Green manures are crops that are grown specifically to benefit the soil. If you have an area of soil that will be bare for at least a few weeks, it is well worth considering sowing a green manure as they have many benefits. As well as suppressing weeds and preventing rapid evaporation turning the soil surface to dust, green manures can improve the soil structure, prevent soil erosion and nutrient leaching and actually add fertility to the soil.
Heavy winter rains, in particular, can wreak havoc with unprotected soil. They can leach nutrients out of the soil (where they’re needed) into rivers and streams (where they cause harm), cause soil compaction and even – particularly on slopes – soil erosion.
When the soil is needed again, or when the green manure has done its job, the plants are dug in and allowed to rot down – adding their nutrients back into the soil.
There are many green manures to choose from, each with their own benefits, but in the autumn your choices are limited to those that will successfully overwinter.
Hungarian grazing rye (Secale cereale) is very good for improving the soil structure on heavy clay soils – it has very strong roots. It will happily grow on other soils too, and if sown between late summer and early winter will cover the soil all through the winter. Hungarian grazing rye contains chemicals that inhibit seed growth – it makes a great weed suppressor – and it should not be used on ground that is intended for a seed bed immediately afterwards. The effect wears off in a couple of weeks, and in the meantime transplants are not affected.
Field beans (Vicia faba) prefer heavy soils and can be sown in autumn and early winter. The large seeds make it easy to grow field beans in rows, and because field beans are in the Legume family they are able to fix nitrogen in the soil and make it available as a nutrient to future crops.
Winter tares (Vicia sativa) will also overwinter if sown in late summer or early autumn. They dislike dry and acid soils and, like grazing rye, inhibit seed germination. On the plus side, they will fix nitrogen in the soil and can be used to undersow (grow underneath) hungry fruit bushes like blackcurrants, to suppress weeds and add feed the fruit bushes.
A different selection of plants are grown as green manures in spring.