Many farmers ran out of time to seed cover crops last fall due to wet soils. I’m frequently asked, “Should I frost-seed my cover crops this spring?” The best response is, “It depends,” but let’s look at some information regarding this longstanding yet seemingly new-to-many practice of frost seeding covers.
First, let’s clearly identify what it means to frost-seed. Frost seeding is not dormant seeding. Dormant seeding is broadcasting seed over the surface of frozen soil that then lies there dormant until spring. This typically will occur during the coldest time of the winter, and the seed is there long enough to get covered with snow.
Frost seeding occurs in late winter or very early spring. It’s a good time to frost-seed when you can drive over the field in the morning because it’s firm but not in the afternoon because it’s too soft. During the morning hours, soil aggregates heave upward, giving a resting spot for seed. As temperatures warm, the heaved, frozen aggregates “melt” and settle back into the soil surface.
Seeding over the top during this heaving action helps work seed into the soil. To ensure successful seeding, it may be best to seed the cover crop at night or very early in the morning.
Second, look for cover crop species that have a low germination temperature. This includes grains such as oats and cereal rye, and clovers such as red, berseem and crimson. It also includes field peas and many brassicas, such as rapeseed or mustards. They germinate at a minimum soil temperature of 42 degrees F down to 34 degrees F.
The goal is to have frost-seeded cover crops germinate and start to grow at a minimum soil temperature, yet also be able to possibly withstand cooler overnight temperatures. Get growth initiated as early as feasibly possible while still minimizing the risk of a killing frost taking the new stand out. Species that do well in cooler soil temperatures will have better success.
This early-season growth can provide some very important things: protection and food. We still receive a lot of early-spring rains. If covers weren’t seeded last fall, frost seeding is the next best option to getting that soil surface covered and protected.
Frost-seeded covers will also start activating soil biology earlier than a bare field. Soil biology is activated by root exudates. An early-spring cover crop can grow with the express intent of “waking” the soil up, so it is ready and active for the following cash crop.
The most important soil species to benefit from frost-seeded cover crops is mycorrhizal fungi. Cover crops such as oats are highly mycorrhizal, and oats growing in a field before your cash crop will ensure that mycorrhizal fungi populations are actively growing. Brassica species are not mycorrhizal, so make sure these are in a mix with other species that support mycorrhizal activity.
There is some risk to this practice. Choose your species wisely, and be aware of how quickly these plants can go to seed. Brassicas will go to seed very quickly in the spring. Be aware of potential agronomic issues, and work with a crop consultant or agronomist knowledgeable about cover crops.
Seek sound guidance on termination options, nutrient management considerations and pest scouting needs for these green fields. As always, stop at your local Natural Resources Conservation Service office if you have any additional questions regarding frost-seeding cover crops.
McLain is the state soil health specialist with the Natural Resources Conservation Service. She writes on behalf of the Indiana Conservation Partnership.