Sowing green manure: benefits, types, and planting schedule

Green manure is not a plant that you harvest: it is a plant you sow for the soil, and then return to it. Between two crops, on a bed that would otherwise remain bare, it protects the surface, nourishes underground life, improves deep soil structure and retains nutrients that would otherwise be washed away by rain.

This is probably the gardening task with the best effort-to-benefit ratio. You still need to choose the right species for the right purpose — they do not all do the same thing.

What green manure actually does

There are four distinct effects, and no single species combines them all. That is why the question is never “what is the best green manure?” but “what am I trying to correct?”

Retaining nutrients (the least-known and most immediate effect)

Bare soil loses nitrogen with every rainfall: highly soluble nitrates move beyond the reach of roots. Plant cover captures them and stores them in its tissues. This is known as a nitrate catch crop. When the green manure is destroyed, this nitrogen becomes available again at the surface, exactly where the following crop needs it.

Crucifers — mustard and rapeseed — are the most effective in this respect, thanks to their rapid growth late in the season.

Fixing nitrogen from the air

This is characteristic of legumes: clover, alfalfa, broad bean, fenugreek. In their root nodules, they host bacteria of the Rhizobium genus, which can transform atmospheric nitrogen into an assimilable form.

A nuance is important here: this nitrogen initially benefits the plant itself. It enriches the soil only after decomposition. Hence the rule: mow the above-ground part and always leave the roots in place — that is where most of the benefit lies.

Structuring the soil in depth

Some roots break through compacted layers where no tool can reach without damaging the soil structure. Alfalfa grows down more than a metre, while rapeseed and fodder radish create vertical channels that earthworms later use. This is sometimes called “biological ploughing”, and the image is apt.

Feeding and covering

The biomass produced, left on the surface, becomes mulch and then humus. During its growth, the dense cover also smothers weeds and protects the soil from crusting — a key benefit on loamy soils.

Choosing the right species

Phacelia — the versatile one

Very fast growth (6 to 8 weeks), spectacular bee-friendly flowering, and a fine root system that improves surface soil structure. Its great advantage: it belongs to no crop family grown in the vegetable garden, so it can be included anywhere in a rotation without the risk of sharing diseases. Sow from March to September.

If you were to grow only one, this is the one.

White mustard — the emergency option

It emerges within a few days and provides cover within a month. An excellent nitrate trap, it freezes at around -5 °C, effectively providing free winter termination.

The important restriction: it is a brassica, like cabbages, radishes, turnips, and arugula. Never sow it before or after these crops — you would perpetuate clubroot. Black mustard and rapeseed share this limitation.

White clover — permanent ground cover

Slow but dense and long-lasting growth. It tolerates foot traffic, making it an excellent cover for paths between beds or a permanent cover at the base of fruit trees. Nitrogen-fixing and highly attractive to bees.

Red clover and alfalfa — restoring the soil

These are the species to choose for a tired or compacted plot. Deep roots, high biomass production, and significant nitrogen fixation. Their purpose is multi-year: leave them in place for one or two full seasons. Reserve them for areas that can be temporarily taken out of production.

Fenugreek — the summer legume

Short cycle, nitrogen-fixing, and dual-purpose: the seeds are used in cooking, and the young shoots are eaten in salads. A good option for a short summer intercrop.

Summary table

Sowing: simple, but two details matter

1. Prepare the soil roughly — scratching the surface to a depth of 2 to 3 cm is sufficient. There is no need to dig.
2. Sow densely, by broadcasting. This is the key point: sparse sowing leaves room for weeds and fails to provide adequate cover. Use approximately 2 g/m² for phacelia or mustard, and 3 g/m² for clovers.
3. Cover very lightly — rake once or pass a roller over it. Small seeds should not be buried deeper than one centimeter.
4. Water if the soil is dry, until emergence. A summer sowing in dry soil will not emerge.

Spring-sowable species are listed under Spring sowing, and late-season species under Autumn sowing.

Destroying it at the right time — the decisive step

This is where all the benefits are realized—and where people most often get it wrong.

When

At the beginning of flowering, never afterward. There are two reasons: beyond this stage, the plant goes to seed and you will unintentionally reseed it the following year; above all, its tissues become lignified. A young, tender, nitrogen-rich plant decomposes in a few weeks; an older, carbon-rich plant takes months and causes a temporary nitrogen lockup.

One small exception: if your main goal is mulch or biomass, let it flower a little longer—but cut it before seeds form.

How

• Surface mowing — the permaculture method: cut at ground level and leave the material in place as mulch. The roots decompose on their own, while the soil structure remains intact.
• Shallow incorporation — scratch the residues into the top 5 centimeters. Decomposition is faster, but this causes more disturbance.
• Covering — cover with an opaque tarp for 2 to 3 weeks. Effective on tough perennial species such as alfalfa.
• Frost — mustard and phacelia freeze, doing the work for free in cold regions.

How long should you wait before sowing?

Allow 2 to 3 weeks for young-mown, tender material, and 4 to 6 weeks if it was already fibrous. Sowing too soon after a large amount of biomass can lead to uneven emergence.

The five classic mistakes

1. Sow too thinly. Green manure must smother competing plants: denser is better than sparser.
2. Let it go to seed. You turn your green manure into a weed for three years.
3. Plant mustard before cabbages. Same family, same diseases. A very common mistake.
4. Bury it deeply. Matter buried deep ferments instead of decomposing.
5. Sow only one species. A grass–legume mixture (for example, oats and clover) combines soil structuring and nitrogen fixation, with a naturally balanced C/N ratio.

Frequently asked questions

Can you sow green manure between two rows of vegetables?

Yes — this is the associated cover crop. Dwarf clover sown between rows of already well-established cabbages or tomatoes covers the soil without competing. Sow it once the main crop is well developed, never at the same time.

Do you really need to wait before replanting?

Yes, unless you have simply mowed it and used it as surface mulch without incorporating anything: in that case, you can plant seedlings (but not sow seeds) almost immediately, moving the mulch aside where each seedling is planted.

Can green manure replace compost?

No, they complement each other. Compost provides already stabilized organic matter; green manure provides fresh matter that feeds biological activity and works the soil through its roots.

Is it useful for a small vegetable garden or container?

Yes, even in a planter. A handful of phacelia or mustard between two potted crops occupies the substrate, limits its depletion, and prevents it from becoming compacted. See Balcony vegetable garden.

Key takeaway

Sow densely, destroy at the beginning of flowering, and always leave the roots in the ground. Choose the species according to your goal: phacelia for versatility, mustard for urgency, and clovers and alfalfa for nitrogen and soil restoration.

Going further

• The basics of living soil
• Composting: feeding the soil by imitating nature
• How to create a permaculture vegetable garden
• The month-by-month sowing calendar

Discover our collection of green manure seeds — phacelia, mustards, clovers, alfalfa, rapeseed and fenugreek, all capable of producing seed.