Every bed you clear in the next six weeks has a choice ahead of it: sit bare through 200 days of rain, or grow something. Bare winter soil loses nutrients to leaching, caps and slumps under heavy rain, and hands the weeds a free run. A green manure — a fast crop grown to be dug in rather than eaten — turns that dead time into a working season. Sown from mid-August to the end of September, it will build organic matter, lock up nitrogen that would otherwise wash away, break up compaction with its roots and, in the case of clovers and vetches, actually manufacture fertiliser out of thin air.
- Sowing window mid-August to late September
- Sowing rate roughly 5–20 g per square metre, broadcast
- Dig in before flowering, 2–4 weeks before sowing again
- Golden rule never leave winter soil bare
What a green manure actually does
Four things, and they are worth separating because they lead to different plant choices. It protects: a leaf canopy takes the impact out of winter rain, so the surface does not cap and the soil structure survives. It captures: deep, hungry roots hold on to nitrate that would otherwise leach straight past the root zone by February. It fixes: legumes — clovers, vetch, lupins, field beans — host bacteria in root nodules that convert atmospheric nitrogen into a form plants can use, genuinely adding fertility rather than just recycling it. And it builds: everything you dig in becomes organic matter and worm food.
None of that replaces compost, but it complements it neatly. Compost adds bulk from outside the bed; a green manure builds structure and fertility in place, using only sunlight and a packet of seed. If you already run a heap, our composting guide pairs well with this — the two together do far more than either alone.
Which green manure to sow in August and September
Choose by what your soil needs, not by what looks prettiest — although phacelia manages both.
- Phacelia tanacetifolia — the best all-rounder. Germinates fast, makes huge amounts of soft leaf, is easy to dig in, and if you let a patch flower it is the finest bee plant of the lot. Frost-tender in a hard winter, which conveniently kills it for you.
- Mustard (Sinapis alba) — the fastest of all, ready to dig in six to eight weeks from sowing. Superb at smothering weeds. It is a brassica, though, so keep it out of any rotation slot that will carry cabbages, and avoid it where clubroot is a problem.
- Crimson, red and Persian clover — nitrogen fixers with dense, fibrous root systems. Slower to establish than mustard, so sow these at the start of the window rather than the end. Red clover will overwinter and can be left for a full year.
- Common vetch (winter tares) — the classic hardy nitrogen fixer for a cold garden. Comes through frost, bulks up again in early spring, and produces the largest nitrogen contribution of any autumn sowing.
- Blue lupin — a deep taproot that pushes through compaction, plus nitrogen fixing. Best on light and acid soils; unhappy on wet clay.
- Lucerne (alfalfa) — a long-term choice for a bed you can spare for a year or more. The roots go down over a metre and mine minerals nothing else reaches.
- Field and broad beans — sow thickly in October and November when everything else is too late. Tough, nitrogen fixing, and easy to chop down in spring.
- Fodder turnip — quick winter cover with a fat root that breaks up the top 20 cm. Also a brassica, so mind the rotation.
The whole selection sits in our green manures collection and the overlapping cover crops collection, with pack sizes scaled for beds rather than fields.
How to sow a green manure, step by step
- Clear and rake, but do not enrich Pull the spent crop, remove perennial weed roots and rake to a rough tilth. No compost, no feed — the point is to make fertility, not spend it. See preparing your soil.
- Broadcast the seed by hand Scatter in two passes at right angles for even cover. Fine seed like phacelia and clover goes at around 5 g per square metre; large seed such as vetch and beans at 15–20 g.
- Rake in and firm Cover lightly — half a centimetre for small seed, 3 cm for beans — then tread or tamp. Good seed-to-soil contact matters more than depth; our germination guide explains why.
- Water once, then leave it One good soaking to trigger germination in dry August soil. After that autumn rain does the work, and a green manure never needs feeding.
- Cut and dig in before it flowers Chop the top growth down with shears, leave it to wilt for a day, then dig it into the top 15 cm — or leave it on the surface as a mulch if you garden no-dig.
Nitrogen fixers or biomass builders?
Legumes (clover, vetch, lupin, beans)
Add nitrogen from the air — real, new fertility.
- Best before hungry crops: brassicas, sweetcorn, squash
- Slower to establish, so sow early in the window
- Most of the nitrogen sits in the roots — leave them in
Non-legumes (phacelia, mustard, turnip)
Hold on to what is already there and build structure fast.
- Best after a heavily fed crop, to stop leaching
- Very fast — usable cover in three to four weeks
- Easier and quicker to dig in
In a bed you plan to fill with brassicas next May, sow vetch or crimson clover now. In a bed that has just finished a heavily manured crop of courgettes, sow phacelia to catch the leftovers. Fitting that into a wider plan is exactly what rotation thinking is for.
Common green manure mistakes
- Letting it set seed — the single biggest error. Mustard and phacelia go from useful to weed in a fortnight once flowering starts. Cut at the first buds unless you are deliberately feeding bees.
- Digging in too late — decomposing green material temporarily locks up nitrogen. Allow two to four weeks between digging in and sowing your next crop, or the seedlings will go yellow.
- Digging in tough, woody stems — an overgrown crop takes months to break down. If it has got away from you, cut it, compost the top growth and dig in only the roots.
- Ignoring the rotation — mustard and fodder turnip are brassicas; clover and vetch are legumes. Slot them in as if they were a crop, because to soil-borne diseases they are.
- Sowing too late in a cold garden — after the end of September only field beans, vetch and grazing rye will establish properly. Elsewhere, a thick mulch is the better option.
- Forgetting the slugs — lush green cover is a slug hotel. Clear it well before sowing anything small and vulnerable; our pest guide has the rest.
Frequently asked questions
When should I dig a green manure in?
Just before it flowers, which for mustard and phacelia is six to ten weeks after sowing, and for overwintering vetch, clover and beans is late March or April. Cut, wilt for a day, then incorporate into the top 15 cm and wait two to four weeks before sowing your next crop.
Can I use green manures in a no-dig garden?
Yes. Cut the top growth at ground level and leave it on the surface as a mulch, or move it to the compost heap and cover the bed. The roots decay in place and feed the soil life without any disturbance — the approach fits neatly with mulching and weed control.
How much nitrogen does a legume green manure add?
A good stand of overwintered vetch or clover contributes roughly the equivalent of a light dressing of general fertiliser — useful, free, and released slowly as the residues break down. Leave the roots in the ground: that is where most of the nodules are.
Will a green manure stop weeds?
Largely, yes. A dense crop shades out annual weed seedlings and mustard in particular is fiercely competitive. Perennial weeds with established roots are unaffected, so remove those before you sow.
A packet of phacelia costs less than a bag of compost and does more for the bed it covers. Sow the empty patches as they open up over the next few weeks, keep the flowering beds going with biennials like foxgloves, and by spring you will be digging into dark, crumbly, worm-riddled soil instead of a compacted crust. Browse the green manures and cover crops, the organic vegetable seeds to follow them, and everything tagged for September sowing — then take your pick from the rest of our growing guides and the full sowing calendar.
