What Is Green Manure and Why Should You Plant It?

Fertilising, green manure crop lolium perenne (rye grass) organic gardening, march, rhs gardens, Wisley
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Organic gardeners use green manure to add much-needed nutrients to the soil, and any gardener can employ the same method. It is as simple as selecting the right plants that will be turned under into your garden's soil after the growing season. You can even plant green manures in the off-season when your herbs and vegetables are not growing. These plants are a fantastic and natural way to keep your soil fertile over the years.

What Is Green Manure?

Green manure, also called a cover crop, is already withered or uprooted plant material or sown crop that is turned over into the soil and later serves as mulch and soil amendment (anything added to soil to improve its ability to be a hospitable environment to plants). Green manure can also be referred to as green undecomposed material.

Green manures can be planted in the fall after the herbs have been harvested. You can also plant your green manures as part of your crop rotation during the growing season. If you plant your green manure to grow all fall and winter, it also doubles as a cover crop and helps keep the rich topsoil from being washed away.

Gardeners turn or till the green manure crop into the soil in the early spring. This is done when the soil is yet not warm enough to plant but is dry enough not to get compacted while you are working with it. You should not walk on or try to work with wet soil if at all possible.

If you have heavy soil like clay, you will want to turn the green manure into the soil late in the fall so that it decomposes over the winter.

How Do I Plant Green Manure?

The best time to plant green manure is right before it is going to rain. The seed must not dry out during the germination period.

For a small herb garden, you can broadcast (or spread) the seeds evenly by hand. 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.

Types of Green Manure

There are two types of green manures: legumes and nonlegumes. Legumes are plants whose roots work with the bacteria in the soil to grab nitrogen from the atmosphere. This is called nitrogen fixating and is helped along by an inoculant or treatment medium to help the legumes work. Inoculant is available at garden centers in powder form, and it will significantly improve your yields if used. Legume green manures include vetch, alfalfa, clover, and soybeans.

Nonlegumes make up all other green manures like ryegrass, buckwheat, and oats. There is a form of rye called Winter Rye that will grow in the coldest of zones and be ready to turn under in early spring.

No matter which hardiness zone you are gardening in, you will find numerous green manures to fit your needs. This is another reason why your county extension office is a treasure of information. Your county extension agent can identify the best green manures for your area.

Fun Fact

Many green manures (especially clover varieties) will eventually flower, helping to attract bees and other pollinators to your garden.

How Do I Choose the Green Manure to Plant?

Choosing your green manure depends on when you want to seed and what type of soil you have. Some green manures require you to sow in the winter and others in the spring. It is best to combine more than one green manure and rotate what you plant from year to year.

Whichever you choose, green manure is a great way to add organic nutrients to your herb garden.

Article Sources
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  1. Cover crops and green manures in the home garden. University of Minnesota Extension.

  2. Soil Inoculants. University of Georgia Extension.