Frost, a Gardener's Good Friend

By Barbara Damrosch
Special to The Washington Post
Thursday, November 23, 2006

This time of year the Earth does the old possum trick of playing dead. Not easily fooled, we know the leafless trees are merely dormant (scratch a twig with your fingernail and you'll see the green layer just beneath) and the daffodil bulbs just biding their time. When snow falls, tracks reveal that multitudes of creatures are still out and about. Snow fleas, a type of hopping insect that feeds on pollen grains scattered on snow and ice, appear as sooty dustings within those footprints.

The soil's secrets, however, are more closely kept. We can't see the buried pupae of hibernating insects or the earthworms burrowing deeper into the earth to find unfrozen ground. However, the most barren-looking garden, if it contains plenty of the organic matter on which bacteria and fungi feed, still has a living soil.

When people talk about the winter garden, they usually emphasize protection. It's true that certain trees, shrubs, perennials and roses benefit from insulating wrappings or mulches. And an important strategy in vegetable growing is learning how to use mulches and cold frames to coax a harvest out of the cold months. Right now you might be laying straw or evergreen boughs over spinach, carrots, leeks, parsnips and other frost-tolerant crops.

Frost, however, can be a force to be put to work.

If you look closely at your garden's soil in spring you'll see that something interesting has happened to its surface. It looks pitted, with little cracks here and there, as if tiny hailstones had rained on it from above. But it all has happened at ground level, from the action of frozen water in the soil. As water freezes, it expands, creating a powerful force capable of breaking up even heavy clods of earth. That, of course, is what you do to the soil when you till it, whether it's with a plow, a rototiller or an ordinary garden spade.

You loosen the soil so plant roots can penetrate it easily, so water can move freely through it and so air -- vital to roots and soil organisms -- can permeate it. Your goal is a perfect "crumb" structure, with tiny aggregates of soil particles and spaces in between. Veteran gardeners know soil like that when they see it, but it is often hard to achieve, especially if your garden is heavy clay.

Frost will work on its own to create that structure, but it will accomplish even more with a little help from you. It might seem counterintuitive, but forking up the soil to a depth of several inches and creating a surface of lumpy clods is an excellent way to expose more soil surfaces to frost's action. The fissures you've opened in the soil will also do their part to prevent runoff during winter rains.

You'll double the benefit of this "frost-tilling" if you spread a layer of organic matter and work it into the soil. The action of freezing and thawing during the winter will incorporate it still further. Even compost that has not fully decomposed can be spread and forked in this way. By spring it should be mature -- broken down by microbes so that its nutrients are available, ready for use by plants. Spoiled hay or partly decomposed straw might also be spread as a thin layer, or some storm-cast seaweed if there's a beach handy. The shallow tilling-in of dead autumn leaves will condition the soil well for a number of crops, notably carrots and brassicas, which love the nitrogen the leaves provide. Cover crops such as clover or vetch are also excellent if turned under in fall.

Spring is rarely a good time for tilling; the earth tends to be waterlogged then, and easily compacted. But when the weather warms, the earthworms, beetles, ants and other volunteers will wake up and start carrying all your fall-applied organic matter farther down into the soil. If the soil disturbance you've done is shallow, and by hand, few of these creatures will have been harmed. Certain pest organisms, on the other hand, will have been unearthed by your efforts and been frozen, or eaten by winter birds. In that way, frost often has a cleansing effect on the garden.

I try to leave mine in a neat, end-of-season state by raking up debris and removing weeds. Forking in amendments is the last thing I do before I turn my back on the garden, placing it in the expert hands of winter.

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