Every autumn the same thing happens on veg plots everywhere. The summer crops come out, the beds are empty, the weather closes in, and the instinct is to tidy up, dig it over and leave it to the frost. I understand the instinct. It's also about the worst thing you can do for the soil.

What a bare bed loses over winter

North Wales is wet, and winter rain does real damage to soil with nothing growing in it.

Nutrients wash away. Nitrogen and other soluble nutrients are carried down below where next year's roots will reach, or run off the surface altogether. With no roots in the ground, there's nothing to catch them.
The surface slumps. Heavy rain hammers bare soil into a crust, and the fine, crumbly top layer, the bit you most want to keep, is the bit that goes first.
Weeds move in. Chickweed and the rest are happy to grow all winter, and they set seed that you'll be pulling out all spring.
Digging it in March costs you more. Turning soil over lets air rush in, and soil life burns through the organic matter you've spent years building.

The answer is one simple rule: keep something living in the soil for as much of the year as you can. Over winter, that means a green manure, a crop you grow not to eat but to look after the ground.

Three jobs a green manure can do

Different green manures do different jobs, and it's worth knowing which is which, because a plant that's brilliant at one may do almost nothing for another.

1. Add nitrogen

Legumes, such as winter tares, field beans and clovers, carry bacteria in little nodules on their roots that take nitrogen out of the air and turn it into plant food. It's the only way a bed gains nitrogen without you buying something in. When the crop is finished in spring, that nitrogen is released for whatever you plant next.

2. Bring nutrients back up

Deep-rooted crops, grazing rye especially, chase nutrients that have washed down below the reach of most vegetables and pull them back up into their leaves. When you cut them down in spring, the nutrients go back on the surface. They don't add anything new, though. They just put back what was drifting away, so a genuinely hungry bed needs a legume as well.

3. Build the soil's structure

A thick green cover shields the surface from rain and shades out weeds. Below ground, roots push through compacted layers, and when they rot they leave channels that let air and water down. Field beans are good on heavy clay. Tillage radish drives a fat root 30 to 40cm down and then rots away in winter, leaving a hole where it was. Grazing rye makes a dense mat of fine roots that holds wet clay together.

A late-summer border of asters, salvias, rudbeckias and heleniums in front of a polytunnel — The border by the polytunnel in my own veg garden. Phacelia, if you let some of it flower, feeds the same bees and hoverflies.

What to sow

Green manure	What it does	Sow	Over winter
Grazing rye	Lifts nutrients, holds structure, smothers weeds	Aug to Nov	Very hardy
Winter field beans	Adds nitrogen, breaks up heavy clay	Sept to Nov	Very hardy
Winter tares	Adds nitrogen, forms a weed-smothering mat	July to Sept	Very hardy
Crimson clover	Adds nitrogen, good on lighter soils	Up to Aug	Fairly hardy
Bristle (black) oats	Quick cover, lots of growth	Up to Sept	Checked by hard frost
Phacelia	Fine crumb structure, feeds pollinators	Up to Aug	Usually killed by a hard winter
Tillage radish	Breaks up compaction deep down	Aug to Sept	Killed by frost, rots in place
Mustard	Fast cover, can suppress some soil pests	Up to mid-Sept	Killed by frost

Reading this in October? It isn't too late. Grazing rye and winter field beans can both go in through October and into November. They're what I'd sow this week. The clovers and phacelia have missed their window for this year.

Mixing it up

A mix does more than any single crop, because each plant covers the gaps in the others. These are the combinations I'd use, chosen by what the bed needs and what's going in next:

All-rounder, for a tired bed going into roots or salads: grazing rye, bristle oats, winter tares, crimson clover, a little tillage radish and phacelia. Sow in August or early September.
Hungry beds, ahead of hungry crops like leeks, courgettes, sweetcorn or cabbages: mostly winter tares and crimson clover, with some field beans and a few oats.
Heavy, compacted clay: grazing rye, field beans and tillage radish. Roots do the digging for you.
Going into brassicas next year: rye, oats, tares, clover and phacelia, with no radish and no mustard.

Use the sowing rate on the packet for each crop on its own, and in a mix sow each at a fraction of that rate, so the total comes to a little over one full sowing. Rake the bed to a firm, fine surface, press big seeds like beans and rye into shallow drills, scatter the small seeds over the top and rake in lightly.

The brassica rule. Tillage radish and mustard are both members of the cabbage family, and both can host clubroot, a soil disease that lasts for years once you have it. Keep them out of any bed that's going to grow cabbages, kale, sprouts, swede or turnips next year. A lot of green manure advice leaves this out.

Finishing it in spring, without digging

The old advice was to chop the green manure and dig it in. I'd steer you away from that for three reasons. Digging tears up the fine networks of soil fungi your vegetables rely on. It burns off organic matter as air floods in. And a tough, mature crop like grazing rye, buried, makes soil bacteria grab nitrogen to break it down, so your next crop goes pale and sulks. Gardeners call it nitrogen lock-up.

Finish it on the surface instead, and leave the roots where they are to rot down in place:

Chop and drop. Cut the crop down as close to the ground as you can just as it starts to flower, when it's at its biggest but before it sets seed. Leave the cuttings on the surface as a mulch.
Flatten it. On a small bed, walking a plank along the rows crushes the stems without pulling the plants out.
Smother it. Grazing rye and tares regrow from the base when you cut them. Cover them with cardboard and 5 to 15cm of compost or woodchip, and they're done in a few weeks.

Crimson clover won't regrow once it's cut to the ground, so it's the easiest to finish. With grazing rye, give it three or four weeks after cutting before you sow small seeds like carrots into that bed, because the residues can hold back germination.

The roots you leave in the ground are the point. They've done the digging for you.

Nothing new under the sun

None of this is a fashion. F. C. King, head gardener at Levens Hall in Cumbria, was asking whether digging was necessary at all in the 1940s, long before soil science could explain why leaving the soil alone worked. Modern research has largely proved him right.

If you keep hens, there's a way to take this a step further and let them finish the green manure for you. That's in the next post.