Frost seeding cover crops
Frost seeding is broadcasting cover crop seed over a frozen soil surface and letting the freezing and thawing incorporate the seed into the soil profile. For hard seed (seed that is difficult to germinate), the freezing and thawing may break down the seed coat and improve germination. Most small grain fields (wheat, oats, barley, spelt) can be frost seeded but also fields that either had a poor cover crop stand or were not planted in the fall can also be frost seeded. Generally, small grain fields that have less than ideal stands are the best candidates for frost seeding cover crops. When the small grain crop is thick and yields are high, there is little sunlight and the grain competes with cover crop establishment.
There are several benefits to frost seeding. Soil damage is minimized by driving on frozen ground with lighter vehicles. Small tractors and ATVs with broadcast spreaders can cover a lot of ground very efficiently, which saves fuel. The best time to frost seed is early morning, sometimes before or just as the sun rises. The frost seeding window can be short before the soil starts to smear. The freezing and thawing incorporates the seed naturally, giving good seed-to-soil contact and minimizes the need for tillage. Damage to the grain crop is minimized because the leaves are frozen. Spreaders need to be calibrated and watch the spreader width. Generally, spread seed when the wind is calm to get good coverage.
Weeds love bare soil so an emerging, growing cover crop generally provides competition to weeds and reduces weed pressure. Usually there is not a lot of top growth biomass but more below ground biomass which stimulates microbial activity and increases nutrient availability. Other benefits include improved soil structure to relieve soil compaction, improved drainage, reduced soil erosion, improved water quality and it provides some wildlife food and shelter.
Spring planted cover crops (Mid-February through April) is a great time to plant cover crops early. Early spring-planted cover crops can have 40 to 60 days of growth before another crop is planted (corn, soybeans) or 90 to 120 days of growth if inter-seeded into a grain crop (oats, wheat, barley). Frost seeded cover crops should be able to tolerate low and freezing spring temperatures (32 degrees Fahrenheit to 42 degrees). The goal is to plant cool season cover crops that germinate quickly in the spring at low soil temperatures and cover crops that can tolerate cold or freezing night time temperatures. The best time to frost seed is on top of snow or when there at least four to five expected frost/ melting events remaining to allow adequate seed incorporation. Broadcasting on 2 to 3 inches of snow makes it easy to see the broadcast seed pattern but avoid deep snow because seed may be lost during snow melt off. Clay and loamy soils can be effectively frost seeded but avoid sandy soils which may not have enough moisture for seed germination.
The best cover crops to consider are the clovers (red, crimsom, balansa, berseem, sweet), peas, lentils, some brassicas (radish, kale, rape, turnips) and grasses (oats, barley, cereal rye) and flax. Oats and flax are highly mycorrhizal, promoting beneficial fungus in the soil. Oats is a great nurse crop that makes manganese available and reduce many diseases. The clovers, peas and lentils can supply extra nitrogen if properly inoculated. Cover crops prime the soil for the next crop and add soil organic matter. Consult the Midwest Cover Crops Field guide for frost seeding rates. If you want to frost seed cover crops, plan to order or acquire seed now because it may take several weeks to acquire cover crop seed.
To increase germination on hard seed like clovers, double or triple bag the seed in burlap bags and tightly close the bag. Soaks the seeds in the burlap bags for three days in pond or stream water. Then spread the seed out to dry to remove excess moisture and then broadcast the seed within three days. The microbes in the pond water break down the seed coat and greatly improves germination.
Frost seeding can be risky. Successful stands depend on adequate moisture, good seed to soil contact and good spring growth. In small grains, sometimes the cover crop may outgrow or compete with the main crop, reducing crop yields. Some cover crops may even try to go to seed. Adding a spring cover crop though generally adds soil diversity and improves soil health.