Cover Crops for the Home Garden
For most home gardens, the simplest cover crops are oats or forage radish sown in late summer, which usually die over winter and leave a mulch you can plant through in spring. If you want something that survives winter and does more for the soil, winter rye is the hardiest option, and hairy vetch or crimson clover adds nitrogen. Sow 1 to 4 ounces of seed per 100 square feet, depending on the species. The hard part is termination: rye and vetch need to be cut when they're flowering, and then you have to wait before planting.
Why grow one in a garden
The University of Maryland's guide to cover crops for gardens lists the benefits: less erosion, more microbial activity, added organic matter, and, from legumes, nitrogen fixed from the air. Grasses such as rye also soak up leftover nitrogen. SARE's handbook reports that cereal rye typically scavenges 25 to 50 lb of nitrogen per acre and can take up as much as 100 lb, holding it over winter instead of letting it leach.
There's one more reason that matters in long-composted gardens: a cover crop builds organic matter without importing any phosphorus. UMN Extension recommends cover crops as a way to build organic matter in beds that have had too much compost or manure. If your soil test shows high phosphorus, a cover crop is a better way to add organic matter than more compost. See can you use too much compost?
Species for home gardens
Seeding rates and notes below are from the University of Maryland guide. When you mix two species, Maryland suggests using about 60 percent of each one's single-species rate.
| Species | Oz. per 100 sq ft | When to sow | Winter / termination | Notes |
|---|---|---|---|---|
| Spring oats | 3–4 | Spring or late summer/fall | Usually winter-killed | Tolerates acidic soil; easiest to manage |
| Forage radish | 1–2 | Late summer/fall | Usually winter-killed | Deep taproot helps break up clay |
| Buckwheat | 3–4 | Spring or summer | Not hardy | Grows fast; will reseed |
| Winter rye | 3–4 | Late summer/fall | Very hardy | Massive root system; hardest to kill |
| Winter wheat | 3–4 | Late summer/fall | Terminate in spring | Large root system |
| Barley | 3–4 | Spring or late summer/fall | Less hardy than rye | Drought tolerant |
| Crimson clover | 1–2 | Spring or late summer/fall | Terminate fall or spring | Legume; showy spring flowers |
| Hairy vetch | 3–4 | Late summer/fall | Very hardy | Legume; slow to establish; can become weedy |
Legume seed needs the right inoculant, the nitrogen-fixing bacteria for that crop. Maryland recommends mixing the seed with inoculant powder in a bag or bucket just before sowing.
When to plant
Most home garden cover crops go in as beds come empty in late summer and early fall. SARE advises sowing cereal rye from late summer to mid-fall in Hardiness Zones 3 to 7, and from fall to midwinter in Zones 8 and warmer. Hairy vetch needs more lead time: SARE suggests sowing it 30 to 45 days before a killing frost. UMN Extension's page on cover crops and green manures in home gardens suggests quick-growing ryegrass, rapeseed, or oats for late-summer gardens, and heat-loving buckwheat or beans for new beds in spring or early summer.
A realistic caveat about nitrogen: UMN notes that to create enough fixed nitrogen to feed future crops, a legume cover crop has to grow for an entire season. A clover sown in September and turned under in April contributes something, but less than the "free nitrogen" pitch suggests.
Choosing by how much work you want
- Least work: oats or forage radish in late summer. They die in a cold winter and leave residue you can plant through or rake aside. Maryland notes they need less waiting time before planting than other covers. They depend on a hard freeze, so in mild-winter areas they may survive.
- Most soil benefit: winter rye, alone or with hairy vetch. Rye is extremely hardy and roots deeply; vetch adds nitrogen. Both require deliberate termination in spring.
- Summer gap filler: buckwheat between spring and fall crops. It grows fast, but mow it before it sets seed or it will come back.
Terminating by hand
Without a tractor, you have three options, per Maryland: cut it with a mower or string trimmer, smother it under a tarp, or turn it under with a garden fork. Timing is what makes each one work.
Cereal rye. Cut rye too early and it grows back. SARE warns that rye cut before about 12 inches tall risks regrowth, and recommends waiting until it has begun flowering before mowing it down. UMN Extension notes the same thing for mowing and crimping in general, that they work best at flowering or heading, but that in Minnesota this often pushes termination into late May and can delay planting. Mature rye is also more carbon-heavy: UMN lists a C:N ratio of 37:1 at flowering versus 26:1 in the vegetative stage, so it breaks down more slowly and ties up more nitrogen as it does.
Hairy vetch. SARE reports that vetch is easily killed by mowing or cutting once it reaches about 50 percent bloom. Cut it before it flowers and it tends to recover. Vetch also produces hard seed, about 10 to 20 percent of which can lie in the soil for a season or more, so letting it go to seed creates a weed problem.
Tarping. Maryland lists smothering under a tarp as an option; it kills the cover crop by depriving it of light, and it saves digging. How long it takes depends on the species and the weather, so check under the tarp before you plant.
How long to wait before planting
| Situation | Wait | Source |
|---|---|---|
| Cover crop winter-killed (oats, radish) | Little or none | UMD, UMN |
| Living cover crop cut or turned under | About 2 weeks | UMD, UMN |
| General field recommendation | 7–14 days | UMN |
| Small-seeded crops (carrots, onions) after rye | 3–4 weeks | SARE |
The rye rule is the one to remember. SARE reports that rye residue is allelopathic, releasing compounds that suppress germination, for about 30 days after termination. Transplants and larger seeds tolerate it much better than direct-sown small seeds like carrots and onions. If you plan to sow small seeds, put rye somewhere else or plan to transplant into that bed.
Cover crops fit into the broader approach in building garden soil organically, alongside a soil test, measured compost, and mulch.