Green manure

A green manure is simply a crop you grow to feed the soil rather than yourself. You sow it onto bare ground, let it grow for a few weeks or right through winter, then chop it down and either dig it in or leave it on the surface as a mulch. As the leaves and roots break down they add valuable organic matter, the same stuff a good compost heap gives you.

Why bother with bare soil?

Empty beds are a missed opportunity. Left uncovered over a wet British winter, rain washes nutrients out of the soil and beats the surface into a hard cap. A living green manure protects that surface, keeps weeds from taking hold, and holds onto nutrients that would otherwise leach away. When you dig the crop back in, those nutrients return to the soil along with extra organic matter that improves structure and drainage.

Common UK green manures

A few reliable choices cover most situations:

• Field beans — a hardy winter legume. Like other legumes they fix nitrogen from the air into nodules on their roots, leaving the soil richer for hungry crops the following year.
• Phacelia — fast, pretty, and loved by bees if you let a little flower. Good autumn cover, though it is not fully hardy in a harsh winter.
• Clover — crimson or red clover is another nitrogen-fixing legume, useful for both short gaps and longer spells.
• Grazing rye — the toughest of the bunch. It germinates in cool soil, stands through the worst of winter, and its deep roots break up heavy ground.

When to sow in the UK

Most green manures are sown as autumn cover, going in from roughly August to mid-October once a bed is cleared. They grow slowly through winter, then you cut them down and dig them in two to three weeks before you want to sow or plant in spring — that pause lets the material start breaking down. Quick-growing types like phacelia can also fill short summer gaps between crops in just six to eight weeks.

How they feed and protect soil

Green manures work in three ways. Their leaves shield the soil from rain and frost; their roots hold the structure together and reach down for nutrients; and once incorporated, the whole plant rots down to release those nutrients and build long-term fertility. Legumes do the extra trick of adding nitrogen for free.

If you garden the no-dig way, simply cut the crop at the base and leave it on top, or hoe off the tops and let the worms pull everything down rather than turning it in with a spade.

Green manures are one of the cheapest ways to keep soil healthy between crops — a packet of seed costs little, and the payback is better soil year after year.