Green manures are a great, big mixed bag of plants from many different plant families, but the one thing they all have in common is that they benefit our gardens in one way or another. They can be used to suppress weeds between established plants like fruit bushes or artichokes, to protect a bed that is being left fallow over winter and to improve soil structure and condition land that is being converted to a new use – a piece of lawn being turned into a vegetable plot for instance -and they can be used to control pests and diseases and feed the soil. Green manures have been used in agriculture for a very long time but their value in gardens is still new and exciting.

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 Legumes such as clovers work by converting atmospheric nitrogen into a form that can be used by plants. Within little nodules on their roots the plant – a clover, pea or bean, for example -  in partnership with soil bacteria, converts N2 into usable N4H+ and stores it in the plant tissues for a short time.

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Grasses, though they aren’t able to capture nitrogen from the air, are able to hold on to it, thereby stopping it from being washed away. Planting a mix of clover and grazing rye, for example, allows you to capture nitrogen from the air and hold it in the soil where it can be used by your crops.

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Plants like Daikon radishes and Westerwold rye improve the soil by mechanical means; their hefty taproots break through hard and compacted soil, improving aeration and introducing organic matter into the substructure when the crop is turned in, in spring.  For compacted and heavy soils this is a huge benefit. And of course, as they are growing they are preventing the invasion of unwanted weeds simply by covering the soil.

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Other plants suppress weeds by allelopathic means, whereby the plant exudes a chemical that is transmitted through the soil. This fascinating ability is well-recognised in the walnut (not much grows beneath a walnut tree!) but not so well known is that other plants can have similar effects. Some plants weaken or kill the opposition by exuding chemicals that change the pH of the soil, some by exuding herbicides that kill seedlings or stop seeds from germinating and others interfere with the life-cycle of pests. Mustards, for instance, reduce the number of nematodes in the soil, while grazing rye, chopped and dug in, inhibits germination of small- and soft-bodied seeds. This can be exploited if you intend to plant hard-seeded species or young plants, allowing a window of 3 or 4 weeks for your chosen plants to get a foot hold and become established before the rye’s allelopathic effect wears off.

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There are numerous green manures to choose from, each with their own special effects...it is just a matter of choosing the right species for your needs.

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The legumes 

These plants will fix nitrogen, add bulk and suppress weed growth.

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Alfalfa (Medicago sativa) is a good, lush choice for a quick crop but don’t plant it in the same place every year as alfalfa is autotrophic (poisonous to its own kind) and chemicals exuded by the plant will build up and reduce alfalfa yields over time. Sow April to July for 2 to 3 months

 

Field bean (Vicia faba) is a good choice for heavy soils which it will help to break up...will overwinter and can be incorporated in early spring. Sow September to November for the winter

 

White clover (Trifolium repens) is good long-term, low-growing weed suppressor for between permanent crops such as artichokes and fruit bushes which will fix nitrogen into the bargain. Sow March to August for 6 months

Vetch (Vicia sativa) is an excellent choice for winter weed suppression, a quick nitrogen boost and nutrient holding capacity. Likes heavy soils. Sow March to May for 2 to 3 months or July to September for overwintering.

 

Red clover (Trifolium pratense) is a quick, leafy ground cover that can be cut in 3 months or left as a cover crop for a year or so. Sow April to August for 3 to 18 months

 

Sweet clover (Melilotus officianalis) is perfect for nitrogen fixation on compacted soils where the roots will break up the soil and forage for nutrients making them available to successive crops. Sow March to May for 3 to 18 months

Trefoil  (Medicago lupulina) is perfect for planting between tall crops (like beans and corn), for weed suppression and nitrogen fixation. Tolerates light, dry soils. Sow March to August for 3 to 12 months.

 

Grasses

Grazing rye (Secale cereale)  is a good choice for late sowing, to hold nutrients and to suppress winter weeds. Will continue to grow in cold weather and provides large quantities of biomass in a short time. Dig in early in the year to avoid woody stems. Sow August to November for 3 to 6 months

 

Italian Ryegrass (Lolium multiflorum)  and Westerwolds Ryegrass (Lolium westerwoldicum)  are fast growing, good for weed suppression and nutrient foraging and they are most beneficial when teamed with red clover and dug in, in the spring. Sow March to April or August to September for 2 to 18 months.

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Brassicas

These plants, members of the cabbage family, should be grown in the brassica part of a rotation system to avoid build up of pests and diseases.

 

Caliente Mustard (Sinapis ‘Caliente’) is the leader for pest and disease control, weed suppression and biomass production.  For optimum allelopathic effect the crop must be cut, shredded and incorporated within 20 minutes of harvesting. As the active ingredient, glucosides, are water borne, the soil must be wet at the time the plant is dug in.  Sow March to October for 2 to 3 months.  Sinapis alba, ordinary mustard, is almost as good.   

 

Daikon Radish (Raphinus sativus longipinnatus) Good for breaking up heavy soil and for improving poor, light soils. Can be sown in autumn as a winter cover crop and the long tap roots allowed to rot in the ground to provide pockets of organic matter.  Sow May to August for 2 to 6 months

 

Others

 Phacelia (Phacelia tanacetifolia)  is a quick growing hardy annual and is so attractive to beneficial insects that if allowed to flower en masse the insects  may not venture onto your other crops. Instead cut the majority down before flowering and leave a tempting clump for the bees and flies. Will suppress weeds and provide good biomass. Sow March to September for 2 to 6 months

 

Buckwheat (Fagopyrum esculentum) is largely pest and disease free and a good weed suppressor, which makes it a good choice to provide a break where a patch of ground is ‘dirty’ from soil borne pests and diseases, and it will die at the first sign of frost. Buckwheat will forage for phosphate and make it available to the succeeding crop. Sow May to August for 2 to 3 months.

 

 

• To properly exploit the effect of your green manures always incorporate your plants into the top few inches of your soil within 20 minutes of cutting and always cut them before they flower when the useable nitrogen is at its peak!

• 2500 years ago Chinese farmers conducted tests to establish if it was more beneficial to manure the land or to use legumes as green manure, and farmers have been using them both before and since.