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Green Manures 101

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There are a variety of reasons to plant green manure (also known as cover crops) in your garden. Simply, green manures make for healthy soil. These crops do a variety of things for your garden:

  • They increase nitrogen levels.
  • They smother weeds.
  • They can help break up compacted soil.
  • They can help bring subsurface minerals up to where your garden plants can use them.
  • They can help control soil nematodes and fungi.
  • They add organic matter to your soil.

Certain green manures are more suitable for some situations than others. Some are best planted at certain times of the year. The list below describes the most common green manures, as well as what they are used for and when they should be planted in your garden.

10 Green Manures and How to Use Them

Field Peas: This green manure is most often used for adding nitrogen to garden soil, which is something legumes are known for. Field peas are also excellent at smothering weeds. They can be planted at any time during the growing season, and mature within 75 days. For the optimum amount of nutrients and organic matter, field peas should be tilled into the soil when they are at the blooming stage.

Alfalfa: Alfalfa is a great all-around, basic cover crop. It fixes nitrogen in the soil, provides quite a bit of organic matter, and its deep roots help break up soil. It is best sown in midspring through early summer, then till under when it begins to bloom.

Ryegrass:

Ryegrass is great for increasing the level of nitrogen in your soil, as well as adding plenty of organic matter. It germinates and grows quickly, so it can really be sown anytime during the growing season and then turned under in 50 to 70 days. Fall-planted rye will suffer from winterkill, but usually start growing again in the spring. One caveat: there is some evidence that the decomposition of ryegrass may prevent some small seeds, such as those of lettuce or carrots, from germinating. If you plan on using ryegrass as a green manure, this is definitely something to keep in mind.

Mustard: Mustard not only adds lots of organic matter to your soil, but it also does a great job controlling soil nematodes and fungi. Sow, and 80 to 90 days later, till it in. For optimum nematode and fungi control, mustard should be tilled in before it sets seed.

Oilseed Radish: If you have compacted soil, plan on planting some oilseed radish seed in the fall. The long, thick taproots do a great job at not only breaking up compacted soil, but at bringing nutrients up from the subsoil as well. Plant in fall, let it grow, and, just before it blooms (60 to 90 days), till it under to add organic matter and help protect your crops from soil nematodes. This green manure is hardy to 20 degrees Fahrenheit.

Soybeans: Soybeans are great for adding nitrogen to the soil, smothering weeds, and even for acting as a "trap crop" for deer if planted around the perimeter of your garden. Sow soybeans after the soil has warmed in the spring, and throughout the summer months. For optimum nutrition, till the plants in when the plants have begun to set pods.

Hairy Vetch: This is a winter-hardy annual green manure that is used mainly for increasing the nitrogen level in garden soil. It also suppresses weeds and helps loosen compacted soil. Plant in late summer or early fall, then till it in the following spring.

Crimson Clover: Crimson clover is another great all-around cover crop: it grows quickly, fixes nitrogen in the soil, smothers weeds, increases organic matter while reducing erosion, and is shade-tolerant. In bloom, it also provides forage for bees and other beneficial insects. It can be tilled in any time during the growing season, or left in the fall garden to die over the winter. You can even leave it as a permanent cover crop in your vegetable garden or orchard, where it will reseed and continue growing all season long.

Oats: Oats are a quick-growing green manure, most commonly planted in fall to provide winter cover for garden soil and add organic matter.

Sudangrass: Sudangrass is a very fast growing green manure that is ideal for adding organic matter to your soil. It can be sowed after the soil has warmed, and can be tilled into the soil in as little as 45 days. It also helps control soil nematodes.

If you'd rather not have to decide which individual green manure crop is best for your garden, many seed catalogs sell spring green manure (or cover crop) mixes or fall green manure mixes. These are a combination of the above seeds, selected based on whether they grow best in warm or cool conditions.

Green manures are definitely worth incorporating into your garden plans, especially if you are trying to go the "deep organic" philosophy described by Eliot Coleman, which means you are producing as much as you can on your own property, including soil amendments. Green manures are an efficient way to build your soil.

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