What is green manure, and is it worth sowing?

Bare soil over a British winter loses nitrogen to leaching, loses structure to the impact of rain, and grows weeds. Green manure addresses all three for the price of a packet of seed. The catch is timing — most of the value comes from sowing early enough in autumn for the crop to establish before the light goes.

What you'll need

• Green manure seed suited to the season
• Rake
• Hoe or shears for cutting down (optional)

Step by step

1. Decide whether you actually have bare ground.Green manure earns its place on beds that will be empty for six weeks or more. If the plot is continuously cropped or permanently mulched, a compost mulch does the same job with no sowing and no clearing.
2. Sow autumn crops early enough.Late August to mid-September for most of the UK, a fortnight earlier in the north. Sown in late October there is not enough light or warmth left to make worthwhile growth, and you get a thin crop and bare soil anyway.
3. Choose by what you want it to do.Grazing rye and winter tares for winter cover and bulk. Field beans for winter cover on heavy soil. Phacelia for a fast summer gap and superb bee forage. Buckwheat for a quick summer crop on poor soil. Clovers and vetches to fix nitrogen over a longer period.
4. Avoid legumes before or after beans and peas.Clover, vetch, tares and field beans are legumes and share diseases with your pea and bean crops. Keep them out of that part of the rotation.
5. Avoid mustard where clubroot is a risk.Mustard is a brassica and hosts clubroot. On any plot with a clubroot history, or in a tight brassica rotation, choose something else.
6. Sow it like a lawn, not like a crop.Rake the surface to a rough tilth, broadcast the seed evenly by hand, rake lightly to cover, and firm it. It does not need rows, spacing or thinning.
7. Cut it down before it flowers and sets seed.This is the point at which the nitrogen is highest and the stems are still soft. Once it seeds you have created a weed problem and the stems have gone woody.
8. Cut and drop, or chop and dig in shallowly.On a no-dig plot, cut it at the base and leave it as a mulch to break down in place, or take the tops to the compost heap. On a dug plot, chop it and turn it into the top 150mm.
9. Leave two to four weeks before sowing or planting.Fresh green material ties up nitrogen while it breaks down, and can inhibit germination of small seed. Give it time, and longer in cold weather.
10. Deal with grazing rye deliberately.It is the best winter cover crop for bulk and root mass and the hardest to kill. Cut it down in early spring while it is still soft, and be aware it releases substances that suppress small seed germination for several weeks.

Tips

• Phacelia is worth growing for the bees alone. It flowers in about eight weeks and is covered in pollinators.
• A quick summer sowing in a gap between crops does as much good as an over-winter one and is far easier to clear.
• On a small no-dig plot a 50mm mulch of compost does most of what a green manure does, with no sowing, no clearing and no waiting.

Common mistakes

• Sowing too late in autumn — There is not enough light after early October for the crop to make useful growth, so the ground ends up thinly covered and you have spent seed for nothing.
• Letting it flower and set seed — Nitrogen content peaks before flowering, and after seeding you have a self-sown weed for the next two years.
• Sowing straight after digging in — Decomposing green material temporarily locks up nitrogen and can suppress germination. Wait two to four weeks.
• Using mustard on clubroot ground — It is a brassica and it hosts the disease. There are plenty of alternatives.

Questions people ask