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.
When Spring arrived and the snow melted the rye began to grow again. It grew and got greener even though the rain has been sparse and night temperatures have not always been above freezing. In about three weeks I will spread rotted sheep manure over the rye, till it under and a few days after that plant flour corn.
There are other areas of my gardens that will be planted to cover crops this year. One is my main potato garden which will be planted with successive crops of mustard, buckwheat, and in late summer, winter rye. The purpose is to give the garden a rest from potatoes and to control fungus pests and weeds. Other areas will be planted with a mix of buckwheat, peas, dill, mustard, cilantro, oats, and corn poppies. This cover crop will be allowed to flower all summer in order to attract and feed wild bees.