Cover cropping means making sure the ground produces more biomass by covering it with a crop canopy for as much of the growing season as possible. — Steve Solomon
Cover crops are planted between seasons or between rows to help protect your soil and keep your garden productive. They can be a ‘mini’ crop to add nutrients, diversity, or attract pollinators. Or perhaps a crop to improve a problem you have noticed in your soil or habitat.
What is your goal?
When choosing a cover crop, think about what you want to accomplish or improve. All cover crops help to retain moisture, prevent soil erosion, and suppress weeds. They can also trap nutrients and contribute organic matter to the soil - when they die they become nutrients for microorganisms and other plants.
Depending on what you plant, the cover crop may have other benefits:
Fixing Nitrogen
Soil aeration
Food!
The crops below have been grouped by the main function or benefit you might seek.
When and How
Generally - sow a cover crop as soon as possible after harvesting or cleaning the garden bed. Planting sooner gives the cover crop a head-start against weeds. This usually means sowing in the late fall or early spring - check the directions or chat with a gardener about your specific crop.
You can also sow cover crops between rows of your main crop, after the main crop has started.
Risks and Downsides
You do need to learn what cover crops work well in your area, and adjust for your climate, soil, and weather. Using the wrong crop can ruin your timing for planting. e.g. turning in an overwintered cover crop in early spring with lots of moisture will probably work well - the moisture and young, nitrogen rich plant will rapidly decompose (say, in about two weeks).
However - If you let the same cover crop grow until it forms seeds and flowers it becomes woody. It will take much longer (perhaps five weeks) to decompose, and this period will prevent other seeds from sprouting (because the decomposing produces so much CO2). If you have bad weather that year, or are unable to tend your garden in time, having the cover crop may be more of a setback than doing nothing.
Don’t plant a large cover crop directly before - or in front of - the main food you want to grow. Otherwise the cover crop may grow taller, shading out your food! (ask me how I know)
Prevent Soil Erosion - Only
Leaves, straw, hay, mulch.
Tarp, burlap.
If you have nothing else at hand or available, covering your bare soil with _something_ helps to retain moisture, retain nutrients, and prevent erosion and leeching from wind, snow, and rain. This could be a layer of mulch, or even a simple tarp. Even the master Ben Falk admits to using tarps.
Many organic cover materials will also break down, adding nutrients or organic mass.
Fix Nitrogen
If you need to add nutrients to revitalize your soil, choose a nitrogen-fixer such as:
Alfalfa
Clover, sweetclover, red clover, crimson clover
Fava beans (both edible, and non-edible varieties)
Peas, cowpea, field peas
Vetch / hairy vetch
These will help the following crop to grow stronger and healthier. Nitrogen fixers can make an especially good cover crop before planting a heavy ‘feeder’ crop, such as potato.
Adding Organic Matter - Fibrous Roots
Organic matter is the foundation of good soil and plant health. —Falk
To add large amounts of organic matter, choose cover crops such as:
Barley
Buckwheat, winter wheat
Oats
Phacelia
Red clover (note: appears in two lists, also fixes nitrogen)
Rye / cereal rye, annual ryegrass
Sorghum / sudangrass
These crops are usually subjected to “chop-and-drop”: killed and left on the ground after growing, to add organic matter through their roots underneath the soil, and as mulch on top.
Organic Matter + Deep Taproot Aeration
Plants with deep, strong taproots include:
Okra
Radish (especially daikon variety)
Sunflower
Sweetclover (note: appears in two lists, also fixes nitrogen)
Turnip
These cover crops grow farther down into the soil. When they are harvested or die, the hole they dug remains, which helps to aerate the soil and let in oxygen. According to Strickler, oxygen penetration is actually the main limit on how deeply roots can grow, so getting more oxygen can help future crops to grow deeper and more easily. Over time this helps to pump up carbon and other nutrients from deeper down in the earth.
Deep-rooted cover crops that are left in the field will then also add organic matter.
Food!
Arugula
Collard greens
Fenugreek
Mustard
Spinach
Finally - you can retain your soil health and also grow food! Choose a leafy green cover crop that grows quickly and can be harvested early. This can help to prevent erosion, while being light on the soil and making it ready for your next main crop.
If you herd or tend animals you can also choose a cover crop to serve as a backup source of animal food - e.g. in case of crop loss or drought.
Mix and Match
For maximum benefit, blend fibrous-rooted cover crops .. with deep taprooted crops. —Strickler
[A mixed cover crop] gives us a good variety of different root types and rooting depths to fill the soil profile, increasing organic matter. The different leaf shapes maximize solar energy collection, and the variety of flowering species attracts beneficial insects. —Brown
For more results consider mixing several cover crops of different types - this may help to provide multiple benefits and increase biodiversity.
Happy gardening!
References
The Endless Arugula Bed, Falk
Growing Edible Cover Crops, Leaf For Life
Cover crops to reduce pests, Penn State agricultural study
Gardener’s Path (quite spammy on referral links, but detailed plant descriptions)
“Cover Crops For All Seasons”, SARE (6 page PDF).
“Managing Cover Crops Profitably”, SARE (248 page PDF) - LOTS of detail on many plants and climates, discussion of what works well and poorly.
"The Drought-Resilient Farm", Strickler (read my book review here)
“The Resilient Gardener”, Deppe
“Gardening When It Counts”, Solomon (read my book review here)