Once a bed comes empty — potatoes lifted, beans cleared, onions in store — bare soil left through winter washes its nutrients out in the rain and grows nothing but weeds. A green manure is a fast-growing cover crop sown thickly over that soil: it holds the ground together, keeps weeds down, and gets dug back in before spring planting to feed the bed that follows. It needs a few weeks of growth before the soil turns cold, so this is a job for early-to-mid autumn, not midwinter.

Sow a Green Manure Over a Bed You're Leaving Bare This Winter

What you need

50 g green manure seed (grazing rye, phacelia or field beans) roughly enough for 2.5 square metres at label rate — check the packet, rates vary by species
1 length garden netting or fleece optional, keeps pigeons off the seed until it germinates

Step by step

Clear the bed of old crop debris and roots, and fork over any compacted or trodden patches to break up clumps.
15 min
Rake the surface to a fine, level tilth so the seed will sit in good contact with the soil rather than on top of clods.
10 min
Broadcast the seed by hand as evenly as you can across the bed, at the rate given on the packet for the species you've chosen.
5 min
Rake lightly in two directions across the bed to cover the seed to about a centimetre, then firm it with the flat of the rake or by walking over it.
5 min
Water well if the soil is dry, and peg netting or fleece over the bed for the first week or two if pigeons strip seed in your area.
5 min
After seven to ten days check for even germination, and hand-sow any bare patches again while there's still time before the first hard frost.
5 min

Afterwards

Lift the netting once seedlings are through and past the stage birds bother with. Hand-weed any perennial weeds that come up alongside the cover crop so they don't get established over winter. Leave it growing until three to four weeks before you want to use the bed again, then fork or dig it into the top few centimetres so it has time to break down before you plant or sow.

Questions

Which green manure should I choose?

Grazing rye is the most frost-hardy and keeps growing through a mild winter, giving the longest cover. Field beans and phacelia are killed by a hard frost, which makes them easier to dig in but means they stop protecting the soil once cold weather sets in.

Is it too late to sow now?

Most green manures need three to four weeks of growth before soil temperatures drop below about 5°C, so early-to-mid autumn is the practical cut-off in most areas. After that, a mulch or compost layer protects bare soil better than seed that won't germinate in time.

Do I need to dig it in before it flowers?

Yes for annual types like phacelia and field beans — fork them in once flowering starts and before they set seed, or they'll self-sow and turn into a weed in next year's crop. Grazing rye is cut or forked in while still leafy, well before it heads.

Will the roots make the bed harder to dig in spring?

A dense root mass like grazing rye takes more digging than compost alone. If you know you'll want to sow or plant into that bed early, choose phacelia or field beans instead — both break down faster once forked in.