By this time of year, I have pretty much closed up my summer garden. It's true that I do grow a significant number of vegetables over the winter; but the majority of my garden is not actively used for crop production during this time. That's not to say the garden is idle! Fall and winter can have a lot to do with how your garden grows next spring. Autumn is a good time to add limestone, if you are on acid soil (or gypsum, if your soils are alkaline). Compost added now will break down over the winter, without tying up nitrogen next spring. But an actively growing component, known as a cover crop or a green manure crop, can be a big help.
Green manures provide several benefits. Their roots work through the soil, holding it together while they are growing, yet penetrating and breaking it up once they rot. Some green manures have very deep roots which reach down into the subsoil and pull up nutrients which are not available to most garden plants. The above-ground portion of a green manure crop covers the soil, protecting it from erosion and compaction due to heavy rains. These rapidly-growing plants also shade the ground, which suppresses the growth of winter weeds. Other green manures are nitrogen-fixers. This means that certain bacteria colonize these plants' roots. These microbes have the ability to pull nitrogen out of the air and convert it into a form which they (and your plants) can use.
In the spring, when a cover crop is killed and turned back into the soil, it helps feed the soil microlife which is so important to your garden. After a short period nitrogen and other nutrients, collected by these plants through the fall and winter, are then released so your vegetables can use them.
Now I'm going to briefly digress, and tell you want green manures do not do. They do not create nutrients or minerals. For example, you'll sometimes read that winter rye is a good cover crop if your soils are low on phosphorus. If you have a phosphorus-rich subsoil, that is an accurate statement. The rye roots grow very deeply, and can get at those phosphorus reserves. But, if the subsoil is also deficient in that mineral, winter rye will not provide any benefit in that respect.
Green manures can usually be divided into two types: Legumes and non-legumes (usually grains). Leguminous green manures include clover, vetch, peas, fava beans, and alfalfa. Legumes have the advantage of adding to the nitrogen reserves of your soil. Their disadvantages include slow fall growth, a smaller boost to the soil's organic matter content, and the fact that they are less winter hardy.
Winter rye is the most commonly grown non-leguminous cover crop; but oats, wheat, canola (rapeseed), and buckwheat (which is not frost hardy) are also used. These plants do not add nitrogen to the soil, but they do keep it "in the system". They tend to grow faster in fall, which results in better weed suppression. Also, since they break down more slowly than legumes (due to their relatively higher carbon content), they add more organic matter to the soil.
Some gardeners use a combination of green manures to balance the benefits of the different types. In the past I'd used Crimson Clover by itself. But it must be sown early in order to get a good winter cover, and also is very expensive. So I've switched to the common combination of winter rye and hairy vetch. The rye grows fast and supresses fall weeds, plus it gives good cover against the winter rain. In spring the vetch starts to grow quickly and fixes nitrogen in the soil.
Which green manure crop will work best for you depends on where you live. Talk to other local gardeners, and see what they grow. If you have a local agricultural extension agent, he or she can be a great resource. Or just walk into a local feed and farm supply store, and find out what they are selling.
A really nice, inexpensive pamphlet is available from Johnny's Selected Seeds. It is called "Green Manures - A Mini-Manual". This little pamphlet sells for $0.95 US, and provides a concise reference on this subject.
Another great resource is your local Master Gardeners program. Most of their county offices provide literature on many subjects, including this one.
All contents © Travis Saling
This page was last updated November 18, 2013