Cover Crops for Cannabis Beds: What to Plant Between Seasons
Quick answer: Cover crops for cannabis are crops you sow on a bare outdoor bed between seasons and never harvest. Oats, winter rye, winter wheat, crimson clover and hairy vetch are common fall choices, sown in late summer or early fall. Cut them down before they set seed in spring, then wait about 2 weeks (University of Maryland Extension) to 2 to 4 weeks (University of Illinois Extension) before you plant.
What Is a Cover Crop and Why Use One in a Cannabis Bed?
University of Illinois Extension describes cover crops as non harvested crops that add organic matter to the soil, transfer nitrogen to plants and break up heavy clay or compacted soil. After an outdoor cannabis harvest, a bed can sit bare until spring. A cover crop neatly fills that gap.
University of Maryland Extension lists the main benefits:
- Less winter erosion. Roots hold soil in place while no crop is growing.
- Better soil quality. Cover crops feed the beneficial soil microbes that build good soil texture and keep drainage working.
- More organic matter. The debris adds organic material when it is turned under in spring.
- Nutrients held in place. Cover crop roots pull up and retain nutrients that might otherwise leach out of the soil.
UMD notes that cover crops are also called green manures. Illinois adds that they suppress weeds, create habitat for beneficial insects and increase biodiversity. If you are planning what grows next to cannabis during the season, that is a different topic covered in our guide to companion plants for marijuana.
Which Cover Crops for Cannabis Work Best?
Cover crops fall into two groups. Grasses such as oats and rye build roots and organic matter. Legumes such as clover and vetch partner with bacteria. UMD explains that legume roots work with special bacteria that convert nitrogen in the air into a form plants can use, and that decomposing legumes return a lot of nitrogen to the soil.
| Cover crop | Type | When to sow | Notes |
|---|---|---|---|
| Spring oats | Grass | Spring or late summer to fall | Not cold hardy, killed by the first hard freeze (UMD) |
| Winter rye | Grass | Late summer to fall | Very hardy, massive root system, can be planted the latest in fall (UMD) |
| Winter wheat | Grass | Late summer to fall | Easier to manage in spring than rye or barley (UMD) |
| Crimson clover | Legume | Spring or late summer to fall | Fixes nitrogen, works as a warm or cool season cover (Illinois) |
| Hairy vetch | Legume | Late summer to fall | Very hardy, slow to establish, can become weedy if it sets seed (UMD) |
| Buckwheat | Buckwheat | Spring or summer | Not winter hardy, grows quickly, will reseed (UMD). A warm season option in Illinois |
| Cowpeas | Legume | Spring or summer | A warm season legume option (Illinois) |
UMD suggests oats for a first try, since winter kills them and leaves a brown mat in spring. It also says to combine legumes and non legumes when possible, using about 60% of the single crop seed amount for each in a two species mix.
What a Cannabis Field Trial Found
An NC State Extension field trial grew cannabis for CBD after three fall planted legume cover crops: crimson clover, hairy vetch and Austrian winter pea. The cover crops were planted in early November 2020 and mid October 2021 and cut down in early to mid May. NC State found that Austrian winter pea and hairy vetch had higher nitrogen content than crimson clover. The crimson clover had also reached seed production by the time the field was prepared.
When to Sow a Cover Crop Before Cannabis
UMD says most cover crops are planted in late summer or early autumn, at the end of the harvest season. In Maryland, fall sown cover crops go in during a two month window starting in mid August and ending no later than mid October. The plants need time to sprout and grow before the first hard frost.
Illinois says cool season cover crops are planted in late summer or early fall after harvest and need to sprout and grow before winter. Warm season cover crops such as buckwheat go in during spring or summer before the garden is planted. Your county Extension office can give dates for your area.
How to Grow Cover Crops for Cannabis
- Clear the bed. UMD says to remove old plants, weeds and summer mulch.
- Loosen and level. Loosen the soil with a garden fork and rake it smooth (UMD).
- Choose your seed. Pick from the table above. UMD suggests buying seed from a local farm supply store or garden center.
- Broadcast the seed. UMD says to scatter seed by hand or with a hand held seeder, preferably before rain. Mixing seed with soil or compost helps spread it evenly.
- Press it in. Gently rake the seed into the surface and walk lightly over it so it touches the soil, as UMD advises.
- Let it grow until flowering. Illinois says to let the plants grow until they begin to flower.
- Cut it down before seed. Illinois says to mow or cut as close to the ground as possible once flowering begins. UMD says this usually happens in late April or early May.
- Wait, then plant. Leave the residue for about 2 weeks (UMD) to 2 to 4 weeks (Illinois) before planting. Before you plant cannabis outdoors, check your state’s home grow law.
Leave It on Top or Turn It In
Illinois says both options have benefits. Leaving the cut plants on the soil surface can suppress weeds, while tilling helps work the organic matter into the soil but disrupts the soil ecosystem. UMD says turning under means using a fork or shovel to flip or partly bury the debris so it begins to compost. For more on covering soil around growing plants, see our guide to mulch for cannabis.
How Long to Wait Before Planting Cannabis
UMD says to allow about 2 weeks for the cut cover crop to break down before planting vegetables, except after oats and usually daikon radish. Illinois follows a “mow, wait, plant” rule: leave the plant debris for 2 to 4 weeks, so the residue can break down and the flush of microbial activity can slow before new plants start growing. In the NC State trial, the cover crops were left to partly decompose for two weeks before bedding and transplanting.
Start by counting back from your planting date. If you usually plant out in late May, cutting in early May leaves you inside the 2 to 4 week window.
Troubleshooting
The cover crop went to seed. Illinois says a crop cut too late may reseed. UMD warns hairy vetch can become weedy. Cut at the start of flowering next time.
It grew back after mowing. Illinois says a crop cut too early may regrow. UMD notes scattered regrowth after mowing may need cutting again, and that covering the stubble with a tarp can also help.
Young cannabis plants struggle after planting. The residue may not have broken down yet. Illinois says the microbial flush needs to slow before new plants go in, so wait the full 2 to 4 weeks.
The seed did not sprout. UMD says seed must directly contact soil to sprout. Rake it in and press it down.
The bed is too wet or cold to plant on time. Choose oats next fall. UMD says they winter kill and leave a mat that you can plant through or rake aside.
You want living soil in containers. Growing plants in pots year round is a different method, covered in our guide to living soil and cannabis. For bed soil in general, see the best soil for outdoor cannabis.
Sources
- University of Maryland Extension, Cover Crops for Gardens: https://extension.umd.edu/resource/cover-crops-gardens
- University of Illinois Extension, Cover Crops in the Home Garden (PDF): https://extension.illinois.edu/sites/default/files/cover_crops_for_the_home_garden.pdf
- NC State Extension, Leguminous Cover Crops in Organic Cannabinoid Production: https://content.ces.ncsu.edu/leguminous-cover-crops-in-organic-cannabinoid-hemp-production
- SARE, Managing Cover Crops Profitably, 3rd edition (further reading): https://www.sare.org/resources/managing-cover-crops-profitably-3rd-edition/
FAQ
What is the best cover crop for cannabis beds?
There is no single best choice. University of Maryland Extension says oats are the easiest start because winter kills them. Winter rye is the hardiest and builds a massive root system. Legumes such as crimson clover and hairy vetch add nitrogen. UMD suggests mixing a legume with a grass when you can.
When should you plant a cover crop before cannabis?
In late summer or early fall, after harvest. University of Maryland Extension says fall cover crops are usually sown from mid August to no later than mid October in Maryland, so they can grow before the first hard frost. University of Illinois Extension says warm season options like buckwheat go in during spring or summer.
How long after cutting a cover crop can you plant cannabis?
About 2 to 4 weeks. University of Maryland Extension says to allow about 2 weeks for the cut plants to break down. University of Illinois Extension says to leave the debris for 2 to 4 weeks so the residue breaks down and microbial activity slows. An NC State Extension trial waited two weeks before transplanting.
Is clover a good cover crop for cannabis beds, and which clover works best?
Clover can work. Crimson clover is the common choice in University of Maryland and Illinois Extension lists, and it fixes nitrogen. UMD lists Dutch white clover as a spring or summer option. In an NC State Extension field trial, crimson clover had lower nitrogen content than hairy vetch and Austrian winter pea.
Do you need to cut a cover crop before planting cannabis?
Yes, unless winter has already killed it. University of Illinois Extension says to mow or cut it as close to the ground as possible once it starts to flower, before it sets seed. University of Maryland Extension says oats usually winter kill, leaving a mat you can plant through or rake aside.
