Green manures are plants that are sown specifically to improve fertility and are a great benefit to the soil. They are also known as 'cover crops'.
By growing a green manure, you are simply mimicking nature’s own recycling programme of plants living, dying, rotting down and becoming part of the soil once more. A valuable natural fertilizer and soil conditioner, green manure has an important role in organic gardening. These manures are not harvested or allowed to flower instead they are chopped down and dug into the soil. Many plants can be grown as a green manure but some do better than others so it is best to buy a seed especially for this purpose.
A green manure can be especially good if you have a light soil, they help prevent erosion and can stop the nutrients being washed out of the soil by the rain. Some varieties are a fast growing crop and are ideal if you have bare patches of ground or empty beds, they are also useful for suppressing weeds. If you have very heavy soil then try varieties like buckwheat or Italian rye grass, they both have good root systems for breaking up heavy ground.
TYPES OF GREEN MANURE
OVERWINTERING
Varieties such as winter beans rye grass, winter tares and grazing rye can be sown in early Autumn when patches of bare ground are common, these will be ready for digging in to the soil in the following spring.
SHORT TERM
Fenugreek, mustard and buckwheat are fast growing and very leafy plants which can be slotted in six to eight week gaps when you clear the ground between crops.
LONG TERM
Use of green manures long term if you have any soil that is overused and needs a rest or if you don't want to leave any patches of bare soil. Varieties that do well long term are trefoil, red clover and alfalfa. These should be trimmed back occasionally to stop them becoming woody.
FACT
Flowering green manures such as clovers and phacelia benefit helpful insects so keep a few back when chopping them down. Crimson clover is especially good for attracting bees and hover flies.
Broadcast your seeds and gently rake them into the soil, make sure the seeds are in firm contact with the soil. Water well and seeds should germinate quickly covering your bare patches of ground. Dig them in when they are leafy and before they start to flower, if you leave them until after flowering they become woody and can use up the soils nitrogen when you dig them in.
FACT
Rotting green manures can check plant growth, allow at least 2 weeks between the digging in and planting and sowing.
Autumn grown manures are very good at mopping up any nutrients that have been left behind by your crops, therefore preventing them being washed away, clay soils are always best dug in the Autumn. Green manures that have been grown in the summer months i.e. fenugreek and buckwheat can leave your soil very dry, they do however have quite a thick foliage that is ideal for smothering those weeds. Some varsities of green manure come from the legume family, they store nitrogen with a little help from bacteria in their root nodules. These crops include lupins, clovers and peas and they do best if sown in the summer. Some legumes have the ability to take nitrogen from the air. Nutrients which otherwise would be washed away are taken into the plants and are then released back into the soil when the plants are cut down.
TOP TIP
Another benefit of green manure is it provides winter shelter and protection for small insects, you may need to go on slug patrol though after using green manure.