What are green manures, and how do I use them in my garden?
Matt Connors, Carterville, IL
Green manures give nutrients and structure to soil, even if they're turned into the ground at a very young age.
Photo/Illustration: Allison Starcher
Jennifer Brown, a former associate editor of Fine Gardening, responds: Green manures are not, as the name suggests, the by-products of animal waste. Rather, they are crops that are grown solely to be turned into the ground while they’re still green. By doing this, you add a good dose of organic matter to the soil, which improves soil structure, increases microbial activity, suppresses weeds, adds nutrients to the soil, and—in certain instances—conserves water, prevents soil erosion, and breaks up severely compacted soils.
The term “cover crop” is often used interchangeably with the term “green manure,” but the two are not the same. Cover crops are usually harvested for sale, whereas green manures are incorporated into the soil rather than treated as a commodity.
Green manures are most useful in vegetable gardens where an entire area or bed can be sown and the growth can be turned under by hand or with a tiller. It is difficult to use them in foundation plantings or borders since the roots of permanent plantings shouldn’t be disturbed. Using green manures in a new garden, however, can be a great way to improve the soil before planting begins.
Green manures can be planted in all parts of the country and in any season. Most are quick growers, and some can be mixed into the soil less than two weeks after planting. During the warm times of the year, or in warmer climates, plants like hairy vetch, many kinds of clover and peas, and soybeans, oats, buckwheat, barley, or alfalfa can be used. During the winter months, especially in colder areas of the country, winter rye or wheat can be used. No matter which crop you choose, turn it before it goes to seed to keep the plants from self-sowing.
Finding green manure seeds can be a challenge. Some mail-order sources, like Johnny’s Selected Seeds (www.johnnyseeds.com) and Peaceful Valley Farm Supply (www.groworganic.com), carry them, but shipping can be costly if you order a large quantity. You can also try your local feed and grain store or larger garden centers and hardware stores.