Green manures are plants grown specifically to be cut down and incorporated into the soil or compost heap -- a way of growing fertility rather than importing it. In a temperate maritime climate with a long, mild growing season, green manures are highly practical and have been used across northwest Europe for centuries.
What Green Manures Do
- Fix atmospheric nitrogen -- leguminous green manures (clover, vetch, beans) host Rhizobium bacteria in root nodules that convert atmospheric nitrogen into plant-available forms; when dug in or composted, this feeds the following crop.
- Prevent nutrient leaching -- growing plants capture nutrients that would otherwise wash through soil in winter rain, released slowly when incorporated.
- Suppress weeds -- a dense green manure smothers weed germination, particularly valuable on empty beds over winter.
- Improve soil structure -- roots open channels in compacted soil; deep-rooted species like phacelia work particularly well.
- Feed the compost heap -- cut green manure can be added as a nitrogen-rich green material.
- Support biodiversity -- flowering green manures (phacelia, buckwheat, mustard) are outstanding for pollinators.
Best Green Manures for a Maritime Temperate Climate
| Plant | Sow | Duration | Key Benefit | Notes |
|---|---|---|---|---|
| Red or white clover (Trifolium spp.) | Spring-summer | 6-18 months | Nitrogen fixation; long-term fertility | Excellent under-sow in brassicas or sweetcorn |
| Winter tares / vetch (Vicia sativa) | Late summer-autumn | Over-winter | Nitrogen fixation; winter cover | Hardy; excellent pre-brassica crop |
| Phacelia (P. tanacetifolia) | Spring-summer | 8-12 weeks | Rapid cover; pollinator plant | Not a legume but excellent all-rounder |
| Field beans (Vicia faba) | Autumn | Over-winter | Deep nitrogen fixation | Very hardy; thick stems, chop well |
| Mustard (Sinapis alba) | Spring-summer | 6-10 weeks | Fast growth; brassica pest break | Do not use in brassica rotation beds |
| Buckwheat (Fagopyrum esculentum) | Late spring-summer | 8-12 weeks | Phosphorus release; pollinator magnet | Not frost-hardy; use in warm months |
| Rye (grazing rye) (Secale cereale) | Autumn | Over-winter | Excellent weed suppression | Very hardy; difficult to dig in, cut first |
| Crimson clover (Trifolium incarnatum) | Spring-summer | 3-6 months | Nitrogen; spectacular flower | Less perennial than red clover, easier to manage |
How to Incorporate Green Manures
- Cut before flowering (or just as flowers open for legumes) -- stems become woody and slow to decompose if left too long, and this also prevents unwanted seeding.
- Chop finely with a spade before incorporation.
- Leave on the surface for a few days to wilt before digging in.
- Dig in shallowly -- the top 10-15cm is where decomposition is fastest; deep burial can cause anaerobic pockets.
- Allow two to four weeks between incorporation and planting -- decomposing green matter can temporarily tie up nitrogen.
- Or add to the compost heap rather than incorporating directly -- particularly useful for thick-stemmed species like field beans and rye.
Frequently Asked Questions
What's the easiest green manure to grow for beginners?
Phacelia. It isn't fussy about soil, germinates fast, gives rapid ground cover within weeks, and its flowers are outstanding for pollinators. It isn't a legume, so it won't fix nitrogen, but as an all-round easy first green manure it's hard to beat.
Why should I wait before planting after digging in a green manure?
Freshly dug-in green matter is broken down by soil microbes that temporarily draw on soil nitrogen to do the job, competing with whatever you plant next. Waiting two to four weeks lets that initial breakdown finish, after which the nitrogen is released back for the following crop rather than being tied up against it.
Can green manures replace compost entirely?
No -- they complement it rather than replace it. Green manures mainly recycle and fix nutrients that are already present in the system, whereas compost inputs like seaweed or manure bring in nutrients from outside it. A well-run maritime system uses both.