Green Manures Explained

A green manure is simply a crop you grow to feed the soil rather than yourself. You sow it on a bare bed, let it grow, then dig it in or cut it down before it sets seed. It's one of the cheapest, most low-effort ways to keep an organic plot in good heart.

What green manures do

Bare winter soil is a problem. Rain batters it, nutrients wash away, and weeds move in. A green manure covers that gap with living roots and leaves instead.

A good cover crop earns its keep in three ways:

• Protects the surface from heavy rain and capping, and stops weeds getting a foothold.
• Feeds the soil with fresh organic matter when you dig it in or let it rot down — boosting your soil's structure and fertility.
• Improves the soil physically, as deep roots break up compaction and bring up nutrients from below.

It's a natural fit for an organic plot, where you're building healthy soil rather than reaching for the fertiliser bag.

Nitrogen-fixers vs the rest

Green manures split into two camps, and it's worth knowing which you're sowing.

Nitrogen-fixers are legumes — plants in the pea and bean family. With help from bacteria on their roots, they pull nitrogen out of the air and store it in the soil. Dig them in and that nitrogen feeds your next crop for free. The main UK choices are:

• Field beans (a hardy cousin of the broad bean) — the classic over-winter nitrogen-fixer.
• Winter tares (vetch) — quick, leafy, good on most soils.
• Clovers — crimson clover for a season's gap, red clover if you can leave a bed a year.

The rest don't fix nitrogen, but still add bulk organic matter, smother weeds and hold the soil together:

• Grazing rye — the toughest winter cover, with roots that break up heavy ground.
• Phacelia — fast, pretty, and its flowers are loved by bees if you let a little run on.
• Mustard — very fast in summer gaps, though avoid it if clubroot is a worry, as it's a brassica.

Which to sow when

The biggest win is covering beds over winter. Sow from late summer into early autumn — roughly August to October — once a crop comes out. Field beans, grazing rye and winter tares are all hardy enough to sit through a UK winter and grow away again in spring.

For gaps during the season, use the fast growers. Phacelia and mustard will fill a few spare summer weeks between crops and can be chopped down before they flower. Crimson clover suits a slightly longer gap.

Slot these into your beds the same way you'd plan any catch crop — they fit neatly around the rest of your month-by-month jobs.

Digging in vs no-dig chop-and-drop

You've grown the green manure — now you return it to the soil. There are two routes.

Digging it in is the traditional method. A few weeks before you need the bed, chop the top growth down with shears, then turn it into the top 15cm or so of soil with a fork or spade. Leave two to four weeks before sowing or planting, as fresh material can briefly lock up nitrogen as it rots.

Chop-and-drop suits a no-dig plot. Instead of turning anything in, you cut the growth down at the surface and leave it lying as a mulch, or cover it with cardboard and compost. The roots rot in place and the worms pull the top layer down for you. It's less work and keeps the soil structure intact.

A couple of points either way:

• Don't let it set seed. Cut green manures down while still soft and green, before they flower and seed, or you'll be weeding them out next year.
• Tough crops need a head start. Grazing rye and overwintered field beans are best cut down 3–4 weeks before you want to plant, as they take longer to break down.

Where green manures fit

Green manures are a core part of organic, no-spray growing: they build fertility, crowd out weeds and keep the soil alive without anything from a bottle. Sow them in the gaps, let them do the work, and you'll head into each season with better ground than you started with.