Most green manures grow over winter when the ground is free of crops. Sow them in late summer or autumn to help mop up

To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key plant nutrients usually derived from soil are nitrogen, phosphorus and potassium, while carbon, oxygen and hydrogen are absorbed from the air. Other vital soil nutrients include magnesium, calcium and sulphur. Gardeners can add nutrients by applying fertilisers (either artificial or naturally derived) to boost plant growth and improve flowering and fruiting.

Use green manures to cover bare patches of soil in the spaces between crops, or during intervals between one crop and the next. For example if you sow fast-growing mustard sown before mid-September you be can be digging it in by October, for example. Alternatively, leave the frosted remains left as mulch.

Summer-grown green manures such as buckwheat and fenugreek form dense foliage that will effectively suppress weeds.

Green manures belonging to the pea and bean family (legumes) have the additional capacity of storing (fixing) nitrogen from the air to their root nodules, but only in summer. Nitrogen is a valuable plant nutrient.

Advantages and disadvantages

Advantages

• Keeping the soil covered helps prevent soil erosion
• Insulates the soil
• Encourages soil activity by bacteria
• Legumes such as vetches and clovers increase nitrogen
• Supressing weeds
• Adds to soil fertility
• Protects the soil surface from compaction by rain
• Much better for wildlife than bare soil
• May attract frogs and toads to keep slug and snail populations down
• Some green manures such as tagetes can keep soil nematodes at bay

Disadvantages

• Potential increase in slugs and snails from dense foliage
• Brassicas and legume green manures (the clovers tares, lupins and vetches) need to be built in to your crop rotation and can suffer from the same diseases as vegetables in those families
• You may need to chop up the top growth before digging or rotovating in
• Perennial green manures such as alfalfa (Lucerne) can grow from the roots if dug in less than 25cm deep
• It takes time, effort and cost, to grow and green manures may need watering to establish in dry summers

Can green manures be used in no-dig regime?

Yes, you can grow green manures such as  bristle oats (Avena strigosa) in late summer. When you cover them with cardboard in spring, the foliage will

Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

Over-wintering green manures

It is highly likely that mixtures are more effective than single species. You can buy mixes, but homemade mixtures will be effective – bristle oats, crimson clover and phacelia for late summer sowing for example. 

Winter tares (Vicia sativa): This annual legume is hardy and overwinters well, even in heavy soils; sow either in March to August and leave for two or three months before digging in, or sow in July to September for overwintering.

Winter field bean (Vicia faba): This annual legume can be left for two or three months after sowing (up to flowering) and is good for heavy soils; sow in September to November.

Grazing rye (Secale cereale): This annual crop is good for soil structure and overwinters well; sow in August to November and dig in the following spring.

Trefoil (Medicago lupulina): This legume can be annual or biennial and overwinters well but needs light, dry alkaline soil; dig in after two or three months or leave for one or two years after sowing; sow in March to August.

Essex red clover (Trifolium pratense): This hardy perennial legume overwinters well and can be left in for two or three months or for one or two years after sowing; good for loamy soils; sow March to August.

Crimson clover (Trifolium incarnatum): This perennial legume is good for light soils; sow in March to August and leave in for two or three months up to flowering.

Poached egg plant (Limnanthes douglasii): This hardy annual is low growing, supresses weeds well and is easy to dig in. Sow up until September. It attracts pollinators if you leave some to flower.

Italian ryegrass (Lolium multiflorum) very quick growing and can be overwintered from September to November sowing.

Non-hardy green manures

These are quick germinating and useful cover when you need to rest some ground between planting crops, during the growing season.

Mustard (Sinapis alba): This annual crop from the brassica family should not be followed by other brassicas, as it could encourage build up of the disease clubroot; sow in March to September and leave for two or three months before digging in.

Phacelia (Phacelia tanacetifolia): Later sowings of this annual crop may overwinter in mild areas. Sow from April to August and dig in after two or three months; its flowers attract pollinators.

Buckwheat (Fagopyrum esculentum): This half hardy annual will only grow in spring and summer best sown from April to August. Leave it for two or three months after sowing; grows well on nutrient-poor soils.

Bristle oats (Avena strigosa): This is a fast-growing fast half-hardy annual is a good non-brassica alternative to mustard. Sow from April to September and dig in after a couple of months.

Fenugreek (Trigonella foenum-graecum): This annual legume will only grow in the spring and summer; it is unlikely to fix nitrogen in the UK.

Bitter blue lupin (Lupinus angustifolius): This perennial flowering legume suits light, sandy, acid soils; sow in March to June and leave for two or three months before digging in.

A dense carpet of green makes a perfect environment for slugs and snails, so you may need to use control measures after green manuring.

Decaying green manures can suppress plant growth, so allow a month between incorporation and planting or sowing.

Club root can be a problem with green manures in the cabbage family such as mustard.