Autumn green manures: what to sow in October-November

Late October, and most of the vegetable beds are emptied and about to spend the winter bare. Bare soil under winter rain loses nitrogen to leaching, is pounded by the rain, and comes out of the season poorer and more compacted than it went in. There are two answers: cover the soil with mulch, or sow a green manure in autumn that covers and works it all winter. The second option has a deadline — and it is close.

A note on location: the dates below are for the cooler, northern half of the range. In mild southern areas add two to three weeks; in upland or continental cold areas, subtract as much. Either way, a cover must have been up for five to six weeks when hard frost arrives — that is what sets your true deadline.

Which green manures to sow in October-November, and at what dose

Sow broadcast on a cleared, raked bed: scatter the seed, bury it with a pass of the rake — then firm or roll the surface. That last move, which beginners almost always skip, ensures seed-to-soil contact and makes the difference between even emergence and patchy germination. No need to water: autumn moisture is enough. The density for a closed cover without overcrowding:

These rates in grams per square metre assume broadcast sowing; for 100 m² that means 0.8 to 1.5 kg of seed depending on species — a range you can read straight off seed packets. Vetch, being a bigger seed, is measured per metre of row. Below the range the cover stays patchy and weeds take it; above it, mustard and the brassicas germinate so well that overcrowding becomes the problem.

Rye: the only cereal still in its optimal window

Rye is the only cereal whose sowing is still optimal this week. Sown between 20 and 30 October, it emerges after most autumn weeds (foxtail, rye-grass, bromes), which no longer compete with it; sown before mid-October, conversely, the young plants suffer more from frost and aphids. Its strong, deep roots loosen the soil and it covers the ground well all winter.

The catch: rye regrows — so the mowing rule is stricter than for the others: cut it young and green, before heading — in late winter for an October rye, well before its May flowering. Left to grow ears, it turns woody: its carbon-rich straw then causes three to six months of nitrogen hunger in the soil if you dig it in. Second caution, specific to rye: incorporated, it inhibits the germination of small seeds for several weeks — after rye, wait three weeks before sowing, or transplant established seedlings, and never follow it directly with a sowing of small seeds (carrot, lettuce, lamb's lettuce, radish). Both problems disappear if the cut rye stays on the surface as green mulch rather than being dug in — that is the method we recommend.

Vetch and field bean: the nitrogen fixers

Hairy vetch (Vicia villosa) is among the most useful covers: it grows through the winter while the land is free, produces a large amount of green matter, develops an extensive root system, and fixes nitrogen. Dig it in or mow it in early spring. Do not substitute crown vetch (Securigera varia), which self-seeds and becomes a weed itself.

The field bean is a very good green manure — abundant organic matter, fixed nitrogen — provided you respect its climate map. In Mediterranean and Atlantic zones, October-November sowing is unproblematic; further north, you need a winter cultivar (the Aquadulce type), well-drained soil and a sheltered spot, sown early enough for plants to be established before serious cold. In cold continental climates or heavy wet soil, the February-March sowing remains safer. One more nuance: sown in autumn, the field bean's cycle lengthens considerably — treat the bean harvest as a possible bonus, not a goal. In mild climates it partners well with cereals as a climbing cover.

The classic: rye + vetch as a mix

Rather than choosing, you can sow both together — the most proven autumn combination: rye supports and structures, vetch climbs and fixes nitrogen. Sow rye at its normal rate (8-10 g/m²) and vetch over it (3-5 g/m²) in a single pass. The result is a denser cover, better balanced in carbon and nitrogen, and easier to kill in spring than rye alone. The "zero spring work" variant for mild zones: oats + vetch + field peas, entirely winter-killed.

Mustard, rape, turnip rape — and the phacelia trap

White mustard, rape and turnip rape still germinate under cool temperatures and can be sown until the end of October, classically after harvesting the bulb alliums (garlic, onions, shallots). They close the ground in late season and limit the winter leaching of nitrogen. Mustard germinates from 8-10 °C soil temperature; turnip rape is the only genuinely hardy one of the trio.

Two cautions: these are brassicas — do not sow them where the garden carries clubroot, they maintain the disease; and phacelia, which appears on every green-manure list, is not sown in autumn: it does not withstand cold and is sown May to August.

And in spring, what becomes of it?

Three outcomes, by species:

• The frost does the work — if winter is cold enough: mustard, rape and winter oats are killed by the hard frosts of January-February; in spring a simple raking is enough. But if winter stays mild, the cover survives: mow or pull it in March, and never let it run to seed — a mustard that seeds resows itself and becomes a weed.
• Mow at the right stage: the common rule is to cut the cover always before seed set, while the vegetation is still green and tender — that is when it breaks down fastest and the fixed nitrogen stays in the soil (with legumes, once seeds form, the nitrogen goes into the seed). In practice: cereals at knee height, legumes at the start of flowering.
• Do not bury deep: the cut material stays on the surface as green mulch, or is worked into the top 10 cm only, once wilted. Burial causes the nitrogen hunger described above and, for rye, blocks the germination of the next sowings. After any dug-in cover, wait three weeks before planting.

What the cover can attract — and what to sow after

A dense, damp cover is also a refuge: slugs and voles find what they need to overwinter, especially near a hedge or grassy strip. Plan a check in spring (traps, close mowing around the bed to be resown) before setting out your young plants.

For succession, the rule is simple: do not replant the same family in the same place. After a legume cover (vetch, field bean), the nitrogen-charged soil welcomes the hungry crops: lettuce, spinach, lamb's lettuce, cabbage, tomato, potato — brassicas actually do very well there, since they are unrelated to vetch. After a brassica green manure (mustard, rape, turnip rape), however: no cabbage, turnip, radish or other crucifer — clubroot travels with the family. After rye: no small seeds for three weeks, but transplanted seedlings are fine.

Too late to sow? Mulch remains

Past the end of November, it is too late to sow a green manure in autumn — switch then to dead cover: 20 cm of dead leaves mixed with shreddings protects the soil to -6/-7 °C of night frost, as detailed in the winter protection guide. The principle is the same: bare soil should spend the winter neither without plants nor without cover.

Further reading: the broad bean growing guide, the winter indoor sowing calendar — and to keep a head start in spring, the heating mat and the LED grow light.