Cover Crops for February: Oats if you have a 6–10 week gap
In February, you’ll hopefully have made your crop plans and maps. Perhaps you’ve discovered some beds with no winter cover crop, that you are not using for early spring crops? Those winter annual weeds, chickweed, dead nettle and henbit, will shed lots of seeds if you let them.
If food crops were harvested too late to sow any winter cover crops, in early spring you will be looking at weeds, or “spontaneous vegetation” as I’ve heard them euphemistically called. The first year I gardened in Virginia I had lots of beds in April with purple flowers (henbit, purple dead nettle, some ground ivy) and I couldn’t think what to do – I didn’t need those beds for a few more weeks, so I wasn’t ready to till them. Oh, so wrong! I had squandered an opportunity to improve the soil as well as deal with weed seeds.
Now, when we have the crop plans made, we tag any beds that won’t be used for six weeks or more, till in the weeds and sow oats. In February or March here, with a last frost date of April 30, six weeks is just enough time here to make enough growth to out-compete the weeds and add to the organic matter in the soil.
Examples of crops in our gardens that occupy beds too late for us to sow winter cover crops include late cabbages, the last lettuce, leeks and fall Asian greens. Examples of late spring crops the next year include eggplants, peppers, tomatoes, edamame and chard. In some cases, we might even have no food crop planned until August or the beginning of September.
Six–ten weeks (depending on your climate) is long enough in early spring to get worthwhile growth from oats before prepping for the food crop.
Once we get to March 31 here, it is too late in the year for oats (they will quickly head up after making very little growth) and too soon to rely on frost-tender cover crops. See the section below on Stale Seedbed Technique.
I wrote about oats as a winter-killed cover crop in August.
Will oats work as an early spring cover crop for you?
Large oat plants will be killed by three nights at 20°F ((–7°C) or by a single plummet to 6°F ((–14°C). Oats seedlings die at 17°F (–8°C). Consider your likely nighttime low temperatures during the period in question, and how likely your seedlings are to die. Oat cover crops of a medium size die around 10°F (–12°C). If they die after growing to adolescent size, no worries – just till them in before planting your next crop.
The minimum soil temperature for germination of oats is 38°F (3°C), and the time required to grow to a worthwhile height in cool weather is 6–10 weeks.
If you are in zone 8 or warmer, oats will not winter-kill, and can be grown in winter too – they may not reliably mow-kill, but are relatively easy to incorporate.
Pros and cons of oats as a cover crop
Like most cover crops, oats add biomass and nutrients, increase the biological activity of the soil, smother weeds, reduce soil erosion (their fibrous roots anchor the soil) and absorb and store rainfall. Oats are easy to establish, are fast-growing and particularly good at shading out germinating weed seeds and at salvaging any nutrients (especially nitrogen) left from the previous crop and making them available to the following crop.
In early spring you won’t get as much biomass as from a fall oat cover crop, when you can get. as much as 2000–4000lbs per acre (2240–4480 kg/ha. Certainly not enough to be a no-till cover crop for your next food crop. Oats provide some allelopathic effect (producing biochemicals which inhibit the growth of other plants) although less than winter rye. Like most plants, oats form arbuscular mycorrhizal associations (fungi penetrate the plant’s cell walls and help capture nutrients from the soil) – a mutually beneficial relationship, although probably not a big feature when conditions are cold.
Oats are not as good as some other cover crops at breaking up compacted subsoil, although they do loosen the upper layers of topsoil nicely. (Just where you are going to sow.) Oats do not add nitrogen, and unlike flowering cover crops, they do not attract beneficial insects (assuming they are turned under or mowed before heading up and shedding pollen).
Buying oats, sowing rates
We used to buy “horse oats” (feed oats) locally and not worry about organically certified seed. But I noticed canola sprouting along with the oats, and I don’t want GMO canola going feral in our gardens! We plan ahead and order extra Organic spring oats in summer when making our big cover crop seed order.
Oats will grow in soils with a pH range of 4.5–7.5, and even do OK in soils without great fertility. They have some tolerance to flooding, but not much to heat or drought (although more than rye).
The sowing rate for drilled oats alone is 80–100lbs/ac (90–123 kg/ha). The broadcast rate is 110–140lbs/ac (123–155kg/ha). On a small scale, this is 4–6 oz/100 sq ft (12–20gm/sq m). Aim to cover the seed to a depth of about 1″ (2.5 cm).
Oats grow to a height of 2–4 feet (0.6–1.2 m) if not killed before then.
Stale seedbed technique and tarping
If you have less than 8 weeks until you need to plant the food crop, you are better off mowing when you can, or weed whipping, to prevent weeds seeding.
You could mow and tarp. I don’t yet have much experience of tarping, but there are good directions in The Lean Micro-Farm and The Market Gardener. There are also online videos. The basic idea is to mow the bed, cover it with a black plastic silage tarp, weight down the edges to get good soil contact and stop the tarp blowing away, and wait till the plant matter has disintegrated, or until you need to use the bed. This will take several weeks in cold weather, (although only a few weeks in summer). If you have a late-finishing fall crop, you could mow and tarp as soon as the crop is finished, and leave the bed tarped until spring. Weeds germinate under the tarp, but then die without light.
For a stale seedbed technique without tarps, you could mow, till several weeks before planting, water (if it doesn’t rain) to germinate weeds, and hoe off those weeds once a week. This can really help reduce the weed seedbank in the soil. It is particularly useful before sowing small seeds of slow-growing crops, such as carrots.
What not to sow in short gaps in early spring
Other quick cover crops, like buckwheat and soybeans, are not at all frost-hardy, so wait until after your last frost date to sow those.
In early spring, the air and the soil are cold, and sowing a fast-growing vegetable crop will not be successful with a gap of less than eight weeks. Crops take too long to grow at this time of year.
Spring gaps longer than eight weeks
If you have more than eight weeks you could try those fast-growing vegetables: kale, spinach, Tokyo bekana, radishes, chard, lots of salad crops, senposai, mizuna, tatsoi, land cress. Or try Eat-All Greens, an idea from Carol Deppe. Patches of carefully chosen cooking greens are sown in a small patch. When it reaches 12″ (30 cm) tall, Carol cuts the top 9″ (23 cm) off for cooking, leaving the tough-stemmed lower part, perhaps for a second cut, or to return to the soil.
Undersowing oats in spring
Another way we have used oats in spring is between rows of peas (grown on the flat, not in beds). We sowed the oats the same day as the peas, and lightly tilled the oats in. We mowed the oats as needed during the pea training and harvest period to make access easy. This reduced the number of weeds, and we quite liked the “lawn” underfoot!
Green fallow (Full year cover crops)
If you have a bed with no crop planned for the whole season, you could grow a Green Fallow. You can plant long-term cover crops to replenish the soil. Start with oats or one of the more cold-hardy grasses, and once we get to warm weather, after the frosts, till that in (or mow if it mow-kills) and sow warm weather cover crops.
Reasons not to do no-till food crops in spring
Untilled soil in spring is colder than tilled soil, and growth of anything you plant in it will be slower, and harvests delayed. You could consider broccoli or cabbage, perhaps, but not warmth-loving crops. You may get more slugs and/or more fungal diseases with no-till. The cover crop could try to regrow, or you could get some weeds anyway.
Also, if you are planting by hand, transplanting into untilled soil is harder work than planting into loose tilled soil. Hand-sowing into untilled soil is tricky – winter snow and ice can leave quite compacted soil. If the notion of organic no-till appeals to you, experiment on a small-scale the first year. Reduced tillage is another option. Till out narrow strips in the oats for your plantings of large food crops normally grown on a wide row-spacing.
Using a push seeder for cover crops
You can drill cover crop seeds using a push seeder. See VABF Using Manually-Operated Seeders for Precision Cover Crop Plantings on the Small Farm. Don’t worry if the seed ends up deeper than ideal. It will still germinate.
Cover Crop Planning
My book Sustainable Market Farming has a chapter on cover crops and 9 pages of charts about particular options.
The book Managing Cover Crops Profitably (third edition) from the Sustainable Agriculture Research & Education Program (SARE), is the best book I know on the subject. You buy the book or download it as a free PDF from SARE.