Here's Ryan mowing a cover crop of oats sown back at the end of July following potatoes. Surprisingly, some of the oats were actually starting to head out. Last winter's oats were killed by a hard freeze in December, which was ideal for planting early spring crops. When a cover crop winter kills, you don't have to deal with knocking back a live cover come spring. With our generally mild winters, I doubt oats will winter kill every year. We shall see. For fields and garden beds that will not be planted until later in the season, and where we want robust spring growth, winter hardy rye is a good choice.
Cover crops are important for protecting soil from winter rains, trapping and holding excess nitrogen, competing with weeds, adding organic matter to the soil as well as contributing to a healthy soil structure. Nature abhors bare ground and so should we.