Cover Crop Type and Time of Seeding
Cover crops are usually classified as cool season, warm season, winter annual, biennial, or perennial and as grass, legume, brassica or other. Often times, cocktail mixes of the various types are used to ensure success or to achieve a variety of goals as the diversity of the mix adds to the benefits. Selecting a single cover crop, the seeding rate or the amount of any type in a mix would depend on the goals and the time of seeding. Most cover crops need at least 30 days of growth to start being effective and many should have 60 days of growth or more for the full benefits.
As examples, a cool season grass like oats would work if seeded as soon as possible after harvest since it will continue growing after a light frost and be killed by a hard frost closer to November. A winter annual grass like cereal rye or winter wheat could be seeded later, become established in the cool fall and continue growing once the spring warms up. However, it would have to be killed before planting the next crop. Mixing oats and rye would give some quick growth in the fall, some additional growth in the spring, and improve the soil building.
Further examples include seeding a cool-season legume like forage peas or lentils now as they can survive a mild frost but will later be killed by a hard frost. Seeded later, a winter annual legume like Austrian winter peas or hairy vetch would be better, but would have to be killed in the spring before planting the next crop. Mixing a grass with the legume would provide more cover and increase the diversity.
While most brassicas are cool-season annuals, they are most effective if they have 45-60 days before a killing frost to allow the taproot to penetrate deeper into the soil. They shouldn’t be used as a single species cover crop because the residue breaks down rapidly, they don’t fix nitrogen and they aren’t very mycorrhizae friendly. Many producers include them in cover crop cocktails as the seeding rate is fairly low and the benefits of the large taproot are great, if the killing frost comes later in the fall. However, after mid-September, the odds of getting much growth aren’t very good.
If unable to seed any cover crops in the fall, a cover crop seeded in the early spring would still provide benefits. For a spring seeding, consider cool-season grasses and legumes – like oats and forage peas – to help build the soil and manage soil moisture. Often these spring cover crops are seeded as soon as the soil conditions allow, before the spring rains really start. Brassicas, mustards and other cool-season broadleafs can be used in the spring, but they won’t develop the large taproots as if they were seeded in the early fall.
Frost seeding of oats, mustards, vetches or clovers may also be an option, once the temperature drops and the soil firms up. These cool-season cover crops will then germinate and grow in the spring, providing some cover and feeding the soil system. However, they may start growing early and be killed by a frost later because of the lack of residue to help protect them from temperature fluctuations.
Proper timing of an herbicide application will be needed to reduce the risk of using too much soil moisture in the spring. Tillage to kill the cover crop often negates the cover crop benefits and destroys soil structure. The tillage also reduces the residue cover and plants weed seeds. Thus, cover crops are best suited for no-till or ridge-till conditions.