I'm a huge advocate of amending soil. Many of my previous articles reference that dedication. Green manure cover crops provide a great way to amend soil in an easy and productive process during the time you're not actively gardening.
Here's the concept: when your garden beds are finished for the season you plant a cover crop; the plants grow during the fall, winter, or early spring; you till the plants into the soil before planting the primary crop again; the incorporation of these plants into the soil improves its fertility and structure; the next season's plants benefit from the improved conditions provided by this green manure.
There are five beneficial components to a cover crop. The first is erosion protection; the cover crop protects from wind and water erosion. Second is weed suppression; the cover crop keeps weeds from growing on bare soil. Third is nitrogen fixation; some plants, like legumes, store nitrogen in their roots, which becomes available in the soil when the roots die or the plant is tilled into the soil. Fourth is soil structure creation; roots, especially in grasses, exude a substance that helps "glue" together loose soil particles. Lastly, pest insect reduction; the cover crop encourages beneficial insects while reducing harmful insect control measures.
While some perennial grasses can be used as cover crops and achieve many of these benefits in varied areas of the landscape, they aren't best suited as green manure for a flower or vegetable garden. For these type of gardens, the cover crops should be annuals or biennials. You don't want perennial plants regrowing after they've been tilled under.
As you might guess, some plants are better than others to use as green manure. Many of the best and most popular green manure crops are legumes. Because of their nitrogen-fixing ability, legumes can achieve all five aspects of a good cover crop. When they're tilled into the soil, the organic matter breaks down quickly and the nitrogen fixed in the roots boosts soil fertility. Their aggressive roots can also help break up some hard soils.
Legumes have limitations as a cover crop, however. They typically are slow growing and probably won't survive winter temperatures. When they're tilled in in the spring, it may be dead plants rather than active green ones that are added to the soil, though that's not a bad thing. They'll just be younger and smaller than if allowed to fully mature before being killed by a freeze. They're often best suited for early spring planting.
Non-legume cover crops are typically grasses or grains. They don't add nitrogen like legumes, but grow faster and do a better job at weed suppression. When they're tilled into the soil they break down slower, adding more organic matter over a longer period of time. Some can survive winter temperatures, but frozen ground is usually enough to kill many of the annual grasses.
Green manure crops have familiar names and are readily available from nurseries and online seed sources. I'm ordering some of my seeds this year from Territorial Seed Company which has a section of green manure plants in their catalog.
Winter rye, clover, pea, vetch, and fava beans are the cover crops usually mentioned first when you ask a gardening expert about green manure. Alfalfa, oats, winter wheat, canola, and buckwheat often follow in the next breath. Which plants to choose depends on where you live and which ones are best for your climate. Typically, late summer and early fall are the times to begin sowing seeds for fall and winter cover crops.
In late summer I plan to sow hairy vetch and winter rye over much of my open vegetable garden, where corn, beans and squash are currently growing. I can sow the seeds while the other plants are growing and use the straw mulch already in place to help increase soil moisture and assist in seed germination. The first frost will whack the beans and squash, but the cover crop shouldn't be affected. They'll have time to grow and establish before freezing conditions hit. They should overwinter fine. I won't be planting again until next June so I'll have plenty of time to allow some spring growth before tilling them in.
In some of my raised beds I'll sow Fava beans and peas in early spring. Those are the beds where I'll plant my warm season crops. The legumes will add nitrogen to the soil and I'll use a hoe to work them into the soil before I place my tomatoes, peppers, and beans.
You can mix the plants within a cover crop; it doesn't need to consist of a single variety. Grasses grow quickly while legumes provide nitrogen, so plant both in the same bed. Each will provide unique attributes.
When it comes time to till or work the cover crop into the soil, it should be done about three weeks before planting. This allows time for the organic matter to begin decomposing and adding soil nutrients. In warm, wet areas with lots of spring rain, you can do it as late as two weeks before planting. In colder, drier areas, till in about four weeks before planting because it will take longer for the organic material to break down. For spring planting the tilling may need to be done while the soil is still cold. Just be careful not to till if the soil is frozen or too wet.
You want to mow, till, or dig in the cover crop before the plants set seed. The concept is to add nutrients and organic matter to your garden soil not create a nuisance weed. If you allow the cover crop to go to seed, you'll continue to have those plants in your garden, but as competition for the primary garden plants. It works well to mow or cut the cover crop a few days before you plan to till or dig them in. Some of the stems and leaves will begin drying out and that's fine.
Using green manure works just as well in a small garden plot as it does for a large commercial operation. You can plant a cover crop in any garden area. If it's a mulched bed, remove the mulch before sowing seed unless it's a mulch with small pieces that you'll till in along with the cover crop.
I plan my plant rotation among my garden beds with green manure in mind. This year I'm growing peas in one bed. Next spring I'll work the dead pea plants into the soil in that bed and plant something new like green beans. The peas will add nitrogen and organic material to the soil and will benefit the beans. Next year I'll plant my peas in a new bed to benefit another future planting.
Though you grow green manure as a sacrifice to other future plants, you still need to treat them with the same care as the rest of your garden. They need sun, water, and weeding. You want them to grow and mature to achieve the best potential benefit. Snow and spring rain may provide enough moisture to sustain cover crops, but check the soil periodically so they don't dry out and die prematurely.
If your soil can benefit from organic matter and nutrients, and most soils do, consider growing green manure. And having fun telling people about using green manure. It's a great way to stop or start a conversation.