Overwintering a Garden with Cover Crops

Choose the next vegetable before choosing a cover crop. An early bed for lettuce or brassica transplants needs to be clear and workable soon after winter. Tomatoes and winter squash leave more time for a living cover and its residue.

Cover crops protect soil from rain and winter weeds while their roots keep growing. They won't correct poor drainage or a compacted planting strip. Fix those problems directly before asking a cover crop to do its job.

Winter-killed covers need enough mild weather to make useful fall growth. Four to six weeks before the usual first frost is a practical starting point, then adjust for your local season. Rye can be planted later, but its workable window still varies by region.

Clear the spent crop and rake the surface before sowing at the packet rate. Cover and press in larger oat and pea seed so it has firm contact with moist soil. Water until rain takes over. A thin stand that barely emerges won't give much winter cover.

Oats and field peas can suit a bed needed early in spring. Their dead tops usually break down faster than a live cereal rye stand. Oats tolerate light frost, but winterkill requires a colder winter and varies with the site. Field peas can add nitrogen when their roots form effective nodules.

If peas or vetch have not grown in the bed recently, use a fresh inoculant labeled for peas and vetch. For a powder inoculant, follow its label for a compatible sticker or the small amount of moisture needed to coat the seed. Plant promptly after coating. The living bacteria in the inoculant help the roots form nodules.

Forage radish, often sold as daikon or tillage radish, is another winter-killed choice. It usually winterkills after several nights in the 17°F to 20°F range. Its large root can help open dense soil and it scavenges leftover nutrients. The rotting roots can smell strong in spring, which is normal.

Winter rye establishes later in fall than many other covers and survives winter reliably. That makes it useful after late crops, but it can produce a thick mat in spring. Winter wheat usually needs an earlier fall start and is generally easier to manage in spring.

Fresh rye residue can slow small-seeded spring crops. Cereal rye releases allelochemicals as its roots and straw break down, but that effect usually fades within about a week of termination. Heavy residue and nitrogen tie-up can last longer. Give high-carbon grass residue about three weeks to begin breaking down before direct-sowing, or use the bed for a transplant or a larger-seeded vegetable. Both rye and wheat are better choices where the following vegetable won't go in until warm weather.

Match the cover-crop family to the crop rotation. Mustard and radish are brassica covers, so skip them before cabbage and its relatives when clubroot or brassica pests have been a problem. The same caution applies to any cover mix that repeats a crop family already under pressure in that bed.

A simple grass or oat-and-pea mix is often easier to fit into a vegetable rotation than a large mix with many species you cannot identify later.

A small bed has two practical paths for ending rye. When you need the bed before rye flowers, mow or cut the green growth close to the soil, then use an opaque tarp to block light and smother regrowth. This light-excluding method is called occultation. Keep a secured tarp in place for three to four weeks.

If the bed can wait, let rye reach the start of flowering, then mow it close to the soil. That gives mowing the best chance of killing the plant without a tarp. Commercial black silage tarps are durable, but any light-blocking cover held down at the edges can work. Keep checking either method for new shoots.

For a later-planted bed, University of Minnesota Extension advises allowing about three weeks after terminating certain grasses before planting the next crop. The delay lets high-carbon residue begin to break down and keeps it from tying up nitrogen around young plants.

When fall harvest comes too late for a cover to establish, leave the bed clean and cover bare soil with a weed-free organic mulch. Pull large weeds before they seed, then spread the mulch over the soil without burying crowns of any overwintering vegetables.

This is a sensible choice for a bed needed at the first workable spring date. Remove enough mulch to warm and plant the row when the time comes, then leave the rest between plants.