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Green manures: the what and the why

As autumn approaches, it’s the ideal time to consider sowing a green manure. It might not sound that pleasant, but a green manure is simply a crop which is sown not for the purpose of harvesting it, or for enjoying it as an ornamental plant, but as a way of improving the soil.
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Green manures: the what and the why
Show more
Green manures: the what and the why

As autumn approaches, it’s the ideal time to consider sowing a green manure. It might not sound that pleasant, but a green manure is simply a crop which is sown not for the purpose of harvesting it, or for enjoying it as an ornamental plant, but as a way of improving the soil.

So what’s the theory behind green manures?

Essentially, if you’re likely to have bare soil for any length of time (for example over the winter), it’s better to have it covered than exposed. Foliage will help keep weeds at bay, while the roots help maintain the soil structure, preventing erosion by winter rain and wind. Sowing fast-growing plants as green manure covers the ground quickly - in a matter of weeks. When you want to use that ground again, dig the young greenery into the soil, around three weeks or so before you're planning on sowing. As the green growth decomposes, it feeds nutrients back to the soil, ready for you to plant into.

There are many suppliers of green manure seeds – you can readily buy them online and they’re also available in garden centres. Crops used as green manures are quite varied, including familiar flowers and veg, as well as some more offbeat plants less often seen in the domestic garden. Green manures include lupins, clovers, some types of bean, mustard, alfalfa, buckwheat, vetches and many more. Like any plant, they all have different qualities, growing habits, and soil preferences, so take some time to research the right type to use for you and your garden.

What are the benefits of green manures?

Green manures improve both sandy soil and heavy clay soil by adding organic matter. Some (like lupins) are very deep-rooted, sending taproots deep into the soil which help to improve the soil’s structure and drainage.

Another interesting characteristic of some green manures is that they help fix nitrogen in the soil. Crops from the legume family such as winter field bean or red clover will do this, helping to improve the soil’s fertility.

A flowering green manure crop like red clover or phacelia also brings significant wildlife benefits, too – although less so over the winter. But if planted over the summer and allowed to flower, it will be buzzing with pollinators.

Who uses green manures?

Green manures are often used on the plot by grow-your-own gardeners. These crops can cover and feed ground left unused, not just over the winter, but between main crops, too. For example, after early-season crops like spring onions or early potatoes have been harvested, a green manure will revitalise the ground, ready to plant winter veg later in the summer. They can also be planted in alternate rows between crops like sweetcorn to help suppress weeds, then dug in after the main crop harvest.

You can also use green manures if you prefer no dig gardening, although of course without being dug into the soil they’ll take significantly longer to break down ready for planting your crop. They can either be cut down and left on the surface as a mulch, or left to die back naturally. They may need to be covered to speed up the process.

Things to watch out for with green manures
Rotation, rotation, rotation: Some green manure crops come from the brassica family – mustard is one example. These shouldn’t be followed immediately by other brassicas, such as cabbage, as it could create the conditions for diseases like clubroot to take hold.
Pesky pests: A green manure can give pests like slugs and snails a place to hide, so be vigilant and take pest control measures as the weather warms up in the spring – and especially if you’re using it to cover ground between rows of other crops.
Right plant, right place: some green manure crops can take a lot of effort to dig in, so choose one which is both right for your soil and appropriate to your love of digging (or otherwise). Ryes, tares and vetches are said to be tough going; fenugreek and mustards are generally held to be an easier option.

So if you’re getting into grow-your-own, get into green manure as well – and now is the perfect time to give it a go.

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