Going green - Lucy Halliday explains how your soil can benefit from the goodness of green manures
GREEN manures, once relatively uncommon on your average plot, are growing in popularity and it's not surprising. Catch crops have long been used to maximise the productivity of areas of soil that would otherwise be left bare between plantings of main crops. Manures and mulches have been used for centuries to nourish and enrich soil. The fantastic thing about green manures is that they combine these two great garden services in one and have other benefits to boot. So get growing a green manure this year and reap the benefits.
Green manures are special types of plants grown solely to improve the soil rather than to provide food or decoration (although some are quite pretty). They act like "in-soil compost" and once they have served their purpose they can be dug into the ground ready for planting the next crop. They all have particular characteristics which make them perfect for this purpose including nitrogen fixing properties, dense foliage which helps to suppress weeds or root systems which act to open up the texture of the soil. Green manures have so much to offer in improving the overall quality of your plot so here are my top six reasons for trying them out:
Pest control - It may not be their most obvious use but green manures can even help control many common garden pests by providing a cool, damp hiding place beneath their leaves for pest predators. The beneficial animals which appreciate green manures include Rove and Ground beetles which eat pest eggs and larvae, frogs which chomp on slugs and snails and harvestmen and spiders, which collect numerous pests. Green manures can also reduce pest attack by confusing the detection mechanisms of flying pests looking for their target plants when they are planted under and around crops.
Green manures can be fitted neatly in to crop rotations either to fill brief gaps between crops or to periodically provide an injection of nutrients to the soil. There are types suitable for sowing most times of year (see below) but autumn and winter are often good times to monopolise on otherwise bear soil. Green manures are best dug in at the very least a week before you want to use the patch. Generally you should leave from a fortnight to over a month, depending on how tough the organic material is, for the manure to decompose and settle into the soil before you commence planting or sowing. Young manures in warm soil from a quick-growing summer variety can be turned around very quickly but hardier overwintered manures will understandably take longer. Chopping the manure up thoroughly with a sharp spade as you dig it in will speed up the process. Ideally, especially with the quicker growing manures, you want to dig in the plants before they flower and set seed as they could interfere with your crops if left unchecked. However, the flowers of some green manures are great for attracting beneficial insects so you may want to leave them to bloom for a while.
There is such a range of green manures to choose from that there's bound to be something to suit your soil type and time scale, even in small plots. A little planning can allow several manures to be used throughout the year to help keep your soil in tiptop condition. Many of the manures are not related to any vegetable varieties (e.g. Phacelia) so can easily be used in a crop rotation without disrupting the order. The nitrogen-fixing green manures are best followed by cabbages or other leafy crops which will make best use of the extra nutrients. The list below is not exhaustive but will let you decide on a suitable type from eleven of the most commonly available varieties. You can even sow a mixture, especially on a resting patch of ground, to combine the beneficial properties of several types.
Faster growing green manures ideal for filling summer soil gaps include:
Buckwheat (Fagopyrum esculentum) - a half-hardy annual for sowing late spring to late summer which will mature in 1-3 months ,even in poor soils but does not fix nitrogen.
Crimson clover (Trifolium incarnatum) - a hardy nitrogen fixer which can mature in 2-3 months or be left to overwinter. Prefers lighter soils and can be sown from early spring to late summer.
Fenugreek (Trigonella foenum-graecum) - a half-hardy annual herb which thrives in well drained soil where it will fix nitrogen if suitable bacteria are present. Sow early spring to late summer and dig in after 2-3 months.
Mustard (Sinapis alba) - a half-hardy annual and one of the fastest maturing manures in only 1-2 months. Happy sown in early spring to late summer in any soil type but does not fix nitrogen.
Phacelia (Phacelia tanacetifolia) is one of the prettiest green manures which, if left to flower will produce purple flowers which are irresistible to many beneficial insects. Grows well in most soil types but does not fix nitrogen. Sow in early spring to late summer and dig in after 1-3 month, though may be left to overwinter when planted late.
Lupin (Lupinus angustifolius) is a good manure for light, acidic soils where it will help to fix nitrogen. Sow early spring to midsummer and dig in after 2-4 month growth.
Hardy green manures that can be used for covering ground over autumn and winter include:
Alfalfa (Medicago sativa) - a hardy longer term manure which can be left for a year or more. Alfalfa will fix nitrogen in soils where there are suitable bacteria but will not thrive in very acidic or poorly drained soils. Sow late spring to midsummer.
Essex Red clover (Trifolium pratense) - a pretty manure for use over 3 to 18 months depending on need. Sow spring to late summer in loamy soil where it will fix nitrogen.
Field beans (Vicia faba) - a great, hardy nitrogen fixer for overwintering and suitable for breaking up heavy soils. Sow autumn to early winter.
Hungarian grazing rye (Secale cereale) - a very common green manure, grazing rye is great for providing ground cover once summer crops have been harvested and keeps weeds down while improving soil structure well. Sow late summer to early winter in any soil type. Very hardy but does not fix nitrogen.
Trefoil (Medicago lupulina) - a hardy biennial with pretty yellow flowers, which can be left from 3 months to a year as it will reproduce using adventitious buds from the roots. Sow this nitrogen fixer from spring to late summer in all but very acid soils. Can be especially useful on light soils.
Tares (Vicia sativa) - a hardy nitrogen fixing annual which can be dug in after 2-3 months or left to overwinter. Sow from spring to early autumn but avoid very acid or poor draining soils.
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