Green Manuring and Green Manure Crops

Green Manuring and Green Manure Crops

Green manuring is the practice of growing plants specifically to build and maintain soil fertility and structure. More usually thought of as a technique used by farmers, it can also be of great value in the organic garden.

green manuring Green manure crops are grown specifically for incorporating into the soil. They can look very attractive in the garden, making a positive contribution to its appearance.

Although green manuring can seem difficult to fit into the gardening year, it is well worth making the effort to include it.

The benefits of green manure When soil is bare rain can wash out plant foods, damage the soil structure and, in extreme cases, cause erosion. A covering green manure crop breaks the force of the rain and allows it to penetrate the soil more easily. It also protects soil life from extremes of temperature and moisture. As the green manure plants grow, they take up plant foods that would otherwise be washed out of the soil. They are also useful in weed control because they help to smother weed growth.

Some green manures, such as grazing rye, are also particularly good at improving soil structure. Others — such as clover, which can take up nitrogen from the air —can add fertility to the soil.

How to green manure

Green manuring is simple. A suitable variety of green manure is sown, allowed to grow for a period of time, then, while still green, the crop is dug back into the soil. There the plants decompose, releasing nutrients for future plants to use.

The green manure plants are dug in when young and sappy, so there is no danger of nitrogen depletion.

Where to green manure

A green manure can be grown in any site in the garden where the ground is not required for other use for several weeks or more — for example, before a late-planted crop such as courgettes, or after the summer bedding has been cleared.

Green manures can also usefully be sown on recently cleared land to improve it before permanently planting it with other crops, or simply to keep it covered while planning what to do next. The performance of a green manure crop can also give an indication of the fertility of the land. In the greenhouse, green manures can be used to help keep the border soil in good heart.

If you require a fine seedbed in the early spring, especially on heavy land, it is not advisable to grow a hardy green manure over the winter. This could mean digging in when the soil conditions are unsuitable.

One alternative is to grow a green manure variety that will be killed by the frost. Any residue left in the spring can be removed rather than incorporated.

Green Manure Crops – Plants for green manuring

The plants used as green manures are usually agricultural crops, designed to cover the ground quickly and produce a good quantity of foliage. They can be annual, perennial or biennial, frost-hardy or tender, and between them they have a range of sowing times throughout the growing season. Some are for short-term use, reaching maturity in a few weeks, while others may be left to grow for a year or more.

Different green manures also vary in their soil requirements. Several green manures belong to the legume (pea and bean) family; these can take up nitrogen from the air. Those in the legume and brassica families are related to vegetable crops, an important factor to consider with crop rotations.

The majority of green manures will compete well with weeds.


Alfalfa (Medicago sativa) Perennial z5  Alfalfa (Medicago sativa) Perennial z5

Sow from spring to mid-summer to grow for a few months, or, preferably, a year or two. Avoid acid and poorly drained soil. Once established, alfalfa can be cut two or three times a year. The foliage of this very deep-rooting plant is rich in plant foods. It can be cut for composting or for use as a fertility mulch.

Winter field beans (Vicia faba) Hardy annual z8 Winter field beans (Vicia faba) Hardy annual z8

Sow from late summer to autumn to grow over winter. This plant is a good choice of green manure for heavy land but do not use on waterlogged soil. It will not tolerate dry conditions, and it is not recommended as a weed suppressor.

Crimson clover (Trifolium incarnatum) Hardy annual z6

Crimson clover (Trifolium incarnatum) Hardy annual z6

Sow early spring to late summer to grow for 2-3 months; it may survive the winter from a late sowing. Crimson clover prefers lighter soils. An added advantage of this type of green manure is that it produces an abundance of glorious crimson flowers which attract many insects especially bees.

Essex red clover (Trifolium pratense) Short-term perennial z6 Essex red clover (Trifolium pratense) Short-term perennial z6

Sow from mid-spring to late summer to grow for a period of several months to two years. Essex clover does best on good loam and does not like acid soils.

Fenugreek (Trigonella foenum graecum) Annual z8 Fenugreek (Trigonella foenum graecum) Annual z8

Sow early spring to late summer to grow for 2-3 months. Small plants from a late sowing may survive the winter. Fenugreek does best on well-drained but moisture-retentive soil and it is one of the fastest-growing green manures. Although it is a legume, this type of green manure is unlikely to fix nitrogen in those areas where there is an absence of the appropriate strain of bacteria.

Bitter lupin (Lupinus angustifolius) Annual z4  Bitter lupin (Lupinus angustifolius) Annual z4

Sow from early spring to early summer to grow for 2-3 months. This is one of the best green manures for light acid soil, and it is best sown in rows rather than broadcast. Bitter lupin is a poor weed suppressor.

Trefoil (Medicago lupulina) Biennial z5 Trefoil (Medicago lupulina) Biennial z5

Sow any time from early spring to late summer to grow for several months or up to a year or so. Trefoil prefers light, but not acid, soils. It will tolerate some shade and so is suitable for undersowing.

Winter tares (Vicia sativa) Hardy annual z5 Winter tares (Vicia sativa) Hardy annual z5

Sow from early to late spring or mid- to late summer to grow for 2-3 months. Winter tares will overwinter from later sowings. This green manure is best sown in rows rather than broadcast. It does not do well on dry or acid soils and is a good nitrogen fixer.


Buckwheat (Fagopyrum esculentum) Annual z3 Buckwheat (Fagopyrum esculentum) Annual z3

Sow from spring to late summer to grow for 2-3 months. This is an attractive, frost-tender plant that will tolerate poor soils. Buckwheat is quick to grow, producing pink flowers that attract several types of insects, and hoverflies especially.

Mustard (Sinapis alba) Annual z6 Mustard (Sinapis alba) Annual z6

Sow from spring to late summer to grow for up to eight weeks. It will not tolerate poor soil and goes to seed quickly in warm, dry weather. Fast-growing but susceptible to clubroot.

Phacelia (Phacelia tanacetifolia) Annual z6 Phacelia (Phacelia tanacetifolia) Annual z6

Sow from early spring to late summer to grow for up to two months, or overwinter from a late sowing. It will survive a reasonable winter if the plants are small and will tolerate most oils. It produces a mass of lavender blue flowers, loved by bees and hoverflies.

Grazing rye (Secale cereale) Hardy annual z5 Grazing rye (Secale cereale) Hardy annual z5

Sow from late summer to late autumn to overwinter. Grazing rye is a cereal rye, not rye grass. It is one of the best green manures for overwintering, producing a mass of weed-suppressing foliage even in cold weather. Its extensive root system is very beneficial to soil structure. Grazing rye is not recommended for use where it would be followed by a direct-sown, small-seeded crop as it can inhibit germination.

29. January 2011 by Dave Pinkney
Categories: Manures and Fertilisers, Organic Gardening | Tags: , | Comments Off on Green Manuring and Green Manure Crops


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