Gardening glossary
Green manure
Green manure and cover crop are often used interchangeably, but there is a useful distinction. A cover crop is anything grown to protect or improve soil; a green manure is specifically a cover crop that you cut down and turn in (or chop and drop) while it is still soft, leafy, and full of moisture. The point is to deliver a quick pulse of nutrients to the soil — particularly nitrogen — rather than long-lasting structure.
Classic green manures:
- **Mustard (Sinapis alba)** — sown in late summer, cut at flowering. Adds organic matter and helps suppress some nematodes via biofumigant glucosinolates. - **Phacelia (Phacelia tanacetifolia)** — fast-growing, frost-sensitive, and a magnet for bees if you let some flower before cutting. - **Buckwheat (Fagopyrum esculentum)** — six weeks from sowing to flowering. Brilliant for filling summer gaps and unlocking soil phosphorus. - **Crimson clover and field beans** — autumn-sown legumes that fix nitrogen and bulk up over winter for a spring cut.
How and when to incorporate. Wait until the green manure is in active flowering or just before — that is when the carbon-to-nitrogen ratio is most favourable. Cut everything at the base, chop the residue into smaller pieces if you can, and either shallow-fork it into the top 5–10 cm of soil or leave it on the surface to break down as a mulch.
The catch: do not plant your next crop the same day you turn in a green manure. Fresh, green material decomposing rapidly can lock up soil nitrogen for two to four weeks (the microbes tie it up while they digest the cellulose). Wait at least two weeks before sowing seed, or three to four before transplanting hungry feeders.
In a tight rotation, green manures slot into the brief windows between crops where the bed would otherwise grow weeds. A square metre under green manure for six weeks is six weeks not lost to bare soil and weed seed germination.