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Cover Crops & Green Manure—the Basics

Q. Hi, Mike: I plan on starting a new vegetable bed and a friend suggested I use white clover as a cover crop this summer. Is this a good idea? And can you please explain terms like "green manure", "catch crop", and "forage crop"? Thanks!!

    ---Robert in Atlantic Highlands, NJ

Q. Hi! About planting cover crops in my vegetable garden for the fall: How do I choose which to plant, when to plant, when to turn them under, etc.? Thanks,

    ----Ruchika; Somerville, MA

Q. I have been reading references on The Rodale Institute's New Farm site about growing a nitrogen-fixing green manure cover crop under a vegetable crop. But the timing seems to vary. One article says to let the vegetable grow for a while and then sow the cover crop. Another method establishes the cover and then transplants the vegetable starts into it. Have you written about this—or tried it? What do you recommend?

    ---Chris in Stamford, CT

Q. Every fall I prepare my veggie and flower gardens for the next growing season with a winter cover crop of vetch and rye. The biggest benefit of using this method is that I never lose valuable topsoil to our strong winter winds. And of course, the rye and vetch provide lots of nutrients after they're tilled into the soil. But I've been wondering if this is the optimum method, or would other cover crops achieve better results? Thanks,

    ----Renee in Wisconsin

A. Renee has just revealed two big reasons why cover crops are an exceptional tool in an organic growers' arsenal: They prevent soil erosion, and return superb nutrients to that soil when they are tilled back in. They also provide an excellent barrier to entry against weeds. In short, I endorse cover crops in general 100%. I also urge Renee to keep doing what she's doing; she's using excellent crops AND has her timing down. (And I always urge gardeners not to try and seize defeat from the jaws of victory.)

The term "cover crop" applies to any plant that is seeded heavily and thickly to protect and enrich the soil. A cover crop becomes "green manure" when it is tilled back into the soil. (Sorry Chris, but even nitrogen-fixing cover crops can't feed adjoining plants while they're still alive and standing; they have to be tilled back into the soil to become usable fertilizer.)

A "forage crop" might mean a cover whose grain heads are harvested for animal feed, with the roots left in the ground to protect the soil (a no-till farming technique.) "Catch crop" is a new one to me, but might refer to the leguminous covers that can 'catch' plant-feeding nitrogen in the air and slow-release it back to other plants after being plowed under. (Or it could simply refer to a cover crop that plays the outfield well.)

Chris' question about growing a cover crop underneath your veggie plants to prevent weeds would more properly be termed a "living mulch"; a term that has also been used to describe me on some sleepy summer days in the garden.

Anyway, as all those descriptives infer, there are many ways to use cover crops.

One of the easiest ways for beginners to give the technique a try is to sow a cover crop in the late summer/early fall that grows rapidly, but will 'winter kill' when the temperatures drop low enough. (The exact crop will vary with your location and what you intend to plant the following season, but might include nitrogen-fixers like sweet white clover (Melilotus alba) and cowpeas, or non-legumes like buckwheat and oats.)

If its green manure you're after, you would till the dead plants into the soil when Spring planting time arrives.

But if your soil is already nicely rich and weeds are your woe, you might want to use the dead plants as Nature's weed block, simply punching holes in the dead cover to install your tomatoes, peppers and such. If you originally sowed the seeds thickly enough, the remaining mat of dead cover should function as a very effective mulch. (I'd call this one a 'living mulch' as well, but it's not alive anymore. In the words of Dr. McCoy from classic Star Trek: "Its dead, Jim!").

Added advantage: Rain and wind can't remove this kind of mulch. There's no immediate fertilization, but there will be if and when you ever plow it back into the soil. For instance, were you to till it under that fall and replant more of it right after, you'd get both the green manure in the soil and a new living/dead weed-preventing cover for the Spring.

And that's just scratching the surface. There are dozens of possible cover crop systems in addition to the one just described: Permanent "TRULY living mulch" plants like white clover that you remove sections of for planting. Covers like red clover that will stay perennial if you mow them before they can flower, but that will behave like annuals and die if you allow them to flower. Covers that sow their own seed, etc., etc....

The choices can be a little dizzying; and the individual proclivities of your gardening zone may turn some theoretic annuals into perennials, or vice-versa, or create other fun situations that are now making me wonder why I decided to answer this question instead of doing something on tomatoes again.

So I'll tell you what. You now have enough basic info to get you into some serious fun gardening trouble. Contact your local county extension office for their Bulletins on cover crops that perform well in your region, combine that info with the basics in this primer, choose a seed and a tactic and try it on a small scale. If nothing else, you'll have a great story to tell your fellow gardeners.

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