Step 3: Killing.
You must kill your cover crops before they set seed and the topgrowth gets out of control. That’s right, kill them. “The best time to kill them is at flowering or when the seedheads emerge on grains. The annuals can all be killed at this point by cutting at the base of the plant,” advises Sarrantonio.
You can mow cover crops with a lawn mower or a weed trimmer, depending on how tall the plants are. Wait a day or two until the leaves and stems dry down, and then dig them in. Longer pieces of stems and vines may interfere with tilling, but it won’t take long before the vegetative growth partially decomposes.
After turning under a cover crop of grasses, wait 2 to 3 weeks before planting vegetables or flowers. The decomposition of the green material can tie up soil nitrogen. And cover crops such as rye are allelopathic, which means they inhibit seed germination.
Finding space in your garden
When I discuss cover crops in gardening classes, a common concern is the amount of valuable garden space they occupy. However, you can fit cover crops right into your garden plan.
Succession cropping is one of the easiest ways to do this. After spring crops of lettuce, radishes, and other early vegetables have been harvested, plant a fast-growing cover crop, such as buckwheat. In most climates, you can allow this cover crop to flower and still have time to plant a crop of frost-tolerant vegetables. Cover crops can also be planted in fall after some main-season crops, such as cabbage, are finished.
Interplanting cover crops with vegetables is possible, though a bit trickier. “Ample water and nutrients available to both cover crops and vegetable crops, and controlling growth of the cover crop, are key to making this approach work," cautions Creamer. It’s also important to delay your planting of the cover crop. “A good rule of thumb is to sow the cover crop seed into the vegetable bed one-third of the way through the vegetable crop’s growing cycle,” says Steve Diver of ATTRA—The National Sustainable Agriculture Service. For example, if you’ve planted a 75-day corn variety, interplant a cover crop about 25 days after seeding the corn. Organic farmers have had good luck with delayed interplantings of yellow blossom sweet clover with lettuce and onions. Sarrantonio suggests transplanting young tomato and pepper plants into a mowed mulch of hairy vetch and rye. The mulch reduces weeds, maintains moisture, and provides nitrogen.