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 to churn it into the soil and improve its nutritional value or texture.

Cover crops are fast-growing plants sown to cover bare soil. Often used in the vegetable garden, their foliage smothers weeds, their flowers attract pollinators, and their roots prevent soil erosion.

When dug into the ground while still alive, these "green manures" return valuable nutrients to the soil, reduce compaction, and improve soil structure.

Popular Plants Grown As Green Manure

Any kind of growing plant material that is dug into the soil rather than harvested and removed can be loosely considered green manure. However, certain types of plants are especially popular for making green manure. These include rapeseed (Brassica napus subsp. napus), mustard (Brassica kaber), and buckwheat (Fagopyrum esculentum).

Two plant categories are commonly associated with green manure, which are legumes and certain grass species.

Using Legumes As Green Manure

Legumes are valued as nitrogen-fixers, and they're often planted as cover crops to add nitrogen to the soil. To add effective amounts of nitrogen to the soil, the legume cover crop plants must grow for an entire season.

Using Grasses As Green Manure

Grasses are fast-growing, allowing you to plant them before or after summer crops. This fast-growing nature also makes them a better choice when weed suppression is the goal.

When planted early in the spring, they grow to maturity and are churned into the soil before the warm-season vegetables are planted. Alternatively, the cover crop can be planted after the harvest, growing to maturity in fall.

They are then dug into the soil the following spring before planting begins. Examples of grasses used to provide green manure include winter rye (Secale cereale), oats (Avena sativa), annual ryegrass (Lolium multiflorum), and winter wheat (Triticum aestivum).

How to Use Green Manure

Using green manure correctly involves considerations such as what season to plant it, when to till it under, and what conditions are best for planting.

What Season Is Best For Planting Green Manure?

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.

For 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.

When Should Green Manure Be Dug Into the Soil?

Depending on your climate, you may be able to plant a grass-cover crop in the late summer after the garden harvest. It will grow 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 by planting the first green manure crop in the early spring, digging it into the garden in the early summer, and then planting another in the late summer or early fall.

What Conditions Are Ideal for Planting Green Manure?

The best time to plant cover crop seeds is right before an expected rainfall. The seed must not dry out during the germination period. After you spread the seeds, rake the soil to cover them sufficiently for germination. Water lightly if rain is not predicted.

Other Ways to Get the Benefits of "Green Manure"

Many 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 foliage 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 through the growing season.

Common Problems

If the look of an unkempt garden through the winter (as a result of leaving plant material in place rather than "cleaning" your garden in fall) bothers you, consider that birds and other creatures may well be feeding on the seeds from flower heads left extending up through the snow. Plus the dead leaves and stems can provide important habitat for pollinating insects.

But there are drawbacks to this approach:

• You may encourage unwanted self-seeded volunteers to sprout up in the spring (although deadheading the flowers and fruit in the fall will prevent such self-seeding).
• Tree leaves and garden debris allowed to remain in place for the winter can harbor pests and diseases.

5 Best Tips for Using Green Manure

• 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 soil bacteria that allow legumes to form the nodules that capture nitrogen.
• When sowing green manure seeds, mix the seeds with sand or soil before spreading them. This helps you have more control over where the seeds go.
• No matter which hardiness zone you are gardening in, you will find numerous green manure cover crops to fit your needs.
• Mow tall cover crops such as winter rye down to a more manageable size before churning them into the soil.
• While it is possible to dig cover crops into the soil manually in small gardens, use a rototiller for larger spaces.