September is a good time to plant over wintering cover crops. If your winters are mild and you garden in Zones 8 or south, you will be able to plant and till under a sowing of a green manure crop before planting your winter cover crop. Make sure to sow the seeds of your winter cover crop at least 30 days before the last frost date in your area. Cover crops add large amounts of organic matter to your soil and provide many other benefits to your garden area. A primary reason to grow a cover crop is to increase the amount of organic material in the soil.
Organic matter improves aeration, water retention, soil structure, and drainage.
Providing compost and other soil amendments to your soil feeds earthworms and microorganisms that maintain the balance and biological activity in the soil. With a healthy balanced soil environment your garden will have a much easier time fighting off pests and other environmental stresses. The flowers and vegetables that you produce will have more color, flavor and nutrients because of their better growing conditions. It is important to use cold hardy cover crops especially in areas with high winds and spotty snow cover. They penetrate deep into the sub soil bringing up valuable nutrients into the region of garden soil that is readily available for your plants to absorb the nutrients.