When vegetables are not actively growing in your garden, planting a cover crop can be an excellent, and potentially better option to both protect and improve your soil.
It’s beneficial to plant a cover crop any time a bed is allowed to go dormant. This could be in anticipation of winter or between plantings of edible crops. For gardeners wishing to pare down the amount of space that they plant and maintain, a cover crop will sustain soil health until they are ready to put the area back into production again.
The best cover crops do not produce a harvest. Instead, they benefit the soil below and future crops by providing aeration and fertility, and there are many more benefits as well.
How Cover Crops Benefit the Soil
Cover crops provide their benefits both while they are growing and long after.
To understand the benefits of cover crops, consider what happens when soil is left bare. Bare soil can dry out and blow away in the wind like dust or it can bake in the sun and become impermeable. Alternatively, in cool and wet conditions, rainwater will carry away valuable topsoil in the runoff. Rain droplets also hammer uncovered soil, causing compaction. And bare soil gives weeds the space and opportunity to germinate, grow, set seed and spread.
The roots of cover crops keep topsoil in place and reduce compaction, and the foliage both blocks the baking sun and slows down the movement of water so it percolates into the ground like it should. Cover crops shade out seeds and weeds, denying them the sunlight that they need to grow.
As if everything mentioned above is not enough of a reason to use cover crops in your garden, cover crops also improve soil aeration, encourage the proliferation of beneficial microbes in the soil, and provide soil fertility by adding nutrients and organic matter when they are turned in. (More on turning in to come, below.)
The Best Cover Crop for Your Garden
The best cover crop for your garden will depend on the problems (compaction, weeds, low fertility, etc.) that you are trying to address. While all cover crops protect the soil surface, some varieties are better than others at providing specific additional benefits above and below the soil.
The various types of cover crops fall into three main categories:
- Cereal grasses: Oat, rye and wheat are annual grasses that build biomass and break up soil compaction with their extensive root systems. Grass blades slow down water infiltration from rain and overhead irrigation systems, which reduces stormwater runoff. Grasses can be planted at the end of summer for a reliable winter cover, and they provide abundant organic material to add to the soil in spring.
- Legumes: Popular nitrogen fixers such as peas, soybeans, clover and vetch are all in the legume family. Legumes interact with soil bacteria to draw nitrogen from the air and turn it into plant-available nutrients. This makes legumes an excellent choice for banking nitrogen, improving soil fertility and enriching the soil food web. Red clover is an example of a legume that can be planted before winter or even be frost seeded — spread on frozen ground — to germinate in spring once temperatures rise and the ground softens.
- Broadleaves: Buckwheat, mustard and alyssum are a few examples of broadleaf plants that germinate quickly to provide extensive shade that prevents slower-growing weed seeds from sprouting and thriving. While all cover crops serve to suppress weeds, broadleaf plants are particularly effective at this. Mustards have the added bonus of repelling nematodes and grubs in the soil, and alyssum attracts beneficial insects. Buckwheat likewise attracts beneficials, including pollinators, and grows especially fast.
In choosing the right cover crop for your garden, also consider the days until they set seed. A cover crop like winter rye will grow for 330 to 345 days before it has mature seeds, so you don’t have to rush to cut it down before it sets seed. Others will set seed sooner and should not be used as long-term cover.
Cover crops may be killed by frost or a hard freeze. If you plant the cover crop soon enough in summer or fall, this isn’t a problem. As long as the cover crop has enough time to grow, spread and provide full coverage before it gets too cold and dies, the dead plant material that remains on the soil surface will be an effective cover. Some cover crops may appear dead in winter but then begin to grow again in spring.
If your soil is very low in nutrients and you plan to plant a heavy feeder there such as squash come spring, legumes are the way to go. If the soil already has high fertility, perhaps pick a broadleaf cover crop for maximum weed control. For maximum coverage and the best erosion control, grasses are the best low-cost pick.
You don’t have to pick one and stick with it. Mixing up cover crops together or from year to year will increase the variety of benefits to the soil. Many seed companies sell cover crop blends that pair together complementary plants for a synergistic effect.
Turning In Cover Crops
To “turn in” cover crops is, put simply, to mix the plants into the same soil they were growing in.
To turn in a cover crop, first, cut it down to the soil level. You can add the cut plant material to your compost bin and then turn just the roots into the soil. A second option is to turn in the foliage and the roots altogether right in place with a garden fork. Mix everything in several inches down into the soil — but avoid deep tilling that disrupts the soil food web and drainage.
Be aware that turning in cover crops, particularly cereal grasses with their deep root systems, can be laborious. Be prepared to fight with those tough roots. Winter-killed crops or crops that have been cut and left in place for a few days (or longer) to die off will be easier to turn in.
Microbes in the soil will get to work digesting the organic matter and releasing the nutrients to the benefit of the next crop planted there. You should not sow seeds into a bed immediately after turning in cover crops. If you do, those active microbes may also digest the seeds that you want to grow. Also, some cover crops, such as rye, have allelopathic properties that discourage other seeds from germinating. This effect will wane after the cut grass and dead roots have had a couple of weeks to decompose.
After turning in a cover crop, it’s best to wait two to four weeks for it to be broken down adequately before sowing seeds or transplanting into the bed. If you don’t want to wait as long, your best option is to remove and compost the cover crop and turn in only the crop’s roots, so the process moves along more quickly.
The best time to turn in cover crops is after they begin to form flowers but before they go to seed — the point at which their nutrient storage is at its peak. Crops that have gone to seed will have less energy and fewer nutrients available to the soil, and the seeds may germinate when you don’t want them to.
If time or other limitations make a cover crop impractical for your garden, organic mulch is your best bet. Shredded leaves, straw and arborist’s woodchips make fantastic mulches that have many of the same benefits as cover crops do, such as preventing compaction and erosion and adding valuable organic matter, while being less labor-intensive.
Trusted seed vendors can help you identify the best cover crop for your climate and your needs. A few reliable vendors with various cover crop seed options and mixes are Park Seed, Johnny’s Selected Seeds, High Mowing Organic Seeds, American Meadows, Gardens Alive! and Hudson Valley Seed Company.
Have you tried a cover crop? Let us know by leaving a comment below.
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Links & Resources
joegardener Online Gardening Academy™: Three popular courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!
joegardener Online Gardening Academy Essential Gardening Fundamentals: The basics on healthy soil, planting, watering techniques, composting, raised bed and other gardening methods, fertilizer, the many benefits of mulch, and more.
Park Seed cover crop seeds
Johnny’s Selected Seeds cover crop seeds
High Mowing Organic Seeds cover crop seeds
American Meadows cover crop seeds
Gardens Alive! cover crop seeds
Hudson Valley Seed Company cover crop seeds
*Disclosure: Some product links in this guide are affiliate links, which means we would get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us. None of the items included in this list have any bearing on any compensation being an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Park Seed, and Exmark. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.