Fall Cover Crops

by Peter Heffelfinger

posted August 25, 2020

The first signs of fall usually arrive in late August: cooler nights, early morning fogs, and a few rain showers to break up the steady stream of sunny days. The change in summer weather is an early reminder that mid-September through October is the time to sow a winter cover crop in areas of the garden not occupied by fall greens or overwintering plants. A cover crop lessens soil erosion and runoff, as well as preventing  compaction on bare ground. This is particularly important given the heavier winter rainfall that has been occurring with climate change. We are getting the same amount of rain overall, but it’s arriving in more intense storms. Not the usual Northwest slow drip anymore.

My standard cover crop is annual rye, since it is readily available, fairly inexpensive, and, if protected by row covers, can be sown in chilly conditions in late October and even into early November. (Note: do not use rye grass seed, which is a perennial for lawns.) Annual rye does not supply much nitrogen to the soil, but it develops a strong root system and stays a vibrant green even in the coldest winter weather. It will get lush and thick early in spring so it is important to till it in as soon as the soil has dried out enough to be safely worked. If the rye gets over 6-8 inches tall, mow or cut the tops before tilling or digging in with a fork, to hasten the decomposition of both the roots and the shoots. It usually takes a week to ten days to completely compost a healthy crop of rye into the existing soil. A second tilling may be needed to grind up any lingering root wads or clumps of ryegrass before preparing the beds for planting. The end result is a fluffy soil structure, high in tilth, that is ideal for the first crops of spring.

There are a variety of other cover crops, such as crimson clover, Alaska peas, or fava beans, but they are more expensive. They supply nitrogen to the soil, if needed, but I find the regular addition of compost and organic fertilizer materials is more than enough to keep a year-round garden adequately supplied with nutrients in order to grow healthy plants. To maintain high soil fertility I use worm castings, horse manure tea, or fish fertilizer during the growing seasons. Annual rye does what is most needed during the winter: holding on to the soil during the rain, and then easily dissolving into the ground just prior to the first spring plantings.

I do find that the rye seeds, which are only lightly raked into the soil, need to be protected against the birds, who will quickly descend on any freshly sown plot. There are always a few seeds left exposed on the surface, an instant food signal to every avian in the neighborhood, whether they are winter residents or preparing to migrate south. I immediately cover any new planting of rye, whether a raised bed or a big patch, with a light row cover material such as Reemay or Agribon, lain flat on the ground directly over the seedbed. To secure the material against the wind, I lay boards or heavy metal stakes around the edges, and distribute light metal stakes or wooden poles in the central areas. The white sheets will readily float away in even a light gust if you are not careful. Any revealed bare spots will instantly be targeted by every hungry sparrow.

The rye will sprout under the protection of the row cover, especially in late fall when the nights get chilly. Let the rye push up against the material until the shoots are 1-2 inches tall and are safe from the birds. Once the cover is off, the rye will be a green lawn all winter.

Note: be careful when raking in the rye seed. If you rake back and forth too much you risk piling up the seed in thick patches in some areas and very thin spots in others. To maintain an even distribution of seed, I use more of a vertical chopping movement with the rake, to bury as much of the the seed as possible in place. It avoids mounding up the seed at the edges of the bed or in little ridge lines in the middle of a bigger area. When I sow the seed by hand, casting it evenly all over the bed, it definitely feels like an ancient agricultural rite of fall.

2 thoughts on “Fall Cover Crops

  1. Harriet Custer

    I want to use a cover crop this year–for the first time. I have raised bed, and don’t want to till the soil in the spring. I just want to be able to plant into the cover crop and let it decompose naturally. Is this a good idea? Is there a better cover crop for this method than annual rye?

  2. Peter Heffelfinger

    What you are looking for involves a whole different system of horticulture, known as permaculture, which you would need to investigate. There is a permaculture center, including a nursery and a variety of gardens, out in the San Juans, I think on Orcas Island. I would contact them for more information.

    At this point I don’t know of a winter cover crop that decomposes in the spring. Annual rye, as well as all the others I mentioned, store up energy all winter long and then in the spring go into high gear to mature and make seed. Other no-till systems cut down a summer crop and leave the mulch in place to decompose over the winter. Then, spring planting is done through the detritus. This is a new development in large-scale farming, but is still a minority view.

    Actually, what you could do is to mulch the new bed heavily to prevent any weed growth, using a mix of straw, manure, and compost. A bottom layer of cardboard also would help seal off any weeds. The mulch will prevent erosion, but it will harbor slugs and snails, as well as rodents. Also, the mulch needs to be pulled off early in the spring in order for the soil to warm up and dry out prior to planting. This is what I did many years ago when I converted a horse pasture into my present vegetable garden. The mulch has to be very thick. If the mulch is too thin, the weeds will survive all winter in the soil and then explode in the spring. Luckily I had access to a horse stable for a steady supply of manure and straw. But ever since then I have used winter cover crops that have to be tilled in prior to spring planting.

    I do realize that tilling has a negative effect on soil structure and micro-organisms. I try to till as little as possible and regularly use a long-handled garden fork or shovel to loosen the soil in my raised beds prior to replanting during the growing season.

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