If you’ve heard the term “cover crop” you may be wondering how it works and whether to give it a try in your own garden. This week, Master Gardener Susie Everding introduced this way to manage soil health in a home garden.

There are many benefits to cover cropping

Cover crops, also known as green manure, are plants grown to enrich and build soil rather than for their direct value in the kitchen. Long known in agriculture, there are plenty of benefits:

• Reduce soil erosion by slowing the movement of water above ground as well as through soil.
• Reduce compaction and improve soil structure by adding organic matter. Better structure means better drainage, aeration, and nutrient retention. Organic matter also creates an ideal environment for beneficial soil microbes.
• Reduce weeds by shading out those pesky seedlings and out-competing them for water and nutrients. Some crops, like rye, secrete chemicals that can inhibit weed growth.
• Recycle nutrients. These crops use up nutrients in the soil that may otherwise leach away. Once the cover crop has been terminated, nutrients return to the soil as the biomass decomposes.
• Improve soil fertility. Some cover crops (legumes such as beans, peas or clover) can increase nitrogen in the soil by taking nitrogen from the air and converting it to a form plants can use (nitrogen fixation).

Cover crop termination

To work as intended, cover crops need to be “terminated” or killed and fed back into the soil to improve structure and provide nutrients for future plantings.

Timing is everything! Do this when the crop is flowering, but before it sets seed. You don’t want your crop producing seeds that may then germinate, leading to an unnecessary weeding job!

Terminating the crop involves cutting down the plants, which are then composted or dug into garden. The latter approach  jump starts soil microbial activity, but the act of turning over the earth can damage soil structure. If you opt to dig in your cover crop, till it only into the top few inches leaving the rest of the soil profile intact.

Alternatively leave some or all of the cut crop on the soil surface to decompose. This will usually take about 2-4 weeks.

Choosing a cover crop

Consider the issues you want to address – soil compaction, low fertility or weed pressure. Think about ease of termination and speed of decomposition of your chosen crop, as well as seed availability. Locally, cover crop seeds are available at TCO Agromart, Selby (north of Napanee) and at  Willows Agriservices, Harrowsmith or through online seed houses.

Most cover crops fall into three categories: 

• Grasses (rye, barley, oats). Fast growing, providing plenty of biomass, with thin fibrous roots, they are good for erosion control. Oats grow to a manageable size and are easy to terminate
• Legumes (peas, soybeans, clover, vetch) increase nitrogen in soil. These require good drainage and initially grow slowly, thus don’t out-compete weeds until well established. Field peas are a favourite of Susie’s with their attractive blossoms and ease of termination
• Broadleaves (buckwheat or brassicas such as mustard, alyssum or radish). This is a good choice for soils that already have good fertility, growing quickly to shade out weeds. Some, such as tillage radishes, break up compacted soils, while alyssum and buckwheat are a magnet for pollinators. 

Instead of growing a single cover crop species, an alternative is is to grow several types together. Seed companies offer blends that pair complementary plants; for example, Susie buys a pea and oat blend to sow in the spring and fall as well as a blend of oats, crimson clover, tillage radish and lentils for planting in late summer.

How to plant a cover crop

Planting cover crops in an empty garden bed is simple – just remove any plant debris, lightly rake the bed smooth, and sow according to the package instructions. Provide good seed to soil contact to promote germination, by either covering with a thin layer of soil or lightly raking, Water thoroughly. Susie covers her beds with row cover, to keep the critters away until the seeds germinate. 

Timing for planting cover crops

Like many vegetables, cover crops are either warm-season or cool- season plants. Warm season cover crops are planted in June through mid-August, and cool season types are planted in early spring or late summer. This allows them to gain reasonable growth before the crop is terminated either by you or by winter kill. 

There are many ways to make this work even if you have a small garden space. For example, after spring crops (peas, spinach, radish, and lettuce) are finished, plant a warm season cover crop like buckwheat. It will shade out weeds and take up nitrogen, then making nutrients available for the next crop in that location

Similarly, plant oats in late summer after vegetables such as beans have been harvested, and allow those to grow until they are killed by the first hard freezes. 

What about alternatives?

If the planning and effort of cover cropping is just not for you (yet!), an effective alternative is to cover bare garden soil with mulch. Leaves, straw, and grass clippings all make terrific mulches. The benefits are similar to cover cropping, preventing erosion and adding valuable organic matter, while being less labor-intensive.

References

Jesse Frost. The Living Soil Handbook: the no-till grower’s guide to ecological market gardening. (Chelsea Green, 2021)