|Winter rye/Austrian winter pea|
Plant cover crops in fall from mid-September to mid-October on the Front Range of Colorado. With this year's warm fall weather you probably could have planted through the end of October. The season before we had subzero weather the second week in November so late plantings likely would not have survived. Plants require at least a month of moderate fall temperatures to establish before winter cold slows growth and soil temperatures drop below 40 degrees F.
Winter rye/Austrian winter pea or winter rye/hairy vetch mixtures work well for the Front Range. Many gardeners plant winter (cereal) rye. Grass (the winter rye) alone works well for increasing soil organic matter but if you want the advantage of the nitrogen adding abilities of legumes, add winter pea or hairy vetch in a mixture. Hairy vetch is hardier than winter pea and winter rye is very hardy. Plant at 4 to 6 ounces per 100 square feet except a lower rate of 2 to 3 ounces for vetch.
Water at planting and perhaps once or twice more to establish. In general winter snows will provide enough moisture for plants although you could always winter water in extended warm, dry winter weather if you feel you need to.
Cover crops should be a routine part of maintaining a healthy and productive garden soil. Online sources for cover crop seed include Johnny's Selected Seeds and Urban Farmer Seeds.