Cool-season cover crops are what many people think of when cover crops are mentioned. While not providing the biomass and carbon gain of warm season C4 crops, cool-season species will give us much of the benefit of cover without sacrificing vegetable growing space.
The most common cool-season cover is cereal rye. This small grain is among the most winter-hardy plants available and has a wide window for planting. You can plant cereal rye anytime from six weeks before fall frost until the ground freezes. When planted late in northern environments, you won’t see much, if any, growth until the ground thaws and spring is beginning. Although it’s rare for rye to winter kill from late planting, once you are past a heavy frost, it becomes questionable whether rye is the best choice. Oats planted early in the spring grow faster than overwintered rye that is planted late.
Although less common than rye, I prefer triticale as a cover crop. It needs to be planted earlier. Around the first frost is where rye might become a better choice because late-planted triticale can winter kill in northern climates. If planted on time, triticale grows thicker and produces more biomass than rye.
Wheat fits into the same window with the same planting dates as triticale but produces less biomass, so is not my first choice in most cases. With any of these small grains, too much growth in the fall can lead to winter die-off in snow country. Generally, we don’t want to see more than 6” of growth before snow cover. If there is more, mowing it down to 4-5” will increase survivability for early spring growth.
Cover crop seeding rate for any of the winter small grains is about 3-4 pounds per 1000 square feet.
Austrian winter pea is the legume that best matches the growing cycle of these winter small grains. It cannot be planted as late; typically the cut-off point is right around your first frost. When planted with a small grain, we typically use a 50-50 blend by weight.
Other legumes can also be used in overwintering cover crops. Most clovers and vetches are winter-hardy but need to be planted earlier, three to four weeks before first frost.
In this article, I’m using first frost as a reference point because Nature Friend is read over a wide geographical area with many different growing zones. I recommend visiting a local farmer or agricultural seed dealer to get local information on best planting dates. In some areas, a county extension agent is a good resource as well. Organic farmers and no-till farmers are most likely to be knowledgeable about the use of cover crops.
There are also non-winter-hardy cool-season crops that are useful covers. Oats are very useful in both fall and spring. Planted in early fall, oats give the most biomass of any common plant in September and October. In northern areas, oats usually winter kill, and we can use that to an advantage. A thick crop of winter-killed oats can be a nice weed-suppressing mulch that we don’t have to handle in the spring.
Oats can also be planted in early spring, typically as soon as solidly frozen ground is past. Late freezes usually don’t kill very young oats. Field pea is a legume that is commonly mixed with oats; their planting times and growing characteristics match well. As with the winter peas and winter small grains, a typical mix is fifty percent of each.
In the North, grain oats are very commonly available. It’s okay to plant oats from a feed store if they are clean without weed seeds. In southern areas, black oats are often used as cover crops and are usually winter hardy. In northern regions, black oats can also survive the winter if they don’t have much growth before the cold sets in. I like to include them in cover-crop mixtures for the sake of diversity.
Brassicas are very good in combination with oats or oats and peas. Daikon radish is the most common cover-crop brassica, but I like to include some other species such as canola or Abyssinian cabbage when I have the opportunity.
When using daikon radish, I will sometimes plant a solid row of radishes with seeds spaced a couple inches apart, and then four to five rows of a pea/oat mix before the next row of brassicas. There are a couple of advantages to this method. When brassicas are planted right in the mixture, you should keep the seeding rate relatively low because they tend to start quickly, and the broad leaves will shade out surrounding plants. Daikon radishes planted at a low seeding rate get huge, often producing a root more than a foot long and three inches in diameter. In this case, half of the root will typically be shoved above the soil surface. That is a lot of good biomass, but may not accomplish our goal of breaking up deep compaction. Radishes planted more thickly in a row will not develop roots as large on the surface, but will send a deep taproot, which is what daikon radishes are famous for.
I like to stress the CROP word in the term cover crop. Often, I have seen growers surface-spread their cover-crop seed after the cash crop was completed and think they’ve done their job. Occasionally it works. If moisture is adequate and weather conditions are right, small grains and even peas and brassicas can sprout and grow from the soil surface. But in most areas, we cannot depend on the weather conditions being suitable for this type of sowing. It is much better to plant cover crops the same way we would plant our produce crop. We should ensure the seed is buried to the proper depth and in a reasonable spacing.
Proper depth for small grains and large-seeded legumes such as peas is approximately one inch. If you’re doing a row of just brassicas, half an inch is good, but most brassicas will come up from deeper if they are planted along with small grains and peas.
It’s certainly possible on a small scale to pull shallow rows with a hoe or similar instrument and plant cover crops by hand. I really like using a push seeder of the type commonly sold for planting vegetable seeds. Using one of those, one can plant a large garden with a cover crop with reasonable time and effort. If we are planting no-till, a push seeder with rolling disc openers works better than the less-expensive type that just has an aluminum shoe which slides through the soil.
