Cover crops are non-cash plants sown in the off-season to protect and enrich the soil when the primary crop is not growing. They function as a living protective layer over the bare ground and are not harvested for immediate sale. The success of a fall cover crop program hinges entirely on planting time, as the plants must reach sufficient development before winter weather arrives. Proper seeding timing ensures the cover crop can deliver its full range of soil health benefits.
The Purpose of Fall Cover Crops
Planting fall cover crops is a strategic move to manage soil health during the period between the harvest of one cash crop and the planting of the next. One significant function is the prevention of soil erosion, as the plant canopy and root systems hold the soil in place against strong winds and heavy autumn rains. This protective layer ensures that valuable topsoil is not washed or blown away.
Many fall-planted varieties, particularly legumes like hairy vetch or crimson clover, perform nitrogen fixation. Specialized bacteria in their root nodules convert atmospheric nitrogen into a form usable by plants, effectively fertilizing the soil for the subsequent spring crop. Other species, known as “catch crops,” such as cereal rye or forage radish, scavenge leftover nutrients, preventing them from leaching into groundwater. This action stores the nutrients in the plant biomass until the following spring, when they are released back to feed the new cash crop.
Calculating the Ideal Seeding Window
Determining the optimal time to plant fall cover crops relies on a simple, yet crucial, calculation tied to local weather patterns. The generalized rule is to plant the cover crop four to six weeks before the average date of the first hard killing frost. This window is necessary to allow for adequate root development and above-ground biomass accumulation, ensuring the plant can survive the winter or provide maximum soil coverage before it winter-kills.
For successful establishment, seeds must have sufficient soil moisture and temperature for germination and early growth. If planting occurs too late, the plants may emerge as small seedlings that cannot withstand the cold, failing to provide the intended benefits. Conversely, planting too early can result in excessive growth, which may lead to seed production and turn the cover crop into a weed. Farmers must also quickly remove the residue from the previous cash crop to prepare the seedbed and maximize the available establishment time.
Adapting Planting Times to Climate Zones
The generalized timing rule must be adjusted based on the location and the type of cover crop selected. In colder climates, such as USDA Hardiness Zone 5, the planting window is compressed. This requires winter-killed species like oats and forage radish to be seeded by early to mid-September to ensure they achieve enough growth for mulch before freezing temperatures kill them.
For regions with milder winters, such as Zone 7 and 8, planting can be extended into late September or early October, allowing for more flexibility. Species like winter rye are highly winter-hardy and can be successfully planted much later, sometimes even into early December, because they germinate in soil temperatures as low as 34°F. These crops aim to establish a small, robust plant in the fall that will survive dormancy and resume vigorous growth early in the spring.
Management Through Winter and Spring Termination
Once the cover crop is established, it enters a period of winter dormancy or winter-kills. Winter-killed crops, such as oats or oilseed radish, simplify spring management because their dead biomass is left on the surface as a mulch, ready for the next crop. Conversely, winter-hardy species like cereal rye or hairy vetch will resume growth as soon as temperatures rise, requiring a specific termination strategy.
Timely spring termination is necessary to prevent the cover crop from competing with the newly planted cash crop for soil moisture and nutrients. Mechanical methods, such as mowing or roller-crimping, are most effective when the plant reaches a specific reproductive stage, such as the anthesis stage for cereal rye. Chemical termination using non-selective herbicides like glyphosate is reliable, often applied 10 to 14 days before planting the cash crop to allow the residue to begin breaking down.