What Is Green Manure and Why Should You Plant It?

The term "green manure" usually refers to plant material that is grown as a cover crop, specifically for the purpose of digging it into the soil to add organic nutrients or improve the soil's texture. The two terms—green manure and cover crop—are often used interchangeably, although technically a cover crop does not become green manure until it is dug into the garden. Cover crops used as green manure are often planted in the off-season, before a garden is planted or after herbs and vegetables have been harvested.

The cover crops grown to provide green manure have other benefits in addition to adding nutrients to the soil:

• Prevent erosion
• Suppress weeds
• Reduce soil compaction
• Attract pollinators
• Provide cover for wildlife

Types of Green Manure

Any kind of growing plant material that is dug into the soil rather than harvested and removed can be considered green manure. But there are some specific categories of plants that are grown specifically for this purpose. Most commonly, these cover crops are legumes such as vetch, clover, beans, and peas, which have the benefit of fixing nitrogen in the soil; or various fast-growing grasses, such as annual ryegrass, oats, rapeseed, winter wheat, winter rye, and buckwheat.

Legumes are planted as cover crops when the goal is to add nitrogen to the soil—especially when the area is being used to grow nitrogen-hungry vegetables. The most common cover-crop legumes include clover (white, red, and sweet), hairy vetch, alfalfa, beans, peas, and soybeans.

Before planting, the seeds of legume cover crops may need to be treated with an inoculant that helps the plant's roots create the nodules that hold nitrogen. Inoculant powder is available at garden centers, and is essential in regions without the soil bacteria that allow legumes to form the nodules that capture nitrogen.

To add effective amounts of nitrogen to the soil, the legume cover crop plants must grow for an entire season. Farmers often rotate crops, covering fields with legumes every other year. Alternating soybeans (a legume) with corn every other year is a common practice in commercial farms. After the soybeans are harvested, the remaining leaves and stalks are plowed back into the soil to retain the nitrogen that has been stored.

Several species of grasses, non-legumes, are also used as cover crops for green manure. Most common among these are winter rye, oats, ryegrass, buckwheat, and mustards. These plants are fast-growing, allowing you to plant them before or after summer crops. This fast-growing nature also makes such a grass a better choice as a cover crop when weed suppression is the goal.

When planted early in the spring, these cover crop grasses grow to maturity and are churned into the soil before the warm-season vegetables or flowers are planted. Or, the cover crop can be planted after the harvest, growing to maturity in the fall. They are then dug into the soil the following spring before planting begins.

No matter which hardiness zone you are gardening in, you will find numerous green manure cover crops to fit your needs.

How to Plant and Grow Green Manure Cover Crops

Proper planting time depends on the type of cover crop you are growing. If you are rotating in legumes for an entire growing season in your garden, they are generally planted in the spring, whenever soil temperatures are suitable for the species. Generally, this means that seeds can be planted when all danger of frost has passed and the ground can be worked. In warmer regions without freezing winters, legumes also can be planted in the fall and grown through the winter.

Remember, though, that if you are growing legumes for green manure, they need a full season in order to properly fix nitrogen. After peas or beans are harvested, rather than pulling the plants from the ground, just chop them up and dig them into the soil with a shovel or tiller—leaves, stems, roots, and all. You'll miss out on much of the available nitrogen if you cut off or pull the plants from the garden in the fall.

For non-legume grasses being grown as green manure, the cover crops can be planted in the spring. They grow quickly and can be dug into the soil in early summer before you plant warm-season vegetables. Or, depending on your climate, you may be able to plant a grass such as winter rye in the late summer after the garden harvest; it will grow to maturity in the fall and can be dug into the garden in spring before you plant the early cool season vegetables. In some climates, you may even be able to grow two rounds of cover crops framed around the other garden crops—planting the first green manure crop in the early spring, then another in the late summer or early fall.

The best time to plant cover crop seeds is right before an expected rainfall. The seed must not dry out during the germination period. Mixing the seeds with sand or soil before spreading helps you have more control over where it goes. After you spread the seeds, rake the soil to cover them sufficiently for germination. Water lightly if rain is not predicted.

If you are planting legumes as a cover crop, the seeds may need to be treated with inoculant powder before planting. The inoculant stimulates the plant into producing the nodules that capture nitrogen. Follow the recommendation of your local extension service regarding the use of inoculants on cover crop seeds.

Green Manure Without a Cover Crop

Many vegetable and flower gardeners enjoy some green manure benefits simply by altering their winterizing routine. Instead of cutting off or pulling out existing plant material as the weather turns cold, leave it in place over the winter, then chop and churn the dead material into the garden soil in the spring. Roots from annual vegetables and plants left in the ground over the winter will help loosen and condition the soil, and leaves and stems dug back into the garden will add nitrogen and improve soil texture.

If you are a vegetable gardener growing peas or beans, it's especially helpful to return that plant material back to the soil rather than cleaning it out. Even fallen tree leaves can be left in place to decompose, then dug into the soil as you prepare to plant the garden in the spring. Gardening in this fashion can reduce your need for fertilizers and compost mulch through the growing season.

If the look of an unkempt garden through the winter bothers you, consider that birds and other creatures may well be feeding on the seeds from flower heads left extending up through the snow. And the dead leaves and stems can provide important habitat for pollinating insects.

The only drawback of this approach is that you may encourage many self-seeded volunteers to sprout up in the spring. But if these volunteers are not wanted, deadheading the flowers and fruit in the fall will prevent such self-seeding.