Backyard Garden Cover Crops
Cover crops offer the same advantages to backyard home gardeners with raised beds as they do to farmers with large acreage, and are a major component of the Three C's of organic gardening (compost, crop rotation, and cover crops).
What are cover crops?
Unlike harvestable crops, a cover crop (aka green manure) is a plant grown to help revitalize the soil and replace nutrients that vegetable crops inevitably remove from the soil during the growing process. It may seem a bit strange to grow a crop only to dig it back into the soil, but that's how cover crops work. Cover crops improve soil quality, increase porosity (aeration), reduce compaction, add organic matter and beneficial microbes, retain nutrients, add nitrogen, suppress weeds, attract beneficial insects (certain cover crops), and generally enhances overall soil biodiversity.
What's a good cover crop?
There are so many soil types and so many gardening zones across the country that there's no way to designate a 'best' cover crop for vegetable gardens. All cover crops offer benefits, but some are stronger in certain areas such as breaking up compacted soils, fixing nitrogen, or attracting beneficial insects. Some are winter cover crops, while others are summer cover crops. Our soil is sandy so we don't need a cover crop that helps with compaction, but I do need a crop that helps prevent nutrient leaching, adds organic matter, retains moisture, and attracts beneficial insects. And since I'm in the Northeast, it's helpful to have a cold-hardy cover crop for early spring planting. The three crops I've used so far are red clover, peas, oats, vetch mix, and buckwheat.
- Clover: In our first season we grew mammoth red clover which worked well, but the clover seed is very small and it was difficult to sow the seed by hand without the seed getting into the surrounding soil. All cover crops will end up in the surrounding soil to a certain extent, but the clover was a bit difficult to control once it was out of the raised bed due to its spreading nature.
- Peas, oats, vetch mix: Peas, oats, and vetch is a cool weather cover crop that can be planted as soon as the ground can be worked in the spring, which is a boon for those in cooler climates. Even though there are three different-sized seeds it's not difficult to sow the seed by hand, but I've found it's helpful to err on the side of too little seed when sowing. The other advantage to the peas/vetch mix is that the early growth provides material for the compost pile, which is usually in short supply early in the season.
- Buckwheat: Buckwheat is my favorite cover crop so far because it's easy to seed, isn't overly invasive, is easy to dig into (or pull from) the soil, and attracts beneficial insects if allowed to flower. The only drawback (for our area) is that it's a warm weather cover crop so unless the bed is going to remain fallow through the summer, there's not enough time to reap the benefit of the buckwheat's flower. Another small drawback if the buckwheat is allowed to flower is that it can be difficult to turn into the soil when it gets too tall. Once it gets to that stage, it's easier to pull the entire plant by hand. The buckwheat shown in the photo below was pulled by hand and added to the compost pile.
When to turn cover crops:
There are no hard and fast rules for when to turn cover crops into the soil but the longer they're allowed to grow, the more difficult it will be to turn the cover crops under. It's not only more work, but the increased digging will inevitably lead to more earthworms getting chopped up in the process. Peas, oats, and vetch tend to create a thick carpet of vegetation, while buckwheat is more of an individual plant which makes it easier to dig into the soil. If the cover crop gets too large you can always pull up the plants by hand or cut the crop with a string trimmer, then either turn the pieces into the soil or add them to the compost pile. The images below show the cover crops being turned into the soil prior to planting corn. Half of the peas/vetch mix was pulled by hand and the rest turned into the soil.
How much cover crop seed?
It's tough to figure out how much cover crop seed you might need, and I can only offer what works for our application. My raised beds are different sizes but are roughly equivalent to fourteen 4' x 8' beds. We sow the peas/vetch mix in every bed in the springtime and use the buckwheat throughout the growing season when crop rotation allows. In February 2015 we bought five pounds of buckwheat and five pounds of the peas, oats, vetch mix at $20 per five pound bag. By the end of the 2016 season the peas/vetch mix will probably be used up, but there's plenty of buckwheat left for the 2017 season. They also sell an inoculant for the cover crops which was used the first season, but the inoculant has to be purchased new every year so we haven't used it again since the first season.
Where to buy cover crop seed:
There are several places that sell cover crop seed but we usually buy our seed from High Mowing Seeds or Johnny's Seeds. Either one would be a good place to start in order to get a list of the various cover crops along with their growing characteristics. If you have a small garden and don't need much seed, Johnny's offers cover crop seed in smaller amounts than most places. When searching for a particular seed such as organic buckwheat, you may need to include 'cover crop' in the search term or you may get sprouting seeds in the search results instead of cover crop seed.
Cover crop videos:
The first video looks at the peas, oats, and vetch in one raised bed, and a buckwheat cover crop in another raised bed that were in place prior to planting a corn crop, and the second video is a buckwheat cover crop in full bloom with scores of insects buzzing frantically amongst the flowers.