When the last of the autumn harvest comes in, you're left with bare soil. It looks tidy, but it's the worst possible state for a bed going into winter. Rain leaches out the nitrogen, the surface bakes into a crust, and come spring you're starting with soil that's more compacted and poorer than it was last year.
Green manure solves all of that in one operation: you sow a fast-growing crop, it puts on growth before winter, and then it either frosts off by itself or gets worked in come spring.
What green manure actually does
- Holds nutrients in place — the roots capture nitrogen that rain would otherwise flush down beyond reach.
- Protects the soil surface — the canopy absorbs the impact of raindrops and prevents capping and erosion.
- Loosens soil at depth — phacelia and rye roots go deeper than a spade, without you lifting a finger.
- Adds organic matter — once incorporated, the biomass breaks down into humus that improves structure and water retention.
- Suppresses weeds — a dense stand simply denies them light.
Which plants to use
White mustard — the fastest option
It germinates within days, grows on in cool weather, and reliably frosts off at the first hard frosts, so in spring you need only rake the residue in. One warning: mustard is a brassica, just like cabbage, broccoli and radish — never sow it on a bed where you plan brassicas next year, as they share clubroot.
Phacelia — the universal safe bet
It belongs to no common vegetable family, so you can sow it anywhere without worrying about crop rotation. It builds a dense root system, loosens beautifully, and if it gets far enough to flower it's superb bee forage.
Rye and oats — for late sowings
When it's too late for mustard or phacelia, cereals will still manage. Rye germinates at low temperatures and overwinters, holding the bed all winter — but it won't frost off, so you'll have to cut and incorporate it yourself in spring. Oats usually die back over winter.
Vetch and clover — when you want added nitrogen
Legumes fix atmospheric nitrogen and bank it in the soil. They're slower, so sow them earlier — ideally by the end of August. Ideal ahead of hungry crops like cabbage, squash or courgettes.
Latest sowing dates
| Crop | Sow by | Frosts off? | When to incorporate |
|---|---|---|---|
| White mustard | mid-October | yes | spring, shallow only |
| Phacelia | early October | yes | spring, shallow only |
| Oats | mid-October | usually | spring |
| Rye | end of October | no, overwinters | early spring, before it runs to stem |
| Vetch | end of August | partly | spring |
How to sow it
- Clear the previous crop's residue — diseased foliage off the plot entirely, healthy material can be chopped and left in place.
- Just scratch the surface — no need to dig deep; breaking the crust to about 5 cm is enough.
- Sow thickly, broadcast — scatter the seed by hand across the whole area, don't bother with rows. A dense stand is the entire point.
- Rake in and firm down — work the seed shallowly under the surface with a rake, then roll or tread the area.
- Water if it's dry — without moisture the seed won't germinate, and by mid-September you have no time to spare.
The most common mistakes
- Sowing too thinly — a sparse stand lets weeds through and doesn't cover the soil.
- Letting it set seed — mustard and phacelia that shed seed become a weed for years. Incorporate them by the end of flowering at the latest.
- Mustard ahead of brassicas — far and away the most common error, see above.
- Deep ploughing in autumn — the biomass should break down at or just under the surface, not 30 cm down.
Green manure is one of the few garden jobs where the ratio of effort to result is unambiguously in your favour. Half an hour of sowing in autumn saves you hours of digging and feeding in spring.
Frequently asked questions
How late can I sow green manure?
White mustard and oats can go in until roughly mid-October, phacelia until early October, and rye right through to the end of October. The later you sow, the less growth it makes before winter — but even a thin stand is better for the soil than a bare bed.
Do I have to dig green manure in?
Not always. Mustard, phacelia and oats die back in hard frosts, and in spring you can simply rake the residue in shallowly or leave it lying as a mulch. Rye doesn't frost off, so that one you do have to cut and incorporate yourself in spring.
Can I use mustard as green manure anywhere?
No. White mustard is a brassica, the same family as cabbage, broccoli, cauliflower, radish and rocket. Don't sow it on a bed where you're planning those crops next season — they share clubroot. Use phacelia instead; it belongs to no common vegetable family.