Overwintering Green Manures for Crop Rotation

At this time of year, patches of my garden start to look bare as crops are cleared. Left unclothed, winter rains and snow will wash out nutrients and strong winds will strip away topsoil, leaving the earth in poor condition by spring. Naked soil is not an option! Fortunately, there's a solution.

Sowing green manures, also known as cover crops, is a time-honoured way to protect soil over winter, while suppressing weeds and providing habitat for ground beetles, frogs and other pest predators. When these green manures are dug in at the start of the new growing season they will help to improve soil fertility and structure. Knowing what to sow late in the year, after edible crops have finished, will help you to give your soil a much-needed boost.

Overwintering Green Manures and Crop Rotation

Late in the season in my cool, northern European climate, I have only a few options. There are just three types of green manures up to the job: legumes such as winter field beans, forage peas and vetch (aka winter tares); mustard, which is a plant of the cabbage family; and grazing rye (Latin name Secale cereale). Lucky gardeners in milder areas can add phacelia and clovers to their list of autumn-planting green manures.

Sticking rigidly to a crop rotation plan while including one of these few overwintering green manures can become a real headache, so it's necessary to bend the rules a little. Just follow this one, simple guideline: grow a green manure that will leave a beneficial effect on the next crop.

Improving Soil Conditions With Green Manure

Fleshy legumes such as field beans and vetch fix the essential nutrient nitrogen from the air then rapidly release it back into the soil when they are dug in for the next crop to use (brassicas, for example, love a boost of nitrogen).

It's important to note that legumes only fix nitrogen when the soil temperature is above 8°C (46°F), so they may be less effective where winters are cold. Nonetheless, they will provide some protection for the soil, although mustard or grazing rye is more effective for this purpose. Legumes are fairly trouble-free, so using them as an overwintering crop does not normally cause problems within a crop rotation.

Mustard is exceptional at smothering weeds because it forms a dense, leafy growth very soon after sowing. Mustard, particularly 'Caliente' mustard, is also a a biofumigant, which means it may be effective in controlling some pests and diseases that affect other crops – for instance wireworm or potato eelworm. Grow it after your brassicas (unless they've suffered from clubroot, in which case avoid using mustard as a green manure at any point in your crop rotation as it can harbour the disease).

Mustard is not frost hardy, so it will be killed off after a hard frost. This is not necessarily a bad thing, as this will make it easy to incorporate into the soil in spring. You can often grow mustard within a greenhouse or tunnel right through the winter, though if it flowers the stems will become tough and harder to dig in.

Grazing rye and phacelia are useful green manures that don't belong to any of the common crop families we gardeners use, so they shouldn't compromise a crop rotation scheme. Phacelia has to be sown quite early in many areas to allow it to establish, and it won't survive a cold winter. Grazing rye on the other hand can be sown later than any other green manure, making it invaluable for following on from autumn harvests.

Grazing rye has several other benefits. Above ground, it produces a dense canopy of foliage that helps keep weeds in check. Below ground, it has an extensive, fibrous root system that loosens soil, laying the groundwork for root crops such as carrots and parsnips to follow. Grazing rye is unsurpassed among green manures in its ability to hold onto soil nitrogen, refusing to let it leach away, before releasing it gradually for the next crop once it's dug in.

It's important to cut grazing rye down at the right stage – too early and it may regrow (some gardeners cover it with black plastic after digging it in to help kill it off), while too late and it will produce a lot of dry material that's slow to decompose.

Be aware that green manures can produce chemicals that inhibit seed germination. Allow four to six weeks between digging in a green manure and sowing seeds.

Sample Overwintering Green Manuring Plan

Below is a sample crop rotation schedule showing which overwintering green manures to follow on from which edibles. Following this schedule should leave the soil in great condition for the next crop in the list.

This is just a guide, so feel free to experiment and find out what works best in your garden.

We'd love to hear your tried-and-tested ways to incorporate overwintering green manures into your crop rotation plan. Share them with us by dropping us a comment below.

By Ann Marie Hendry.

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