Thomas Christopher: What are the best mulches?

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Snow has become a now-and-then feature of our winters, which is a relief to those who have to shovel it, but a detriment to gardeners. As I have noted in this column before, snow is a natural, if dilute, sort of fertilizer, adding a modest amount of fertility to the soil on which it falls.

Its most important function, however, from a gardener's perspective, is the insulation it provides. Ten inches of light, powdery snow can have an insulation value of R18, similar to a 6-inch layer of fiberglass insulation. This is important for dormant plants, both because the insulation protects them against the coldest of night-time frosts, but also because a covering of snow wards off the sudden, temporary thaws that have become such a hallmark of our winters. When the soil melts and then refreezes, it can actually uproot plants, heaving them right out of the ground. A blanket of snow also protects the plants below it from desiccation by dehydrating winter winds.

Is it just my memory? I seem to remember winters when snow covered the ground for months on end. In any event, that is no longer the case and gardeners are well advised to provide some supplementary insulation to their perennial beds. This can take the form of any light, porous mulch — a layer of clean straw or pine needles works well for this. Don't use hay, as that is typically full of weed seeds. Shredded autumn leaves are also effective for this purpose. Plain, unshredded leaves tend to mat and stay wet, suffocating the plants below them. Wait to apply a blanket of this sort until the weather has grown truly frosty, ideally when the ground has frozen. Then spread your insulation 3 to 4 inches thick.

Another way to insulate beds is to cover them with a blanket of evergreen boughs. These are easy enough to harvest if you have access to a woodlot. An alternative source of such boughs are unsold Christmas trees. Typically, vendors are glad enough to have them hauled away gratis on the day of or after Christmas. This, believe it or not, was a mid-winter chore of mine when I was a student at the New York Botanical Garden (NYBG). The then-curator of NYBG's rock garden drove a truck around the Bronx, picking up the trees from street corners. Then, he had us students cut them up with lopping shears and lay the resulting supply of boughs over his beds of alpine plants. Kept cool in this fashion, the high-mountain plants were protected from the vicissitudes of New York City weather.

There is one drawback to a blanket of wintertime mulch. That is, it provides cover for voles. These small, fat, short-tailed rodents tunnel underneath, seeking out vegetable matter to eat. For this reason, it is best to keep your blanket of mulch a foot away from young trees or shrubs, lest the voles gnaw a ring of bark from around their bases. A blanket of snow poses the same hazard, which is why it is good to armor the base of such plants with a ring of hardware cloth or one of the plastic tree guards made for this purpose, though even that is not foolproof. I once lost a whole (little) orchard when a particularly deep snowfall buried the trees above the tops of the tree guards. The voles tunneled up through the snow and ringed the little trees right over the guards, killing all of them.

A blanket of snow takes care of itself in spring, melting away to expose the plants below. If you have applied a mulch or blanket of boughs, plan to remove it when the weather starts to warm in the spring and the danger of a hard freeze is past. Exposure to the sunlight at that season lets the plants know that the growing season is returning.

Be-a-Better-Gardener is a community service of Berkshire Botanical Garden located in Stockbridge. Thomas Christopher, a volunteer at Berkshire Botanical Garden, is the author or co-author of more than a dozen books, including "Nature into Art, The Gardens of Wave Hill" (Timber Press, 2019). His companion broadcast to this column, "Growing Greener," streams on WESUFM.org berkshirebotanical.org.

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