Soil
Green manures
The short answer
Green manures are fast-growing plants sown on bare soil, usually in late summer or early autumn, and dug in the following spring while still green. They smother weeds, stop winter rain washing nutrients away and improve the soil. Legumes such as clovers and winter tares also fix nitrogen, though only in summer. Chop them down, dig them in, and wait about a month before sowing.
A bed that sits empty over winter loses nutrients and grows weeds. A green manure fills the gap. The choice is which one, and it has to fit your rotation like any other crop.
Green manures RHS lists
| Green manure | Sow | Leave for | Notes |
|---|---|---|---|
| Winter tares (vetch) | March to August; or July to September to overwinter | Two to three months | Legume; hardy |
| Winter field bean | September to November | Two to three months, up to flowering | Legume; good for heavy soil |
| Crimson clover | March to August | Two to three months, up to flowering | Legume; good for light soil |
| Essex red clover | March to August | Two to three months, or one to two years | Legume; overwinters well |
| Trefoil | March to August | Two to three months, or one to two years | Legume |
| Bitter blue lupin | March to June | Two to three months | Legume; light, sandy, acid soils |
| Grazing rye | August to November | Until the following spring | Good for soil structure |
| Italian ryegrass | September to November, to overwinter | Very quick growing | |
| Mustard | March to September | Two to three months | Cabbage family: don’t follow with brassicas |
| Phacelia | April to August | Two to three months | |
| Buckwheat | April to August | Two to three months | Spring and summer only |
| Bristle oats | April to September | A couple of months | A non-brassica alternative to mustard |
| Fenugreek | Spring and summer | Legume, but unlikely to fix nitrogen in the UK |
Digging in
- Chop the foliage down and leave it to wilt.(RHS)
- Dig the plants into the top 25cm of soil. Perennials such as alfalfa can regrow from roots buried less deeply.(RHS)
- Wait a month before sowing or planting: decaying green manure can suppress growth.(RHS)
- No-dig: cover with cardboard in spring and the foliage composts where it is.(RHS)
- Mustard: dig in when, or before, the first flower buds show.(Garden Organic)
Cautions
- They are part of the rotation. Brassica and legume green manures can suffer the same diseases as vegetables in those families.(RHS)
- Clubroot can be a problem with cabbage-family green manures such as mustard.(RHS)
- Slugs and snails like the dense cover.(RHS)
- Grazing rye holds back seed germination for a while. Wait a month before sowing into it; planting out young plants is fine.(Garden Organic)
Where the sources differ
Where a green manure fits in a rotation, and what can follow it: vegetable plant families and what can follow this crop?
| Question | RHS | Garden Organic |
|---|---|---|
| How deep to dig in | Into the top 25cm. | No more than 15cm on heavy soil, 18cm on light. |
| Wait before sowing | A month. | Dig in three or four weeks before you need the ground. |
| Mustard and phacelia | Leave two or three months. | Can be dug in within six weeks of sowing. |
Let each empty bed tell you what could go in
In the Vercro Garden plan, a bed that has come free shows what could go there next, using your location’s frost date and what that bed has grown before.
Keep going
Where this comes from
The growing advice on this page comes from the published sources below, each linked beside the point it supports. Where two good sources disagree, we show both rather than choosing the convenient one. Reviewed 19 September 2026.
- Green manures, Royal Horticultural Society. Read 19 September 2026.
- Green manures, Garden Organic. Read 19 September 2026.
- Field beans (green manure), Garden Organic. Read 19 September 2026.