by Erin Marissa Russell
Ready to take your garden to the next level with green manure? Green manure isn’t actually manure at all. It’s a name for plants you use in empty areas to improve and protect the soil while returning nutrients to the soil like manure. Before the plants are cut down and mixed into the soil, they work like a mulch as cover crops or catch crops.
Green manures are planted on empty garden plots to cover the soil the way mulch does. Like mulch, the plants will choke out weeds while the roots hang on to soil and keep it from eroding. While the plants are still green, you dig them into the soil to add nutrients and improve the soil texture.
When you sow green manures, the plants hang on to the nutrients in the soil, keeping wintertime rains from washing the nourishment away. The green plants also prevent soil from becoming compacted by rain and offer shelter for beneficial garden insects.
Plants to Use As Green Manure
There are two categories of plants used as green manure: those that are legumes and those that are not. Flowering manures have the added benefit of attracting pollinators to your garden.
The legumes have the extra benefit of fixing nitrogen in your soil. You can help promote nitrogen fixing by using an inoculant or treatment medium. You can find powdered inoculant at the nursery or garden center. It’s especially important to use inoculant if it will be your first time you grow legumes. Otherwise, the bacteria that help fix nitrogen in your soil may not be present.
Alfalfa (Medicago sativa): Works well with alkaline soil. You can dig alfalfa back into the soil after two or three months, or you can leave it for a year or two.
Alsike clover (Trifolium hybridum): Works well in wet, acidic soil. Sow between April and August. You can dig alsike clover back into the soil after two or three months, or you can leave it for a year or two.
Bitter Blue Lupin (Lupinus angustifolius): Works well in light, sandy, acidic soil. Sow between March and June. You can dig bitter blue lupin back into the soil in two or three months.
Buckwheat (Fagopyrum esculentum): Works well in poor soil. Sow between April and August. You can dig buckwheat back into the soil in two or three months. Find out more in our article Growing Buckwheat as a Garden Cover Crop [https://www.gardeningchannel.com/growing-buckwheat-cover-crop/].
Crimson clover (Trifolium incarnatum): Works well in light soil. Sow between March and August. You can dig crimson clover back into the soil in two or three months after flowering.
Essex red clover (Trifolium pratense): Works well in loamy soil. Sow between March and August. You can dig Essex red clover back into the soil in two or three months , or you can leave it for a year or two.
Fenugreek (Trigonella foenum-graecum): Only grows in the spring and summer.
Field Peas (Pisum sativum): Find out more about using field peas as green manure in our article How to Grow Field Peas as a Garden Cover Crop.
Grazing rye (Secale cereale): Plant between August and November. You can dig grazing rye back into the soil the spring after it was planted. Find out more in our article Growing Winter Rye as a Garden Cover Crop.
Mustard (Sinapis alba): Do not follow mustard with another brassica crop to prevent clubroot. Mustard grows so quickly it can be sown in September and dug back into the soil in October. Or you can sow between March and September. You can dig mustard back into the soil in two or three months. Find out more about using mustard as green manure in our article How to Grow Mustard Greens as a Garden Cover Crop.
Oats (Avena sativa): Find out more in our article How to Grow Oats as a Garden Cover Crop.
Phacelia (Phacelia tanacetifolia): Sow between April and August. You can dig phacelia back into the soil in two or three months.
Red clover (Trifolium pratense): Find out more in our article Growing Red Clover as a Garden Cover Crop.
Soybeans (Glycine max): Find out more in our article How to Grow Soybeans as a Garden Cover Crop.
Trefoil (Medicago lupulina): Works well in light, dry alkaline soil. Sow between March and August. You can dig trefoil back into the soil in two or three months or leave it for one or two years.
Wheat (Triticum): Find out more in our article How to Grow Wheat as a Garden Cover Crop.
Winter Field Bean (Vicia faba): Works well in heavy soil. Sow between September and November. You can dig winter field bean plants back into the soil in two or three months after flowering.
Winter tares (Vicia sativa): Works well even in heavy soil. Sow between March and August if you will dig winter tares back into the soil in two or three months. If you will be overwintering them, sow between July and September.
Tips for Growing and Using Green Manure
- Before sowing your green manure, check the area where you will plant and remove any stones or other large pieces of debris.
- Sow cool season manures at the end of summer or beginning of fall, then dig them into the soil the following spring. Warm season manures grow more quickly and should be sown in springtime or summer.
- You can either sow seeds in rows or sprinkle them evenly across the surface of the soil. For maximum control over how evenly seeds are sown, mix them with some soil or sand before you spread them in the garden. Rake just a bit of soil over the seeds.
- For best results, plant your green manures when it is about to rain. It’s very important that the seeds stay wet while they are germinating. If there is no rain after you sow your seeds, water them lightly.
- At least two weeks before it’s time to plant a new springtime crop where your cover crops are growing, chop the plants down and let them stay in the field to wilt. The perfect time is at the beginning of spring when soil is still not warm enough to plant but has dried out enough to compact some when handled. Avoid working with your green manure plants when the ground is still wet. Cut your green manures back just before they flower or just after so they won’t be able to self-seed in your garden.
- You can cut your green manure plants back using garden shears or a brush cutter, mower, or trimmer.
- If your garden has heavy soil or clay soil, you should cut down your manure in late fall instead of in the spring. Then it will have the winter season to break down and incorporate into your soil.
- Once the plants have wilted, turn them into the top 10 inches of the soil using a pitchfork, rototiller, or shovel.
- Leave the field empty for at least two weeks before planting your new crop. The decaying foliage can prevent plants from growing properly if you plant too early after turning green manure into the soil.
- You may need to take extra measures against slugs and snails, as green manure is a perfect habitat for these creatures. For tips and tricks, check out our article How to Protect Seedlings from Slugs and Snails.
- Green manures in the brassica family can have trouble with club root. Use a different type of green manure in garden plots where brassicas have grown. Brassicas include: bok choy, broccoli, broccoli rabe, broccolini, Brussels sprouts, cabbage, cauliflower, collard greens, daikon, kale, kalettes, kohlrabi, mizuna, tatsoi, turnips, and wasabi.
- Choose a green manure that’s planted in the fall and dies back in the winter (like oats) when you’ll be planting the plot with early spring vegetables like radishes, peas, or salad greens. Choose a green manure planted in the fall that must be cut back in springtime (like winter rye) when you will plant the plot with summer vegetables like peppers, squash, or tomatoes.
After reading this article, you’ve learned about all the different kinds of plants used as green manure. You’ve also learned a lot about how green manure works, along with some tips and tricks for working with green manures. Now you’re ready to improve the soil in your garden with this efficient technique.