Beginning in mid-September, as summer vegetable crops stop producing, each open space in the garden should be sown to a cover crop of either annual ryegrass, winter rye, or oats. These cover crops will protect the soil over the winter when it would otherwise be bare, vulnerable to erosion by wind and water. The roots of the cover crop will absorb nutrients that would otherwise be leached below the root zone, holding these nutrients in place for use by next years vegetable crops.
Which of these three cover crops to use depends on two main factors, time of planting and ease of incorporation in the soil come spring.
Sow Winter Rye in Areas Where Next Summer’s Crops Will Grow
When a spot in the garden opens up in late September and you plan to use that space next year for a main summer crop (tomatoes, peppers, squash, etc.), consider sowing seeds of winter rye. The most cold tolerant of commonly used cover crops, winter rye sown in late September grows rapidly, even into cold weather. It does not winterkill and puts on vigorous new growth the following spring, the extensive root systems preventing the leaching of excess soil nitrogen. Winter rye will grow well in acidic soils.
I do not recommend using winter rye in areas where early spring crops will be grown. To do so means being able to turn the winter rye under very early in the growing season, before its growth gets unmanageable. This requires a garden that warms up and dries out quickly in spring, a rare situation in northern New England.
Also, because winter rye is still in active growth when you turn it under, you need to allow four weeks after turning it under before planting any vegetable crop. There are two reasons for this waiting period. First, the bacteria which decompose the fresh organic matter tie up available soil nitrogen as they grow in number. Once their job is done, most of these bacteria die, returning the nitrogen to the soil. The process of decomposition takes two to three weeks.
The second reason for waiting demands the fourth week. Winter rye has an allelopathic effect on many other plant species, releasing chemical compounds into the soil that can inhibit the growth of those sensitive species. These allelochemicals disappear during the additional week.
Using winter rye only where main summer crops will be grown allows enough time for both bacterial decomposition and the breakdown of allelochemicals. Sow winter rye at the rate of 3 ounces per 100 square feet of bed area.
Sow Oats Immediately and Hope for a Late Freeze
Oats should be sown between mid-August and early-September (not before mid-August or the plants could go to seed, leading to a volunteer spring oat crop). I just sowed one of our raised beds to oats, hoping for a late freeze.
Although oats rank lowest in terms of amount of biomass produced, I have found that they do produce enough vegetative growth to make an adequate catch crop, trapping soluble nutrients in the soil, keeping them in the top of the soil profile. Once killed by freezing temperatures, they form a dense mat of dead vegetation that holds the soil in place through the winter. This oat mulch can be easily incorporated into the soil in spring. Or, to avoid disturbing the soil and help with weed control, you can transplant seedlings directly through the mulch.
Oats are an excellent cover crop for soils with good drainage. They will also grow well in acidic soils. Sow oats at the rate of 4 ounces per 100 square feet of bed area.
Sow Annual Ryegrass Now for a Quick Cover
Annual ryegrass is ranked high among cover crops for its rapid germination and vigorous establishment. It produces an extensive root system that mines soil nutrients, sequestering them in its leaves.
Like oats, annual ryegrass is killed by freezing temperatures and thus should be sown as soon as space opens, certainly by the end of September. Sow annual ryegrass at the rate of 2 ounces per 100 square feet of bed area.
Seed for each of these cover crops can be purchased at local garden centers and feed stores.