A green manure is a plant which is planted to provide nutrition to the soil, to act as a weed suppressant and to prevent bare soil.
By growing extra crops on soil rather than leaving them bare, you protect the soil from the excesses of wind, rain and sun.
You can get many different types of green manure. Mustard, clover, phacelia, and many others are available.
Some can add nitrogen – leguminous types of green manure plants eg: Field beans, winter tares.
Caliente mustard is thought to fumigate the soil.
All plants that put out roots into the soil help aerate the soil. The action of roots can break up tough soils and make the soil easier to work.
For overwintering you should plant in September or October. If the weather is too grim then seeds don’t get started very well.
A thick carpet of green manure will keep weeds down – they can’t compete very well against a flush of green manure. The layer of plants can either be dug in or chopped down and left on the surface as a mulch. You can leave them to fall at the first frost too. If some make it through the winter they may flower and set seed if you don’t cut them back. If you have a corner of the garden you don’t have a use for then just let them. The flowers are often good for the bees – mustard flowers are yellow, phacelia is grown for it’s bee-friendly flowers too.