With the increased interest in sustainable agriculture, the use of cover crops and green manures for the home garden are becoming more common. Now is a good time to start planning for a winter cover crop to be planted late September.
Green manure or cover crop? Although most cover crops and green manures incorporate the same plants, their intended use is a little different. A green manure's primary purpose is soil improvement while the main purpose of a cover crop is soil preservation (protection from wind and water erosion). I plant a cover crop in the fall to protect my garden from the pounding fall rains and the howling winter winds then in the spring that same plant becomes green manure when I turn it under to add organic matter and enrich the soil. So the terms are interchangeable for the home garden.
It is actually better to plan ahead and think about planting a cover crop when you are planing your garden. The first year I decided to plant a cover crop I had a row of carrots and some cabbage in various places of the garden, I did not want to try to work around them so I waited till they were finished. That put the planting of the cover crop about a month late. Although it sprouted and started to grow it was pretty stunted because of cold weather. Usually I try to put any fall crops like broccoli, cabbage, and especially Brussels sprouts at the end of the garden, that way I can get the green manure in the biggest part of the garden in a timely manner. But don't let lack of planning stop you this year. One year I planted the cover crop right in among the cabbage, broccoli, and carrots and it worked great.
Rye, one of the most common cover crops, puts off a compound that inhibits germination of weed seeds and other plants. This is great for spring weed control but it needs about 2-3 weeks after tilling it under for the compound to diminish. Actually any green manure uses nitrogen as it decomposes and can cause a nitrogen deficiency in your new plants, working it into the soil early eliminates that problem too. It can also be helpful to add compost or a nitrogen rich fertilizer to the soil as you work in the cover crop.
Green manures are usually divided in two groups. Legumes and grain/grasses. While winter rye is the most common of the annual grasses; other grain/grass type crops include wheat, buckwheat, oats, and rapeseed. Although these green manures do not add much nitrogen to the soil, they help keep the nitrogen from being lost to the environment. They also have a tendency to grow fast in the fall, which helps with weed inhibition. The big plus is they add biomass or organic matter to the soil. The other group of green manures, Legumes, include vetch, clover, peas, and alfalfa. legumes biggest plus is that they add nitrogen to your soil as well as organic matter.
One year I tried a mixture of rye, for biomass; hairy vetch, a legume for nitrogen; and a type of Daikon radish that is being used as a cover crop. The radish was said to have a long root, up to 24" to pull nutrients up from the subsoil and then when it decays it leaves them near the surface for your garden plants. It was also suppose to helped break up and permeate the soil for water retention. The radish seed was a little expensive so I haven't used it since. The rye/hairy vetch combo is a popular one and I have used it several times since.
I also like planting oats early in the fall. Oats will not survive the winter like the rye does. It will usually die of in December but it will have grown enough to protect the ground but is easier to turn under in the spring since it is already dead.
I have experimented with Sunn Hemp, which is a summer cover crop. Sunn Hemp is a legume and although it puts a tremendous amount of nitrogen and biomass in the ground I have found it to be hard to get incorporated into the ground for the home gardener. It is also a little on the expensive side.
There is a lot of experimentation and research going on with cover crops, they are definitely worth considering for your garden.
An added advantage to a cover crop like rye is that your garden will be covered with a carpet of green all winter with no maintenance, and come spring it will start eliminating weeds while you are sitting in side watching. Now that's the way I like to garden!
Peter Sutter is a life long gardening enthusiast and a participant in the MU Extension's Master Gardener program. Gardening questions can be sent to [email protected]