Sowing green manure: benefits, types, cultivation calendar

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

This is probably the action with the best effort-to-benefit ratio in the entire vegetable garden. 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-trapping 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 oilseed rape — 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 genus Rhizobium that can transform atmospheric nitrogen into an assimilable form.

A crucial distinction: this nitrogen initially benefits the plant itself. It enriches the soil only after decomposition. Hence the rule: mow the above-ground parts 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 oilseed rape and forage radish bore vertical channels that earthworms will 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 smothers weeds and protects the soil from crusting — a crucial point on silty soils.

Choosing the right species

Phacelia — the versatile one

Very rapid growth (6 to 8 weeks), spectacular bee-friendly flowering, and a fine root system that improves surface structure. Its great advantage: it belongs to none of the plant families 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 would be it.

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 include it before or after these crops—you would maintain clubroot. Black mustard and rapeseed share this limitation.

White clover — permanent cover

Slow but dense and long-lasting growth. It tolerates trampling, 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 solution for a short summer intercrop.

Summary table

Objective	Species	Sowing season
Quickly cover an empty bed	Mustard, phacelia	March to September
Add nitrogen	Clover, alfalfa, fenugreek	Spring or late summer
Deep loosening	Alfalfa, rapeseed	Spring or autumn
Protecting the soil throughout winter	White clover, rapeseed	September to October
Attracting pollinators	Phacelia, clovers	Spring to summer
Regenerating an exhausted plot	Alfalfa, red clover	Spring, for 1 to 2 years

Sowing: simple, but two details matter

Prepare minimally — scratching the surface to a depth of 2 to 3 cm is enough. There is no need to dig.
Broadcast densely. 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.
Cover very lightly — rake once or make one pass with a roller. Small seeds should not be buried more than one centimeter deep.
Water if the soil is dry, until emergence. A summer sowing in dry soil will not emerge.

Species that can be sown in spring are listed under Spring sowing, while late-season species are listed under Autumn sowing.

Destroying at the right time — the decisive step

This is where all the benefit lies, and where most mistakes are made.

When

At the beginning of flowering, never afterward. Two reasons: beyond this point, the plant sets seed and you may inadvertently 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 tie-up.

One small exception: if your main goal is mulching 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, leaving the soil structure intact.
Shallow incorporation — scratch the residues into the top 5 centimeters. Decomposition is faster, but it 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 are killed by frost, doing the work for free in cold regions.

How long should you wait before sowing?

Allow 2 to 3 weeks for tender material cut young, 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

Sow too thinly. Green manure must smother competing plants: denser is better than sparser.
Let it go to seed. You turn your green manure into a weed for three years.
Plant mustard before cabbages. They belong to the same family and share the same diseases. A very common mistake.
Bury it deeply. Material buried deep ferments instead of decomposing.
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 well-established cabbages or tomatoes covers the soil without competing with them. 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 and mulched the surface without incorporating anything: in that case, you can plant seedlings (but not sow seeds) almost immediately, moving the mulch aside where each seedling will be planted.

Does green manure replace compost?

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

Is it useful in a small vegetable garden or planter?

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 restoring the soil.

To go further

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