Green Manure is the premium in-place organic soil conditioner created by the practice of cover cropping. Cover cropping with leafy greens and grasses is a traditional natural gardening practice meant to enrich soil structure and available nutrients. It also crowds out weeds! Here are 11 cover crops you can use to create a rich green manure top-dressing for your garden.
“Green manure saves not just on artifical fertilizers but also on precious garden manure – of which no one ever has enough. It has many virtues and practically no drawbacks, but has never really caught on.”The Complete Gardener: A Practical, Imaginative Guide to Every Aspect of Gardening, by Monty Don
1. Crimson Clover
Crimson Clover (Trifolium incarnatum) is a cool-weather clover often planted in the fall as an overwintering cover crop. It grows best in full sun, but can also grow in partially-shady areas. As a green manure, clover is known for its ability to draw nitrogen from the air, which enriches the soil of your garden (since nitrogen is the most commonly deficient soil nutrient).
Crimson clover for green manure is most commonly sown as a cover crop in the fall. Sow it anytime from late August until November, as long as nighttime air temperatures tend to remain above 40°F (4°C) to help seed germination. This tends to be about a month or two before the first fall frost in any given area. Crimson clover can overwinter as a green manure cover crop in Zones 6 and warmer, or with cold-weather protection in cooler zones, as its hardy to around -10°F (-23°C)!
Here are two sources of the classic crimson clover used for green manure:
Crimson clover grows striking red blooms in early spring which work to attract beneficial insects to your garden. Bees are especially fond of this cover crop! Let the clover flower, but mow/trim the crop down before seeds can form. Gently turn the spent crop into the first inch or two of soil as a green manure to prepare the bed for springtime planting (like yummy tomatoes!).
2. Winter Peas
Winter peas (Pisum sativum) are a cool-season green manure crop sown in the fall to condition soil for planting next spring. Winter peas add valuable nitrogen to the soil, suppress autumn weeds, and reduce runoff and erosion during winter precipitation. Winter peas are frost-hardy, and may survive over winter in warmer zones.
Plant winter peas as a green manure about a month or two before the first fall frost is expected in your area. They need about 2 months to fully mature and create optimally-nutritious green manure. Here are two lovely “green manure” options for your garden:
In cooler areas (and snowy, wet climates), winter peas may die off over winter, while in more mild winters the plants may survive to flower the following spring. The important thing is to mow or trim off the spent flowers before they go to seed.
Buckwheat (Fagopyrum esculentum) is a quick-growing, warm-weather leafy green manure crop that can be used as a transition to fill mid-summer bare patches of soil. It grows quite well in poor soil and tends to improve the amount of available nutrients in the soil (particularly phosphorus). Buckwheat does not tolerate frost well, so it’s generally grown in periods when frost is not expected.
Plant buckwheat in the spring when soil is a minimum of 55°F (13°C) and the threat of frost has passed. It can be sown in bare patches throughout the spring, summer, and fall. Use it whenever you expect a bare patch of soil lasting about a month:
Let the buckwheat grow for about a month, and then turn it into the soil as an organic soil conditioner. Buckwheat can be sown as a green manure in this manure up until 4 weeks before the average first autumn frost in your local area.
4. Fava Beans (Field Beans, Broad Beans)
Fava beans (Vicia faba), also called Field Beans or Broad Beans, are a cold-tolerant cover crop sown most often in early spring to enrich soil before warm-weather crops are planted. As a green manure, fava beans are known for their excellent ability to aerate soil and enrich it with nitrogen. They are also an early bloomer that attracts bees and other beneficial insects to your area.
Fava beans planted for green manure are generally planted in early spring in cooler climates, most often with February-May sowing dates. While the seeds germinate best at soil temperatures at or above 50°F (10°C), they can actually germinate at even lower temperatures (perhaps as low as 35°F (2°C))! Here is a type that works well as a green manure:
Fava beans take a month or two to reach maturity and start to flower. Once the beans have flowered, trim back the crop before seed pods form. Work the green plant material into the top of the soil and allow the bed to sit for a short period before planting again so the green manure can start to decompose.
5. Winter Rye (Cereal Rye)
Winter rye (Secale cereale) is cold-tolerant, overwintering cover crop that can be planted later in autumn than some other green manure crop options. Also called cereal rye, this cool-loving plant helps to add nitrogen and organic matter to depleted agricultural soils or naturally-sandy garden plots. It also has a net-like root system and tall above-ground profile which both help to prevent wintertime soil erosion.
Winter rye can germinate at chilly temperatures, down to 34°F (1°C). This makes it an excellent option for green manure when its too late in autumn for other grasses, mustards, and clovers. Cereal rye is generally left growing in spring time to help draw up moisture and dry out waterlogged soil. Just be sure to cut it down and incorporate it into the soil prior to the plants growing seeds.
6. Lacy Phacelia
Lacy Phacelia (Phacelia tanacetifolia) is a warm-weather flowering green manure perfect for summer dry spells. Its lovely purple flowers can be used in bouquets or left on the plants to attract bees, butterflies, and other summertime pollinators. In addition to producing flowers, the roots of lacy phacelia improves soil biology by hosting beneficial mycorrhizal fungi and also works to aerate compacted soil, improving soil structure.
Lacy phacelia is generally planted as a mid-summer transition crop to enrich the soil for early-fall planting of cool-weather vegetables. Use it whenever you expect a patch of soil to lay bare for about 2 months (it has a longer maturity time than buckwheat). Here are the green manure seeds:
Lacy phacelia for green manure grows well in the heat of summer when crops like peas tend to find things a bit too hot out. That being said, healthy lacy phacelia plants can be hardy down to 18°F (-7°C), and so this crop can be used as an overwintering green manure in mild climates.
7. Broadleaf Mustard
Broadleaf mustard (Brassica juncea) is a cool-season autumn cover crop planted to boost nutrients (and repel pests) for the following growing season. As a green manure, these plants work to pull nutrients from deep below the soil surface, making them available for next year’s veggies. Broadleaf mustard also keeps weeds down, repels garden pests, and holds soil in place to protect against erosion from winter precipitation.
Plant broadleaf mustard in late summer as the first of the warm-season veggies finish harvest. Broadleaf mustard will produce leafy greens for a month or two, followed by little yellow flowers. It generally takes about 3 months for mustard to produce seeds. When growing it for green manure, make sure to cut down the crop before the seeds are produced. After trimming, work the organic matter into the soil as green manure.
8. Iron Clay Pea
Iron Clay Pea (Vigna unguiculata), also known as cowpeas, is a warm-weather cover crop used to enrich soil in climates where the spring and fall growing seasons are separated by mid-summer drought. As a cover crop, it suppresses weeds and attracts pollinators. As a green manure, it pulls nitrogen from the surrounding air, making it available for fall-planted vegetable crops. Here is the type of cowpeas used for green manure:
Plant iron clay peas when the soil temperature is warm, around 65°F (18°C). These plants will survive the hot and dry mid-summer weather, and can then be cut down before setting seed to enrich the soil prior to fall planting. To use as green manure, till the cut plants down into the soil or cover the whole area with a silage tarp (Silage Tarps for Green Manure (Cover Crop Decomposition)).
9. Brown Top Millet
Brown top millet (Brachiaria ramosa) is a warm-weather grass that makes a great cover crop and green manure during the summertime. Millet can be used to fill bare patches in mid-summer that are expected to last about 2 months before fall planting takes place. This cover crop crowds out weeds when the weeds are at their strongest (and somehow manages to grow faster than they do – amazing!). This quick growth adds lots of organic matter to the soil when cut down and used as green manure.
Brown top millet is best planted when the soil is a minimum of 65°F (18°C). Let it grow for a couple months, but trim it down before seed heads can grow. As with iron clay peas, brown top millet can be tilled into the soil or mulched and covered with a silage tarp to speed decomposition.
10. White Clover
White clover (Trifolium repens) is a long-lasting perennial cover crop used to crowd out weeds and enrich the soil around long-lived tree and vine crops. White clover pulls nitrogen from the air into its foliage, and can then be repeatedly mowed. This fertilizer-like characteristic makes it a particularly valuable permanent cover crop:
Plant white clover whenever air temperatures remain above 40°F (4°C). Let the plants grow and establish themselves for several months prior to the first mowing.
11. White Mustard
White mustard (Sinapis alba) is a versatile cover crop used to suppress soil-borne diseases/rot and pests like nematodes. As a green manure, this quick-growing brassica adds a great deal of organic matter to the soil (including vital nitrogen).
This specialty cover crop is most often planted in vineyards and other irrigated perennial crops, but it can certainly be used as a fall-planted green manure in home gardens. The leafy greens are not very cold tolerant, and generally die back in freezing temperatures. Till the spent plants into the top layer of soil in the early spring in preparation for spring veggie planting.
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