Green manures in the vegetable garden: which to sow in Belgium and when to cut them

You have just pulled out the garlic, the onions or the early potatoes. The bed stays free until spring. In Liège, between 60 and 90 mm of rain falls on average each month from October to March (RMI, 1991-2020 normals). Bare soil takes all that water head-on. A green manure is a plant you sow to cover and feed the soil. You then cut it to plant something else in its place. Choose it by the date your bed is freed. Buckwheat is sown until the end of August, white mustard and phacelia until mid-September. After that, switch to rye and winter vetch. The guide also gives the rates, the rotation trap and the spring move.

What a green manure is for

• It covers. The vegetation protects the surface from the pounding of rain, which packs the soil down. Natagora points out that silty soils are sensitive to erosion by rain, wind and sun.
• It builds structure. The taproots of brassicas and the dense fibrous roots of grasses crack the soil. Once dug in, fresh matter makes earthworms multiply (GRAB).
• It feeds the next crop. Legumes (vetch, clover, broad bean) capture nitrogen from the air.
• It slows weeds and welcomes insects. Buckwheat and rye slow wild weeds. Phacelia and buckwheat are good for bees (GRAB).

The six species, with their dates

Our Green manures collection offers White mustard, Winter vetch, Summer vetch and White clover. It also contains the Fève Longue de Belgique and the Mélange mellifère et engrais vert (a bee-friendly green manure mix). The mix sheet lists phacelia, buckwheat, mustard, vetch and clover among its species. Broad bean is a green manure you can eat. Sow it in February and March for a harvest from May to July. A hardy variety is also sown in October and gets through winter (Velt).

Choose by the date your bed is freed.

• In August. Buckwheat (until the end of August), white mustard or phacelia. Buckwheat and brassicas need six to eight weeks of growth. In summer, GRAB counts at least two months before the next crop, so one month of growth then one month of waiting.
• Early September. White mustard or phacelia until mid-September, or winter vetch. In Wallonia (Belgium), farmers in vulnerable zones must have a cover crop in place by 15 September on land harvested before 1 September. Take this date as a landmark.
• From mid-September to October. Rye alone or with winter vetch, or white clover.

The rotation trap

• White mustard is a brassica. It is in the family of cabbage, radish, turnip and rocket. Clubroot survives more than ten years in the soil. It hits all the brassicas that follow (DRIAAF Île-de-France). Velt mentions waiting times of up to five years for cabbage and its relatives. Do not sow mustard where you will put cabbage, radish, turnip or rocket, or just before. The bee-friendly mix lists mustard on its sheet. It follows the same rule.
• Do not mix it up with Chinese mustard. Green in Snow and Métis rouge are leaf vegetables grown to eat. They are also brassicas, subject to the same rule.
• Vetch and clover are legumes. They are in the family of peas, beans and broad beans. GRAB advises choosing a green manure from a different family than the crop that follows.
• Phacelia has no vegetable in its family. It can host aphids and thrips. It sometimes comes back in the next crop (GRAB). Cut it before the seeds.
• Rye calls for caution. Its residues are allelopathic, they slow the emergence of other plants (Agriculture Canada). After rye, plant young plants rather than sowing small seeds right behind it.

How to sow

• Prepare the bed. Clear it and rake the surface, as our sheets ask.
• Broadcast the seed. If your packet is a mix, stir the seeds well first, because their sizes differ.
• Stick to the rate. Count 150 to 200 g per 100 m² of white mustard, 100 to 150 g of bee-friendly mix and 50 to 100 g of white clover. For phacelia, sources give from 1 to 3 g per m². For rye, farming references run from 40 to 150 kg per hectare. So follow the rate on the packet.
• Cover lightly. Rake to cover by 1 to 2 cm (2 to 3 cm for vetch). Barely cover white clover. Then firm with the back of the rake.
• Water with a fine spray. Keep the soil moist for the first ten days.

The spring move

When to cut. Cut at the start of flowering, before the seeds form. Otherwise, the green manure comes back in your crop. Summer vetch is cut about two to three months after sowing. White mustard is cut as soon as the first flowers appear. They come in about 30 days in midsummer (GRAB) and in about two months for a later sowing (our sheet). Winter vetch is cut in March and April, before the summer crops go in.

How to cut. Take shears, a sickle or a mower set high. Leave the stems to dry for a few days where they lie. Do not bury them deep. Fresh matter buried deep ferments badly and can deprive the next crop of nitrogen (GRAB). Leave them as mulch or mix them into the soil surface. For broad bean, cut at ground level and leave the roots in the soil.

Frosts do part of the job. Phacelia, white mustard and buckwheat die when the cold passes below the thresholds in the table. Their remains work as mulch. If winter stays mild, they may survive. Cut them then like the others.

How long to wait before planting. Sources do not agree. The Winter vetch sheet says 2 to 3 weeks, Germinance 3 to 6 weeks, GRAB 30 to 50 days in the field. Count 2 to 3 weeks before putting in young plants. Wait up to 6 weeks for small seeds or under a thick layer. If you have to plant right away, take the stems to the compost or use them as mulch elsewhere.

The limits of a green manure

In a planter

In a container, the small volume of soil limits the effect on structure. A container left empty over winter is also protected with a mulch of dead leaves or dried grass clippings (Natagora). You can also put a winter crop in it. Our sheet for the spinach Géant d'hiver Verdil says it can be grown in a deep balcony box in winter.

Compared with compost

A green manure breaks down fast. It makes little humus and mostly feeds the crop that comes right after (GRAB). It does not replace your compost. The two complement each other.

If your bed is freed in September, also read the guide What to sow in September in Belgium.

Further reading

• The collection: Green manures
• The Start page: Tell us which seeds you have, and we tell you when to sow each one where you live
• Three other guides: What to sow in September in Belgium, Sowing calendar for Belgium and A vegetable garden on a balcony in Belgium

Photo: Agnieszka Kwiecień (Nova), Wikimedia Commons, licence CC BY-SA 4.0