Autumn green manures: choose a cover and plan its end

A green manure is not a bag of fertiliser. It is a plant grown on a vacant bed to cover the soil, compete with some weeds and produce roots or foliage. In autumn, choosing one is only half the decision. The seed still needs to establish in the conditions you have, the plant may need to survive winter, and you will eventually need to end the cover in time for the next crop.

Start with the bed’s schedule

First work out when the existing crop will actually come out and when the bed will be needed again. A cover sown after an early harvest has longer to establish than one sown late in autumn. Rainfall, soil moisture and ground temperature matter more than a printed calendar. If the bed is dry, already cold, or needed soon, seed may struggle or complicate the rotation. A surface mulch or another form of cover may be a more reliable choice.

The RHS guide to green manures lists different sowing windows by species. Those dates reflect UK conditions, so treat them as horticultural reference points rather than a schedule to copy into a French garden.

Match the species to the job

Rye is worth considering when the bed will remain occupied for several months and the priority is winter soil cover. In its climate, RHS lists rye as a species that can be sown as late as November. It is winter-hardy, which also means it may not disappear at the first frost. Decide when you sow it whether you will mow, cut or otherwise manage it in spring.

Winter vetch or field beans can add a legume to the rotation, but do not promise an immediate nitrogen feed. Nitrogen fixation depends on plant activity and conditions; the RHS guide says legumes fix nitrogen in summer. A late-sown legume may therefore function mainly as ground cover, rather than replacing a considered fertility plan.

Phacelia or another frost-tender species may suit an earlier autumn sowing if it can germinate before cold weather. It is not an automatic late-season choice: a frost-killed cover leaves a different amount of biomass from a hardy crop that will need active termination.

Also think about plant families. Before a brassica, turnip or radish, do not choose a cover from the same family without checking rotation guidance and the seed-mix label. A green manure should fit the crops planned after it, not simply the empty space available today.

Termination belongs in the decision

A living cover can protect the surface, but it can also occupy space, use water as growth resumes, or set seed. The INRAE work on cover-crop diversity describes the trade-offs among species, weather, soil condition and the following crop. That work focuses on orchards. It helps explain the choices, but it is not a crop-specific vegetable-bed prescription.

In spring, cut the cover before it sets seed. Depending on the species and your gardening method, leave the cut material on the surface as mulch or incorporate it shallowly. If you dig in green material, do not sow straight away: the RHS recommends waiting about a month after incorporation to let decomposition begin. Treat that as an RHS benchmark, not a universal guarantee; a large amount of material or cool conditions may change the rate of breakdown.

A quick decision before buying seed

Ask three questions: will the bed be free long enough for the cover to establish? Does the species suit the cold and moisture in your garden? Can you terminate it before the next crop needs the space? If you cannot answer one of them, pick a cover that is easier to manage or protect the bare soil without a living sowing.

Use the vegetable sowing and planting calendar to compare the bed’s release date with your next crop. The guide to what to plant in October already mentions green manures; this article focuses on selecting and ending the cover. The crop-rotation guide can help you check what should follow.

Record these details in your garden plan and journal: when the bed becomes free, which cover you chose, when you expect to terminate it and what will follow.