Crimson clover is a legume. Many legumes, including peas and beans, have a symbiotic relationship with rhizobia bacteria. The bacteria act like little nitrogen factories for the legumes. They form nodes on the plants' roots and convert nitrogen from the air into a form that the plants can use. In return, the legumes provide the bacteria carbohydrates and minerals. Rhizobia bacteria occur naturally in the soil, but crimson clover fixes more nitrogen in the soil if it is inoculated with rhizobia bacteria. Many crimson clover seeds come precoated with inoculant or you can buy inoculant from mail order seed companies (see Resources below).
Plant crimson clover four weeks before your average first hard frost in fall (this means September or October in most parts of the United States). The colder your climate, the earlier you should plant. Prepare to plant crimson clover by removing any existing vegetable plants growing in the bed. Then, rake the soil to loosen it up and broadcast the seed evenly over the bed. Rake the seed in (most of it should be buried about ¼ to ½ inch deep) and water it in well.
Keep the soil in the bed evenly moist. The clover should germinate within 14 days. In climates that drop to below 10 degrees F in winter, the clover will go dormant and then begin growing again in spring. It will grow all winter in milder climates. By spring the clover should grow to about 18 to 24 inches tall.
You will need to kill the clover in spring so you can begin planting vegetable crops in its place. Four weeks before you want to start planting, cut down the clover's top growth with a string trimmer. Rake off the top growth and add it to your compost pile. Using a shovel, turn the clover under (so the top growth's stubble is down and the roots are up). Allow it to decompose for four weeks before planting.