Journal/Vegetable Garden · 7 min read
Green Manures: How and When to Sow Cover Crops
By Arthur Meade · Vegetable Garden & Seasons
When and how to sow green manures: the best cover crops for late summer and autumn, how to dig them in, and how they feed your soil for spring.
Quick answer
Green manures are fast-growing cover crops sown on empty beds to protect and feed the soil. Late August to October is the key sowing window: hardy types like winter tares, grazing rye, crimson clover and field beans grow through winter, then get cut down and dug in (or composted) 3–4 weeks before spring planting.
By late summer, the vegetable garden starts emptying out — onions lifted, potatoes dug, the first spent beds looking bare. Leaving that soil naked all winter means rain compacts it, nutrients leach away, and weeds move in. Green manures are the old, cheap fix that's come back into fashion: sow a fast-growing cover crop now, let it hold and feed the soil through winter, then return it to the ground before spring planting. Your soil comes out the other side better than it went in.
What green manures actually do
- Cover bare soil, shielding it from compaction and erosion by winter rain.
- Lock up leftover nutrients in living tissue instead of letting them wash away — then release them when dug in.
- Legume types (tares, clovers, field beans) fix atmospheric nitrogen in their root nodules.
- Smother autumn-germinating weeds with dense foliage.
- Add organic matter that feeds soil life and improves structure.
- Deep-rooted types like grazing rye break up compacted layers for you.
What to sow now
The late-August-to-October window is the most useful of the year, because it's when hardy, overwintering green manures go in. Sow onto cleared, raked soil, broadcast or in close drills, and water if the week stays dry — most germinate within one to two weeks while the soil is still warm.
Overwintering green manures at a glance
| Green manure | Sow | Winter-hardy? | Fixes nitrogen? | Best for |
|---|---|---|---|---|
| Winter tares (vetch) | July–September | Yes | Yes | Nitrogen boost on most soils |
| Grazing rye | August–October | Yes | No | Soil structure, weed suppression |
| Crimson clover | July–September | Fairly hardy | Yes | Light, sandy soils |
| Field beans | September–November | Yes | Yes | Heavy clay soils |
| Phacelia | March–September | Only in mild winters | No | Quick cover, brilliant for pollinators if left to flower |
| Mustard | March–September | No — killed by frost | No | Fast 6-week cover; the frosted foliage mulches the bed |
Tip: match the green manure to what you'll grow next. Mustard is a brassica, so skip it on beds destined for cabbages, kale or broccoli — it can host the same diseases, including clubroot. The same rotation logic applies to legume green manures before peas and beans. Phacelia and grazing rye are the safe 'fit anywhere' choices.
How to sow
- Clear the bed of crop debris and large weeds; rake to a rough tilth.
- Broadcast seed evenly (rates are on the packet — roughly a small handful per square metre for most species).
- Rake again lightly to cover, firm, and water in dry spells.
- Protect emerging seedlings from birds with netting if pigeons are a problem locally.
Digging in — the part people get wrong
A green manure gives back most when it's returned to the soil young — soft, leafy, and before flowering or setting seed. In spring, cut the growth down, chop it roughly with a spade, and dig it into the top 20–25 cm of soil. Then wait: decomposing green material temporarily ties up nitrogen and can inhibit seed germination, so allow three to four weeks before sowing or planting the bed. Tough, over-mature rye is the classic offender — turn it in early, while it's still soft.
No-dig gardeners don't have to miss out. Instead of digging, cut the green manure at soil level, leave the roots in place to rot, and either compost the tops or lay them on the surface under a mulch. The soil life does the incorporation for you.
One caution
Dense, lush green manure foliage is a five-star slug hotel. If slugs plague your garden, cut the cover down a little earlier in spring, let the surface dry out, and be ready to protect the first tender transplants that follow.
Not sure whether that self-sown seedling coming up through your green manure is phacelia or a weed? Snap it with PlantMe — the app identifies 35,000+ plant classes from a single photo, seedlings included.
Frequently asked questions
When is it too late to sow a green manure?
Most hardy types establish well until early October while soil stays warm. After that, field beans are your best bet — they'll germinate in cooler soil sown into November.
Do legume green manures really add nitrogen?
Yes — bacteria in their root nodules fix atmospheric nitrogen. The catch is timing: most of it is released for the next crop only when the plants are dug in young, before flowering.
Can I just leave a green manure to grow instead of digging it in?
You can leave phacelia to flower for pollinators, but most types get woody, set seed and become a weed problem. Cut down overwintered green manures by mid-spring at the latest.
Do green manures work in raised beds and small plots?
Very well — a bed empty for even six weeks is worth covering. Fast growers like mustard or phacelia suit short gaps; overwintering types suit beds cleared in autumn.
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