Green manures are quick-growing plants that you sow, leave to grow and turn into the soil before they flower. They can be sown at many different times of year but are best grown in autumn as parts of your plot become bare, and while the soil is still warm.

They’re called green manures because of the way they mulch your soil – improving its structure and moisture retention. They also help suppress weeds, protect the soil from winter rains and frost, and provide habitats for insects.

Like a handful of other green manures, field beans have an additional ‘secret ingredient’ that means they can also add to your soil fertility in the right conditions.

What are field beans?

The field bean (Vicia faba) is a hardy annual that’s often grown by farmers for its soil benefits. The plants (like all beans) store nitrogen in nodules on their roots, which - when turned into the soil before flowering and fruiting - will be ready for the next crop to access.

This means when you plant nitrogen hungry crops such as leafy greens, cabbages and lettuce in spring, they take up the nitrogen left by the beans. Nitrogen is vital for plants because it's a key component of many biological processes, including photosynthesis, growth and water uptake.

Another benefit of field beans is they’re happy in clay, so they’re a great choice for the heavy soils of an allotment plot. Their long taproots, and extensive branching, will help break up the soil and make it more free draining.

How to grow field beans

You can sow field beans from September to November, 2cm deep and 10cm apart, in rows 15cm apart. The plants should establish quickly (within approximately 20 weeks).

If you're following the no-dig method, you can cut them down and hoe off the leaves once they reach 30-45cm tall (and before the beans develop) but you may need to add tougher stems to the compost heap.

You can also dig in the whole plants four weeks before the soil is needed for your next crop. If you are following a crop rotation system, field beans should be grown in the same area beans have been growing, so they can nurture a different crop next year - usually brassicas.

You might want to intercrop them with a green manure such as grazing rye to ensure good coverage and to block out weeds.

For more information about green manures head to gardenorganic.org.uk/green-manures.