I use cover crops in my gardens when they are not mulched or growing food crops. In the photo above is winter rye which was planted last September. It sprouted in the Fall and grew a little bit before the snow and cold weather came. The rye has three purposes. The first is to protect the soil from erosion. The second is to add organic matter to the soil. And the third, because rye is allelopathic, is to kill weed seeds.
Source: Cover Crop | A Really Small Farm