Green Manures and Cover Crops

Growing your own fertility in the gaps

5 minIntermediateUpdated 2026-10-23 Green Manures and Cover Crops

You will learn

Choose between a nitrogen fixer and a biomass builder
Sow at the right rate and cut at the right moment
Plan a winter cover for a Mediterranean plot

⚠️ Wear eye protection and stout boots when cutting a dense stand with a strimmer or scythe.

What a cover crop does

Bare soil leaks. Over a wet winter an uncovered bed can lose 30 to 50 kg of nitrogen per hectare to leaching, plus topsoil to rain splash and erosion. A cover crop holds that nitrogen in living tissue, feeds soil fungi through living roots, breaks up compaction, smothers weeds and returns organic matter when cut. A good stand produces 2 to 4 kg of fresh material per square metre, which composts down to a useful but modest amount of humus, so treat green manures as nitrogen retention and structure rather than as a substitute for compost. The rule is simple: any bed that will be empty for six weeks or more gets sown with something.

Fixers versus builders

Legumes host rhizobia bacteria in root nodules and pull nitrogen from the air. Field beans fix roughly 100 to 150 kg of nitrogen per hectare over a winter, about 10 to 15 g per square metre; crimson clover 80 to 150, vetch 90 to 150, lupins 100 plus and they also mine phosphorus from acid soils. Sow legumes before a hungry crop such as brassicas or sweetcorn. Non-legumes build biomass and catch nutrients instead: grazing rye produces the most root mass of anything you can sow and roots to 1.5 metres, phacelia is fast and superb for bees, mustard grows in four weeks but is a brassica so keep it out of the brassica rotation slot, buckwheat thrives on poor soil and flowers in six weeks. Mixes of one legume and one grass usually beat either alone.

💡 Cut open a root nodule. Pink inside means the bacteria are actively fixing nitrogen.

Sowing rates and timing

Rough broadcast rates per square metre: field beans 20 to 25 g, vetch 10 to 15 g, crimson clover 2 to 3 g, grazing rye 15 to 20 g, phacelia 1 to 2 g, mustard 2 to 3 g, buckwheat 5 to 10 g. Rake the surface, broadcast in two passes at right angles so coverage is even, rake in lightly and water if dry. Sow winter covers four to six weeks before your first frost date so they establish; sow summer gap-fillers such as buckwheat or phacelia any time there is a six-week window. Do not let mustard or phacelia set seed unless you want them forever.

Cutting and the wait before planting

Cut at or just before flowering, which is the peak of nitrogen content and before stems go woody. Chop it down with shears, a scythe or a strimmer, and either leave it on the surface as mulch, the no-dig way, or shallowly incorporate the top 5 cm. Then wait. Soft green material such as phacelia, clover or mustard needs two to three weeks before you sow or plant, while coarse stemmy material such as rye or field beans needs four to six weeks. Sowing too soon into decomposing residue causes poor germination, because breakdown temporarily ties up nitrogen and some residues, rye especially, release compounds that suppress small seeds. Transplants cope better than direct sowing.

💡 Cover the cut residue with a sheet or cardboard for three weeks to speed the breakdown before planting.

A Mediterranean winter plan

In Portugal the growing window for a cover crop is the wet season, not the summer. Sow in October, once the first real rains arrive, and you get free growth on free water. A good mix is field beans at 20 g per square metre with grazing rye at 10 g, or vetch with oats at similar rates; both germinate in autumn warmth, sit through the mild winter and bulk up in February. Cut in late February or March at flowering, at least four weeks before planting tomatoes, peppers and squash in April or May. On beds you want free earlier, use phacelia, which is quicker and softer. Leave the cut material as a surface mulch and it will also shade the soil going into the heat.

Check yourself

Which of these fixes nitrogen from the air?

Field beans are a legume and host rhizobia in root nodules, fixing roughly 100 to 150 kg of nitrogen per hectare over a winter.

When is the best moment to cut a green manure?

Nitrogen content peaks at flowering and stems are still soft, so it breaks down quickly and does not go woody.

How long should you wait after cutting coarse rye before sowing seeds?

Coarse stemmy residue ties up nitrogen as it breaks down and rye releases seedling-suppressing compounds, so allow four to six weeks.

Why keep mustard out of the brassica slot in your rotation?

Mustard is in the cabbage family, so using it there defeats the purpose of the rotation and can build up clubroot.

Try it for real: Cover every empty bed this autumn

Walk the plot and list every bed that will be empty for six weeks or more
Order or save seed for one legume and one grass, such as field beans and grazing rye
Rake, broadcast in two crossing passes at the rates given, rake in and water
Mark the sowing date and the intended cutting date on your plan
Dig up one legume plant at flowering and check the nodules are pink inside