beds, to keep down the weeds, and to help preserve the precious moisture that falls during the dry season.

Well, some of the same logic that told us to mulch in the spring is still appropriate in the fall. This time, however, mulching will serve to get our plants through the winter. And, even though we are doing the same thing, the reasons are a bit different. And the reasons make it critical to apply the mulch at the right time.

As fall comes to a close, we pull out the spent annuals, prune back the dead parts of perennials, clean up fallen leaves and branches. Perhaps we spread some compost on the beds to revitalize them and to give them more organic matter and substance. But, even though it is the last step in readying the beds for winter, we dutifully hold off on putting down that topping of mulch. Why?

Let's look first at why we put mulch on the gardens for the winter. Then, the timing of applying it will become more evident. Some of the reasons are the same as they were in the warm season. We want to top-dress the gardens so they will look presentable through the less colorful winter months. The uniform appearance announces that they are well-tended, cared-for gardens. It enhances the overall appearance of our properties. When spring comes, there will be fewer, or no, weeds. Rain or snow that falls will be readily absorbed and held near the plant roots.

But there is one other reason that we need mulch in winter that is not always apparent. As the ground goes through the freezing and thawing cycles that occur as part of our temperate winters, it can heave plants and bulbs out of the soil, damaging their roots and making the stressed plant vulnerable to disease later on. Since it doesn't happen all the time and to all the plants, we may not even be aware of it. A layer of mulch will help keep the soil temperature variations to a minimum and, thereby, reduce the possibility of heaving.

Timing of application is very important. We do not want to apply the mulch too early. Doing so could create heat around the plant, encouraging it to put out new growth that will be killed by a hard frost. This would cause severe stress in the plant that would unduly weaken it in future years. And once mulched, we do not want to leave the mulch on too late in the spring as this would produce long leggy growth that will be pale since it lacks chlorophyll from sufficient exposure to sunlight. To take advantage of the help mulch will give us over the winter we need to arrange our fall gardening calendar as follows. After cleaning up the beds and putting on some compost we want to wait. Wait? For what? For one or two nights of good, hard frost, that's what. We want the shrubs and perennials that will be wintering over to go dormant. The frost will shock them into dormancy. Then, we mulch.

A hard or killing frost is when the temperature drops below 25? F. The first hard frosts usually occur in the south central Pennsylvania area on or around November 15. When we hear that a hard frost is predicted, we want to make sure the plants have been well watered. Once the ground freezes down around the plant roots, they will not be able to take up moisture as they normally would. Consequently, winter sun and wind will wreak havoc with the foliage and dry it out. Not only is this unsightly, but it opens the way for disease and insect damage and breakage. With the plants properly watered, wait until there have been one or two nights of hard frost. The plants will go into their dormant period. Now is the time to preserve the ground moisture and to assure that there will be sufficient insulation so the ground will not undergo dramatic temperature changes.

Shrubs & Woody Ornamentals: For the winter protection the best mulch material is an organic mulch - shredded bark, rotting leaves, compost. Apply the mulch evenly to a depth of 2"-4" over the entire garden. Do not heap it up around the stems or crowns of plants. They should have very little (a half inch) or no mulch near the stems as we want to avoid making a shelter for disease pathogens, bacteria, mold or, even, rodents that, hiding under a heavier cover of mulch, might strip the bark from the base of the plants. The plants will survive quite well on their own, barring some catastrophic cold snap. We just want to keep the soil temperature from varying wildly. Incidentally, if you do have plants with exposed leaves that could dry out, now is not too late to spray them with an antidessicant, such as Wilt-Pruf.

Bulbs & Perennials: To protect these plants, clean straw, hay (especially salt marsh hay - the seeds will not germinate here) shredded leaves or evergreen boughs provide protection from the temperature changes and yet are loose enough to allow sufficient air circulation and water penetration to keep the plants healthy. The mulch on these beds should not be applied too early, before the soil temperature drops sufficiently to send the perennials into dormancy. Nor should it be of an easily compacted material, such as whole leaves. Compacted material around the crowns of the plant encourages disease and fungal growth. Be sure to remove the mulch from the perennial beds and the bulbs after the last hard frost - on or about April 10. Applying a protective cover of mulch at the right time will assure that the plants survive the winter in a healthy condition to produce their best come spring. Happy Mulching!

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