You sow green manure in autumn as soon as a bed comes free, roughly between early August and mid October. Which type you pick depends on two things: how late you are and what goes on that bed next year. Fast, frost tender types like phacelia and yellow mustard go in until about mid September, and they die off in the first frosts by themselves. Winter rye can still go in from September to early October, stays green all winter and delivers the most organic matter. Source: De Bolster
Sounds simple. And still, a lot of gardeners come unstuck on one point that isn’t printed on most seed packets: green manure belongs in your crop rotation, just like everything else. Yellow mustard and fodder radish are brassicas, same family as cabbage. Sow them on a bed where cabbage follows next year and you are building up clubroot instead of soil life. That is why Velt keeps a 6 year gap between brassica green manures and cabbage crops. Source: Velt
Below is the complete guide: what green manure actually does, a selection table with seed rates per square metre, the latest sowing dates, how to sow, which traps are hiding in your rotation, how to finish it off in spring (digging in or no dig) and when you are better off not sowing green manure at all.
What does green manure actually do?
Green manure is a crop you don’t sow to eat, but to feed your soil with. Six things happen at once.
1. The ground isn’t left bare. Bare soil in winter is soil that caps under the rain, whose top layer washes away and where weeds have a free run. A green mat catches the rain.
2. Nutrients stay put. Nitrogen leaches straight down through your soil profile in winter, off into the groundwater. Green manure takes that nitrogen up and locks it into its leaves and roots. In spring, as it breaks down, it is released again exactly where you want it.
3. The structure improves. Roots drill channels. Fodder radish and lupin go deep and break open compacted layers. Grasses like rye do the opposite and make a fine, dense root network that leaves the top layer crumbly.
4. The soil life gets fed. Living roots exude sugars. Those feed fungi and bacteria, and with them your entire soil food web. This is the effect you can’t see and the one that pays off most in the long run.
5. Legumes pull nitrogen out of the air. Clover, vetch and lupin fix nitrogen from the air through root nodules. Source: Velt That is free fertiliser.
6. Weeds get no chance. A dense green manure wins the race for light, and that saves you hours of weeding in spring.
What green manure does not do: it is no substitute for compost. Green manure mostly moves and holds on to what is already there, and adds carbon out of the air. Only legumes genuinely bring in new nutrition. Think of it as soil maintenance, not as feeding.
The selection table: which green manure suits your bed?
This is the heart of the article. Look at two things in the table: the latest sowing date, and what goes on that bed next year.
| Green manure | Family | Sow until | Seed rate | Winter | Strong point | Watch out |
|---|---|---|---|---|---|---|
| Winter rye | grasses | September to early October | about 10 g/m² | hardy, stays green | most organic matter, deep root network | has to be dealt with properly in spring, not one to put ahead of fine seed |
| Yellow mustard | brassicas | mid September | 2 to 2.5 g/m² | frost tender, dies off | very fast, cheap, smothers weeds | keep a 6 year gap from cabbage |
| Fodder radish | brassicas | end of August | 2 to 3 g/m² | mostly frost tender | deep taproot breaks up compaction | same story: brassica, not before cabbage |
| Phacelia | borage family | mid August | about 1 to 1.5 g/m² | very frost tender | fits any rotation, bees love it | sown too late it comes to nothing |
| Japanese oats | grasses | early September | 6 to 10 g/m² | frost tender | plenty of bulk, dies off, no digging | gives less root mass than rye |
| Common vetch | legumes | mid August | 9 to 12.5 g/m² | moderately hardy | fixes nitrogen from the air | years of gap from peas and beans |
| Italian ryegrass | grasses | September | 2.5 to 3 g/m² | hardy | fine rooting, cheap | turns stubborn if you let it sit |
| Rye and vetch mix | mixed | September | follow the packet | hardy | carbon plus nitrogen in one | inherits both rotation rules |
Seed rates are guidelines converted from common recommendations: winter rye 100 grams per 10 m² Source: De Bolster, fodder radish and yellow mustard around 200 to 250 grams per 100 m², common vetch 90 to 125 kg/ha and Japanese oats 60 to 100 kg/ha. If there is a rate printed on your packet, that one wins.
And if you want to keep it simple? Three rules cover 90 per cent of situations:
- Bed comes free in August, and no cabbage next year: yellow mustard. Cheap, grows like mad and clears itself away after the frost.
- Bed comes free in August, and cabbage is coming: phacelia. It sits in no vegetable family at all, so it always fits.
- Bed only comes free in September or October: winter rye. The only one that still really performs by then.
The latest sowing date: sowing too late is money thrown away
This is where autumn is merciless. From mid September the soil temperature and the day length drop at the same time, and a green manure that still has to germinate then never amounts to anything. You end up with a bed holding a few thin shoots, which does exactly as much as an empty bed.
| Sowing window | What still works |
|---|---|
| Early August to mid August | everything: phacelia, vetch, mustard, fodder radish, mixes |
| Mid August to end of August | mustard, fodder radish, Japanese oats, rye |
| September | winter rye, Italian ryegrass (mustard just about, in the first half) |
| First half of October | winter rye only, and with less bulk |
| After mid October | don’t start: cover the bed with cardboard, compost or leaves |
In arable farming phacelia is sown before mid August, and yellow mustard into September. Source: LG Seeds Fodder radish works until the end of August.
Are you too late? Then sow nothing, but don’t leave the bed bare either. A 5 centimetre layer of compost, a thick layer of autumn leaves with a few branches on top to stop it blowing away, or a sheet of cardboard with compost on it does largely the same job over winter: the ground stays covered, the soil life has something to eat and the weeds get no light.
The rotation traps (this isn’t on the packet)
This is where most of the damage happens, and it is quiet damage, because you only notice it two years later.
Trap 1: brassicas and clubroot
Yellow mustard and fodder radish are related to cabbage, radish, rocket and turnip. Sow those two often in the same spot and you are handing clubroot a host. Clubroot is a soil disease you won’t shake off for years and it makes growing cabbage impossible.
Velt advises a 6 year gap between brassica green manures and cabbage crops. Source: Velt Other sources keep a minimum of 4 years as part of your normal rotation. Source: Diana’s mooie moestuin
In practice: if you have a small garden with four or five beds and you grow cabbage every year, then simply don’t use yellow mustard. Pick phacelia or Japanese oats. They sit in no vegetable family and you never have to think about it again.
Trap 2: legumes and pulses
Clover, vetch and lupin are related to peas, broad beans and French beans. Disease pressure plays a part here too, with soil sickness and foot rots as the result. Keep at least a year’s gap, and preferably more, between a legume green manure and your pulse bed. Source: Diana’s mooie moestuin
Trap 3: nematodes
This is the trap almost nobody in the vegetable garden knows about, and the one that costs millions on the farm.
Some green manures multiply nematodes and some clear them out. There are special varieties of fodder radish that strongly reduce the beet cyst nematode (labelled BCA-1 and BCA-2). Winter rye, on the other hand, lets the maize root knot nematode multiply strongly. Yellow mustard is not a host plant for the root knot nematode. Source: LG Seeds
What does that mean for you? In an average vegetable garden with plenty of variation, nematodes are rarely your first problem. But if you are getting deformed roots, patches in the bed that lag behind or spots where nothing wants to grow, have a quick look at what green manure you have sown there over the past few years. And Tagetes (French marigolds) is the classic choice if you want to target root lesion nematodes specifically, although that is a summer crop running several months, not an autumn job.
The safe route
If you would rather not worry about any of it: rotate your green manures exactly the way you rotate your vegetables. So not three years running of yellow mustard on every bed because it is the cheapest packet. Make a circuit of it: phacelia here this year, mustard there, rye over there.
How to sow green manure: 6 steps
Step 1. Clear the bed. Take out the crop residue from your summer planting. Diseased material (mildew, rust, blight) goes in the bin, the rest goes on the compost heap.
Step 2. Pull the weeds out. Perennial rooting weeds especially. Whatever you leave standing now simply carries on growing under your green manure.
Step 3. Loosen the top layer. A couple of centimetres, with a rake or a cultivator. You don’t need to dig. Seed wants contact with soil, nothing more. If the top layer is compacted and capped, first go over it with a garden fork, pushing in without turning.
Step 4. Broadcast or sow in rows. Broadcasting is easiest for the vegetable garden: split the seed into two halves and walk the bed twice, in crossing directions. That gives a far more even stand than scattering it once.
Large seeds (rye, vetch, oats) do better in rows. Winter rye goes in 2 to 3 centimetres deep, in rows 12.5 to 25 centimetres apart. Source: De Bolster
Step 5. Rake in lightly and firm it down. Fine seed like phacelia should barely be covered, roughly half a centimetre. Coarse seed wants to go deeper. Then go over it with the back of the rake or with your feet to press the seed down. Firming matters more than it looks: it creates contact with moisture.
Step 6. Water, or sow just before rain. Germinating seed must not dry out. If you sow during a dry spell, water in the first week whenever it stays dry. Or do what most gardeners do: check the rain radar and sow the day before it rains.
Cost: a packet of green manure seed covering 25 to 100 square metres usually costs between €3 and €10. So for an average vegetable garden you are done for €10 to €25 a year, which is less than a single bag of potting compost. If you want to do bigger areas, seed bought by the kilo from an agricultural seed merchant is considerably cheaper per square metre than the small garden packets.
What do you do in winter?
With frost tender types: nothing. Phacelia, yellow mustard, buckwheat and Japanese oats die off in a normal north west European winter and stay lying on your bed as a mat. Source: Velt That is exactly the idea: the dead material protects the soil, and the worms pull it down over the course of the winter.
With hardy types (rye, ryegrass, vetch in part): leave them standing and deal with them in March.
One warning. Never let a green manure set seed. What starts as soil improvement otherwise ends as a weed that keeps coming back for three years. Phacelia and yellow mustard especially self seed with enthusiasm. If you see flower buds in autumn, cut it down. If it flowers anyway and sets seed, pick the seed heads off by hand.
Spring: dig it in or no dig?
Two schools part ways here, and both work. The choice depends on your bed and on how you garden.
Route 1: digging in (the classic)
Work the green manure in shallowly, into the top 15 to 20 centimetres. Source: De Bolster Any deeper is pointless, because the soil organisms that have to break the material down sit in exactly that top layer. With coarse, sturdy rye, cut it down first so the material is shorter.
And then wait. Allow 2 to 3 weeks between working it in and sowing or planting a new crop. Source: De Bolster
Why that wait matters: fresh, woody plant material with a lot of carbon and little nitrogen makes the soil life lock up nitrogen temporarily in order to break it down. So during those weeks there is actually less nitrogen available for your crop. With coarse winter rye that effect is strongest. Sow fine seed straight after digging in rye and you will often see poor, patchy germination. Waiting three weeks solves it.
Timing: cut or dig in before or during flowering. Source: Velt After flowering the plant turns woody, breaks down more slowly and starts setting seed on top of that.
Route 2: chop and drop (no dig)
Cut the foliage off just below soil level and leave it lying on the bed as mulch. Source: Velt The roots stay in the ground and rot down there, leaving their channels behind. The foliage breaks down on top.
Why this is often better: you don’t disturb the soil life or the root channels, the ground stays covered, and the effect on structure is generally bigger than with digging.
What you need to know: it goes slower. With a thick mulch layer the bed isn’t ready in March. Practical approach: cut 4 to 6 weeks before you want to plant, and push the mulch aside where you are going to sow. For transplants (cabbage, courgette, tomato) you just make a hole in the mulch. For fine seed (carrot, lettuce) you take the mulch off or move it to another bed.
What if it doesn’t work? Winter rye is the hardest one for chop and drop, because it often regrows after cutting. Two options: cut it when it starts to run up into ear (then it won’t come back), or cover it after cutting for two to four weeks with cardboard or a solid sheet so it dies off without light.
Which route do you pick?
| Situation | Advice |
|---|---|
| Raised beds, no dig garden | chop and drop |
| Heavy clay you loosen anyway | shallow digging in |
| Coarse winter rye, early crop planned | cut, remove or cover, then plant |
| Frost tender type already lying dead | rake the residue through the top layer, or just leave it |
| You are in a hurry and want to sow in March | sow a frost tender type in autumn, not rye |
That last point is a genuine planning trick: choose your autumn green manure based on when you need the bed back in spring. Bed that is due in March for broad beans or early potatoes? Frost tender type, then it is already lying dead by March. Bed that isn’t due until May for courgette or pumpkin? Winter rye is fine, because it has all the time it needs.
When do you skip green manure?
Fair is fair, there are situations where it doesn’t pay off.
- The bed only comes free at the end of October. There is too little growing season left. Cover it up.
- A crop is going in as early as February or March. You won’t have time to break the green manure down. Cover with compost.
- Your soil is heavy and waterlogged. On some wet clay soils you can’t stand on the bed in spring to work the green manure in without wrecking the structure. Pick a frost tender type or cover up instead.
- You have a stubborn perennial weed. Couch grass or ground elder won’t be solved by green manure. Tackle that problem first.
- You have a little patch of 2 square metres. Fine to do, but a layer of compost is less hassle for the same result.
Green manure is an addition, not an obligation. Source: Diana’s mooie moestuin
The 7 mistakes people make most
- Sowing too late. After mid October it is rye only, and even that grudgingly.
- Sowing too thickly. More seed gives thin, floppy plants that crowd each other out. Stick to the rate per square metre.
- Not firming down. Seed without soil contact doesn’t germinate, and the birds find it easily that way.
- Brassica green manure right before the cabbage. The quiet clubroot trap.
- Letting it run up and set seed. Cut before or during flowering.
- Sowing straight after digging in. Wait 2 to 3 weeks, certainly after rye.
- Thinking it replaces compost. Green manure holds on to nutrients and improves structure, but it adds little feed except with legumes.
Calendar: what to do when
| Month | Action |
|---|---|
| July to August | as soon as a bed comes free: weeds out, top layer loosened, sow (all types possible) |
| September | winter rye, Italian ryegrass, and yellow mustard still in the first half |
| Early October | last chance for winter rye |
| Mid October and later | don’t sow: cover the bed with compost, leaves or cardboard |
| November to February | do nothing; frost tender types die off by themselves |
| March | cut hardy types down or work them in shallowly, before flowering |
| March to April | wait 2 to 3 weeks, then sow or plant |
| May | bed that is due later: let the green manure carry on growing until just before flowering |
Frequently asked questions
When should I sow green manure in autumn?
As soon as a bed comes free, with early August to mid September as the best window. Winter rye can still go in from September to early October. After mid October there is little point in sowing, because the green manure won’t build enough bulk. Cover the bed with compost, leaves or cardboard instead.
Which green manure can I sow latest in the year?
Winter rye. You can still sow it in September and the first half of October, it is hardy and it stays green all winter. Sowing later is technically possible, but it delivers less and less bulk.
Do I have to dig the green manure in or can it stay on top?
Both work. Shallow digging in, into the top 15 to 20 centimetres, is the classic method; after that, wait 2 to 3 weeks before you sow a new crop. With no dig you cut the foliage off just below soil level and leave it lying as mulch, while the roots rot down in the ground. With frost tender types you usually don’t have to do anything at all.
Can I sow yellow mustard where cabbage goes next year?
Better not. Yellow mustard and fodder radish are brassicas, same as cabbage, and they can encourage clubroot. Velt advises a 6 year gap between brassica green manures and cabbage crops. On a bed like that, pick phacelia or Japanese oats, because they sit in no vegetable family.
How much seed do I need per square metre?
That varies a lot per type, because seed weight differs. Rough guidelines: phacelia about 1 to 1.5 grams per m², yellow mustard and fodder radish 2 to 3 grams per m², Italian ryegrass 2.5 to 3 grams per m², Japanese oats 6 to 10 grams per m², common vetch 9 to 12.5 grams per m² and winter rye about 10 grams per m². If there is a rate on the packet, stick to that.
Does green manure really deliver nitrogen?
Only legumes (clover, vetch, lupin) genuinely pull new nitrogen out of the air through root nodules. All the other green manures take up existing nitrogen from your soil and hold it until spring, which prevents leaching. That is valuable, but it is retention, not supply.
Why is my new crop germinating badly after I dug the green manure in?
Probably sown too soon. Fresh, carbon rich material (coarse winter rye especially) makes the soil life lock up nitrogen temporarily while breaking it down, and that comes at your crop’s expense. Allow 2 to 3 weeks between digging in and sowing. With fine seed like carrot and lettuce, feel free to wait a bit longer.
Can I just use lawn seed or bird food instead of green manure seed?
Don’t. Lawn seed is more expensive, gives little bulk and is hard to get rid of afterwards. Bird food often contains seeds that either don’t germinate or come back at you as weeds. A packet of proper green manure seed costs €3 to €10 and covers a decent stretch of bed, so the saving isn’t worth it.
In short: look at when the bed comes free, look at what is coming next year, pick a type to match, sow evenly and firm it down. Cut before flowering and let the soil rest a while before you sow again.
Do that a few years running and you will see the difference literally on your spade: darker, more crumbly, more worms. That is the cheapest soil improvement there is.
Want to build your vegetable garden out further with raised beds, a cold frame or a greenhouse? At Fred’s DIY Plans you will find worked out building plans for raised beds, garden sheds and garden structures, complete with a materials list and step by step instructions. Have a look here.
Greetings from Fred Schouten, handyman and DIYer through and through
PS: buy your seed by the kilo from an agricultural seed merchant instead of in garden packets. You pay a fraction per square metre, and green manure seed keeps for a few years if you store it dry and cool. Small tip. Big difference.