Autumn is on the way, and it’s a good time to think about making new beds. More dauntingly, you may have just taken on a whole new garden or an allotment. As you dream longingly of friable loam, your spade invariably hits builders’ rubble, free-draining sand or compacted clay. While there is little choice except to dig out builders’ rubble, there are more options when it comes to the task of enriching the soil. Recycled green waste, animal manure and spent mushroom compost are popular additions, but before you start despairing at the cost of lorryloads of compost, don’t forget the cheapskate’s alternative: green manure.
Green manures or cover crops are an inexpensive way to improve both the structure and nutrient content of your soil. Cultivated primarily to add organic matter to the earth, a green manure crop is usually grown for a specific period (say six weeks, or over winter), and then chopped down and incorporated into the soil. They are a great option when preparing new land for use – forming a green carpet that stops weeds colonising and prevents nutrients from being washed away. Some varieties, such as lupins, fodder radish or alfalfa, even help to give the soil structure because their fibrous tap roots break up the subsoil.
Traditionally, green manures have been used on arable land or allotments, but they are equally useful in gardens. All you need is a packet of seeds, a spade, and patience while they do the hard work for you.
Green manures consist of legumes and non-legumes. Legumes are popular for their ability to fix nitrogen: once the green manure has been dug into the soil, this nitrogen becomes available for future plantings. Non-legume plantings, however, often produce more organic matter. They have a better root system and so are better suited to surviving extreme weather conditions and poor soils. There is a green manure for every ailment: red clover suits sandy soil; field beans and grazing rye are excellent in clay; lupins improve the substructure of the soil; while the best choice for dry soil is alfalfa.
Fast-growing, leafy green manures are often recommended for gardens because they provide the most nutrients when dug into the soil. Just remember, if you are preparing soil to grow vegetables, your green manure should not be closely related to the plants that are to follow. For example, don’t use legumes such as field beans if you are planning to grow runner beans or garden peas afterwards. This reduces the likelihood of disease.
Although many green manures can be sown all year round, it is ideal to sow them in the autumn to overwinter, as vegetable plots are generally empty and people are spending less time in their gardens. Try sowing fast-growing fodder radish or mustard in early September so that it can be incorporated in late October. Alternatively, cut down and chop up the green manure in late October and leave it on the surface as a mulch.
Italian ryegrass and Hungarian grazing rye can also be sown in September because they are very hardy. Leave them growing all winter before being dug in during spring to release nutrients as they rot. If you are growing your green manure on clay soil, however, it’s best to dig it in during autumn. Alfalfa is another good choice for an overwintering green manure. This nitrogen fixer should be dug in during the spring.
Before sowing your green manure, the site should be roughly dug over and weeded. The soil needs to be firmed by lightly treading it down, and then your seeds can be sown in rows about 6in apart, or broadcast and raked in.
All green manures should be dug in while the stems are still soft so that the plant can break down easily. It is a good idea to chop up the green manure and allow it to lie on top of the soil for a few days. It should then be dug in to a depth of about 6in. Your soil should be ready for planting only a couple of weeks after incorporation.
Green manures are often overlooked by gardeners looking for a quick fix, but used wisely, they can play an important and cost-effective role in the establishment and maintenance of a garden.
Hungarian grazing rye. One of the best green manures for winter use. Sow in August/September, or as late as October in the South. A good cover crop to prevent nutrient-leaching. Dig in during the spring.
Field beans. Very hardy, with deep nitrogen-fixing roots. Suitable for overwintering.
Fodder radish. Deep tap root brings up nutrients from the subsoil, and produces plenty of organic matter. Over winter it will gradually die down, making it easy to incorporate into the ground in spring.
UNDERSOWING AND INTERCROPPING
White clover. Low-growing for ground cover and intercropping between rows of vegetables. Use as a living mulch or long-term green manure.
Trefoil. Shade-tolerant and low-growing, so great for undersowing. Sow between March and August and it will stand through the winter into the following season. It does not like acid soil but will tolerate some shade and is suitable for undersowing, for example between rows of sweetcorn. When the corn is cut down, the trefoil will grow to protect the ground over winter.
MULCHING AND WEED CONTROL
Crimson clover. A vigorous, quick-growing clover with wonderful red flowers, which bees love. Sow April-September. Not normally winter-hardy because it dislikes waterlogged and heavy soils.
LONG-TERM GREEN MANURES
Alfalfa. Rich in calcium and the major elements – almost a complete natural fertiliser. It is very deep-rooting – improving substructure and bringing trace elements to the surface. Sow April to July. Can be turned under in the autumn or left to stand through the winter.