Green manures are plants that are grown to protect and improve the soil. Leaving bare soil over the winter is not a good idea as the pounding action of the rain can damage the soil structure, leading to a soil surface that easily sets like concrete in dry weather. The rain can also wash a lot of those valuable nutrients out of the soil into the ground water, where the plant can’t get them.
Most green manures should be sown before the beginning of September when the soil is still warm enough for the plants to establish. Vetch (or winter tares) is a vigorously growing plant that will rapidly cover your plot to protect it. It is also a legume, so it fixes nitrogen, adding some nutrients back to the soil. You could also try phacelia which is also quite vigorous. It will protect your soil and produce abundant purple flowers in the spring that are highly attractive to bees.
If you don’t get around to sowing until late September, because you are still harvesting vegetables from your plot, then you could try field beans that grow at cooler temperatures. However as the seeds are quite large, this can work out quite pricey to cover larger areas.
Winter green manures should be cut down in early Spring (March – April) when you can dig them in. If you practice no-dig, you need to chop them up a bit more finely and smother with a layer of compost. The plant material will break down, releasing valuable nutrients and adding organic matter to improve your soil.
Garden Organic is testing a green manure mix especially designed for gardeners this winter. If you would like to try it out then look here for further details.