Green manures for healthy soil

Cover crops (also known as green manures) are a cheap, easy and eco-friendly way to improve your soil, writes RHS Chief Horticulturist Guy Barter

Bare ground deteriorates, whereas ground that is covered with vegetation becomes more productive. Even in the best-planned veg plots, there's likely to be some bare soil present between October and March.

This means that soil will be deteriorating all winter, so some gardeners cover the soil with compost or manure. However, this can waste plant nutrients which get washed away by winter rain and potentially cause pollution. I prefer to grow cover crops: that is plants sown merely to cover the soil, protecting and enhancing it.

Plants to the rescue

Cover crops (also called green manures) suppress weeds, support wildlife and help hold nutrients that could otherwise be washed away by winter rains. Some, such as vetches (which are part of the legume family), can actually improve your soil's fertility by fixing nitrogen from the air into their roots. 

A family affair

Green manure - mustard 'Caliente'The earlier cover crops are sown the better – ideally sow before mid September. However, it pays to choose them wisely, and pay attention to plant families. Mustard and fodder radish are brassicas and not the best choice to sow where cabbages and other brassica crops have been grown as they can support the same pests and diseases. 

Green manure vetchLegume cover crops; vetches, clover, field peas and beans for example, promote the root diseases that afflict broad beans, peas and French and runner beans. They are best avoided where these legume vegetables have been grown (or where you plan to grow them next year).

Going with the grain

Cereals and grasses work really well for me. In particular, black oats sown before mid-September, after broad beans and peas have been harvested and cleared. They grow fast and smother autumn weeds. Oats are killed by December frosts, leaving the ground loosened enough by their root action to need no digging. The ground is then ready for early sowings of carrots, onions, peas or parsnips.

Rye is a hardier option, but I prefer ryegrasses. Italian ryegrass, especially its extraordinarily vigorous annual form 'Westerwold', works best for me. It can be sown until early November (in the south). 

From March, cover with black plastic to kill it, leaving ground ready for planting in May. So you can then plant courgettes, squashes and sweetcorn for example, with no need to dig. In fact, the dead cover acts as a mulch.

For earlier cropping the cover crop is best dug in before dry April weather. Closely mow the sward using a lawnmower and you can 'half dig', to 15cm instead of 25cm: this keeps the very important biologically active upper root zone nearer the surface than full digging over.

See also

RHS grow your own

Watch a video about growing green manures

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Advice from the RHS

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The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.