Complete guide to green manures

Green manures are a plant-based alternative to animal manures. They are sown as seed on bare ground and grow to cover bare soil, usually on the vegetable patch after a crop has been harvested and cleared, but also to fill the space between crops or even used to underplant taller-growing crops. Green manures are typically fast growing and have deep tap roots, which bring nutrients to the surface, making them more accessible to future crops. What's more they smother weeds and their roots prevent nutrients "washing away" in heavy rains, which can lead to soil erosion. Green manures are usually dug into the ground before flowering, when they rot down and their nutrients are returned to the soil ahead of the growing season. However some can be left in place for a couple of years.

How do green manures work?

Plants need nutrients to grow well. The three main plant nutrients found in the soil are nitrogen, phosphorus and potassium, while magnesium, calcium and sulphur are also important. These nutrients work together to help plants grow strongly, produce flowers and fruit. They are particularly important on the vegetable patch as veg crops need a lot of nutrients to produce the food we eat. Healthy soil is full of invertebrates, bacteria and fungi (fauna and microbes) that work together to break down organic matter such as dead plant material and manure to make these nutrients available to plant roots.

As gardeners, we can boost the level of nutrients in the soil by mimicking natural processes. This includes adding manure or green manure, which feeds the soil fauna and microbes which help keep soil healthy. Synthetic fertilisers are available and offer quick results, however they reduce the fertility of the soil in the long term as they disrupt, and eventually kill off, the fauna and microbes necessary for keeping soil healthy.

Green manures are sown on bare ground. There are different mixes for sowing at different times of year. For example, winter grazing rye and winter tares are hardy and will carry on growing throughout winter, so you can dig them back into the ground before planting crops in spring. Others, such as white clover and mustard, should be sown in spring and summer.

As well as feeding the soil and making your plants grow more healthily, preventing soil erosion and suppressing weeds, green manures help to protect the soil surface from compaction by rain and foot traffic. They also provide food and shelter for insects such as ground beetles. If you let green manures flower they can also provide a valuable source of pollen and nectar for pollinators.

How to sow green manure

In this clip from Gardeners' World, Monty Don demonstrates how to sow grazing rye grass as a green manure, and explains how to use it to incorporate back into the soil:

Learn how to sow green manure in just three steps, below.

You Will Need

• Green manure seed mix
• Garden rake

Step 1

Measure the area and work out the sowing rate according to the packet instructions. If sowing several varieties together mix well for the best results.

Step 2

Scatter the seeds over prepared soil.

Step 3

Rake the seed lightly into the surface and protect from birds.

When to dig in green manure

Ideally, you should dig in your green manure three to four weeks before planting new crops, and at least a month before sowing seed. This gives the manure time to rot down and return its nutrients to the soil before you use it again. However, if the crop looks like it's approaching maturity, you should dig it in sooner. Young green growth decomposes and feeds the soil in a matter of weeks but older, woody material takes longer. What's more, if flowers and seeds develop you could end up with a "weed" problem.

It's easy to tell if green manures are approaching maturity. Some manures, such as clovers and vetches, will start producing flowers. Others, such as grazing rye, form flower buds in the centre of the plant, which you can feel with your fingers. Check your green manure regularly and ensure it doesn't outstay its welcome.

To dig in a green manure, cut the foliage down and let it wilt for a couple of days, then use a sharp spade to unearth the roots and turn them back into the soil, digging everything in in the process. If the manures have been growing only for a few weeks you can hoe them off and then leave the foliage to rot back into the soil. If you'd rather not dig the soil, you can strim the plants down and then cover the whole area with plastic sheeting, for around four weeks.

Problems with green manure

When left to rot back into the soil, green manures can suppress plant growth, so make sure you leave at least least two weeks, ideally a month, between digging in green manure and planting or sowing.

If growing crops in the brassica family you could unwittingly create the perfect conditions for the fungal disease club root to develop. Take care to use green manures as part of your crop rotation, if using brassicas such as mustard.

What are the best green manure crops?

Alfalfa (Medicago sativa): leguminous perennial that can be left for up to two years. Sow from April to July. Best for alkaline soils.

Alsike clover (Trifolium hybridum): leguminous perennial that can be left for up to two years. Sow from April to August. Best for wet, acidic soils.

Bitter blue lupin (Lupinus angustifolius): leguminous perennial that should be left for up to three months. Sow from March to June. Best for light, sandy and acidic soils.

Buckwheat (Fagopyrum esculentum): half hardy annual that should be left for up to three months. Sow from April to August, and dig in before autumn. Best for nutrient-poor soils.

• Buy buckwheat from Thompson & Morgan

Crimson clover (Trifolium incarnatum): leguminous perennial that should be left for up to three months or until it flowers. Sow from March to August. Best for light soils.

• Buy crimson clover from Thompson & Morgan

Red clover (Trifolium pratense): leguminous perennial that overwinters well and can be left in for up two years. Sow from March to August. Best for loamy soils.

Grazing rye (Secale cereale): annual that works well as an overwintering crop. Sow from August to November and dig in the following spring. Best for improving soil structure.

Mustard (Sinapis alba): annual that should be left for two or three months before digging in. Sow from March to September. Don't follow with other brassicas as this could lead to the build up of the disease clubroot.

Phacelia (Phacelia tanacetifolia): half hardy annual that overwinters in mild areas. Otherwise sow from April to August and dig in after two to three months.

• Buy phacelia from Thompson & Morgan

Trefoil (Medicago lupulina): annual or biennial that can be left for up to two years and overwinters well. Sow from March to August. Best for light, dry alkaline soil.

Winter field bean (Vicia faba): leguminous annual legume that can be left for up to three months. Sow from September to November. Best for heavy soils.

Winter tares (Vicia sativa): leguminous hardy annual that overwinters well. Sow from March to August and leave for two or three months, or July to September for overwintering.