For best winter cover, plant early in September or early in October. If the previous crop will not be harvested soon enough, the cover crop can be seeded into some standing row crops. This can be done in August before the last cultivation, which will then cover the seed. Short term cover crops. Soybean and buckwheat are examples of cover crops planted in early summer and tilled in about 2 months or less.
Soybean is a good nitrogen fixer.
Buckwheat makes fast, lush growing and is good at suppressing weeds. Longer term perennials. Alfalfa and the various clovers are biennials or perennials which should be grown at least a whole season, or longer if possible, to get the maximum nitrogen fixing and soil building effects. One common practices is to over seed them into rye or wheat in the spring. After the grain is harvested or just cut for straw, the legume cover crop comes on.