Any organic gardener worth his or her salt should be aware of green manure - essentially one of a number of plant species that can be grown and then dug into your soil to improve its quality or nutrient levels.
For many years I would simply leave my plot bare over the winter months and it was not until I read a simple sentence in a gardening magazine that I became a green manure avocate. The sentence read, 'Plant green manure; you don't want your precious soil left bare'. I pictured the dust bowls of the 'dirty thirties' in America and decided that my allotment's soil was indeed very, very precious.
It is unlikely that our allotments will become dust bowls, or our back gardens barren wastelands, but erosion and drying can be the result of leaving your soil open to the elements. And if you're not using green manure, you're making more trips in the car to the garden center or the nearest stables to pick up manure or compost.
Green manure can provide nitrogen for the soil and will also increase levels of oh-so-important organic matter, improving topsoil depth, water-holding capacity, nutrient content, friability, and soil texture. What's more it can be very cheap, especially if you save the seed year after year.
Another added benefit is that when the ground is covered, weeds find it harder to take hold. Imagine green manures as a good friendly house sitter, one that feeds your cat and puts the rubbish out on the correct day and deters any burglars. Indeed, even wildlife will thank you for growing green manures.
The frogs and beetles really enjoy a layer to hide amongst on my allotment, and they have thanked me by devouring some of the pests that can be problem. In fact I am thinking of opening a frog hostel on my allotment as there are so many living there now, especially considering how harsh the winter has been.
It can seem like a bit of a minefield when considering sowing a green manure as there are many different sorts with varying characteristics, useful for many different applications and sown at various different times. But don't be put off, just consider your soil's needs and you will find the correct green manure below to suit it...
Mustard (Sinapis alba)
Mustard is great if you want a quick-growing green manure, and can be sown from March right through until about September time (depending how far north you are). When grown late the bees seem very thankful for flowers during the dying months of the year. It grows very quickly (in about 1-2 months) and is particually good for soils that lack organic matter due to its fibrous content, which helps give structure to fine soils when dug in. What's more, as mustard leaves are edible they can be used as crop too.
I tend to follow a potato crop with mustard seed as it can help rid the area of wireworm. Grow until the first flowers appear then dig in. Sow thinly in 15cm rows, 3-5g per sq metre.
Winter Tares (Vicia sativa)
Sow in the spring, late summer or early autumn and it will be ready to dig in after 2-3 months. It won't give much ground cover if sown in the autumn as it grows slowly so don't expect it to keep the weeds down for you. Winter tares is not a big fan of acid or overly dry soils but will tolerate most others, and will fix nitrogen in the soil. It should be sown, not broadcast, at a rate of 20g per sq metre.
Alfalfa (Medicago sativa)
Alfalfa is in the pea family and might already be known by many as one of the seeds in sprouting mix. Sow between late spring and the middle of summer - it will take about a year or more to grow. Indeed given the optimum condiditons it can live for up to 12 years!
This makes alfafa an ideal crop to sow on a patch of earth that may not be cultivated for some time. Many allotment societies will have a rule concerning how much of a plot must be used to be deemed to be under cultivation, and, as alfalfa can be sown and left, it will be considered by most as using a plot, thus keeping the officals off your back! However, it should be cut back two or three times over a year to encourage new, fresh growth. Broadcast at 2-3g per sq metre.
Phacelia (Phacelia tanacetifolia)
Phacelia is a beautiful plant with vivid blue flowers and can also be grown on a wildlife-friendly plot in order to attract bees and hoverflies. When used as a green manure it should be cut down before it flowers. It can be sown in early spring and then dug in after about two months or planted in late autumn to be dug in during spring.
Phacelia will thrive in most soils and as it is a bushy plant it should also act as a weed retardant. It is not frost hardy and many of my plants have died off during this cold winter (2009-2010), however it has kept the weeds down and the dead plants have now been dug in. I will allow one or two of the surviving plants to mature before cutting back in order to obtain seed for the next season. Broadcast sow at a rate of 2g per sq metre.
Hungarian Grazing Rye (Secale cereale)
I have sown this in October before, but only because I live in the south of England. Further North it will be safer to sow by September. Hungarian grazing rye is left over winter and its dense, grass-like foliage is enough to frighten off most weeds. Sow thinly in rows 20cm apart - or broadcast the seed at a rate of 16g per sq metre.
Trefoil (Medicago lupulina)
Sow in spring to late summer and Trefoil will make a good ground cover for the winter months. It will take around 3 months to grow and will withstand most light and dry soils. Trefoil will not tolerate acid soil. It is a low-growing plant and can be intercropped with taller crops such as maize.
Andy Hamilton is co-author of The Selfsufficient-ish Bible (£20, Hodder & Stoughton)
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