by Keith Cowling

Now that the summer solstice is behind us, allotment sites are bursting with beans and peas, new potatoes, salads and early soft fruit, and the smell of barbecues pervades weekend afternoons.

The plots that seem to have coped best with the Spring drought followed by a cool wet June have been those with a high humus content in their soil. Most allotmenteers aspire to grow in humus- rich soil, of course, but achieving this on Bristol’s sticky clays can take years of dedicated cultivation and the hauling of much compost, manure and leaf mould.

But fertility and humus content can also be built without imported nutrients using green manures that grow right on the plot from seed or plant offsets.

Green manures return nutrients to the soil

In essence, green manures are plants that are grown primarily to add structure and nutrients to the soil rather than for food. A wide range of plant types are used, especially by organic farmers and gardeners, but several species are particularly useful on allotments to fix nitrogen, gather nutrients from the subsoil, build humus and deal with weeds. Green manures compete for space on crowded plots, so allotmenteers particularly value those that grow well over the winter period, when most food crops have finished and space is at less of a premium. Foremost among these is Hungarian grazing rye. Not the rye from which Ryvita is made, but a variety called Lovazpatoni, specially bred for animal fodder in a climate colder than ours.

Rye is sown after food crops are cleared in the late Summer and early Autumn. The end of the first week in October seems to be an effective last date in Bristol. It will then grow right through the winter cold and reach a height of around 60cm by the following mid April, suppressing early weeds. By then, the nitrogen-rich shoots can easily be dug in before new potatoes, spring cabbages or early carrots. Plot holders with room to spare can leave it growing and take two cuts for the compost heap and dig in the roots in August.

Alternatively, a small area left uncut will produce seed for next year and the experience of a mini grain harvest with a grass hook. Other over-Winter crops can be sown specifically to fix nitrogen and the most useful of these is perhaps winter tares, a member of the legume family which fixes nitrogen from the air in nodules on its roots. Tares grow a good bulk of material between late Summer and early Spring, but they are not so good at suppressing weeds as rye and are loved by rabbits when food is scarce.

Green manuring does not sit too comfortably with no-dig systems of gardening but it is possible to cut rye and tares as shallow turfs, which are then merely turned over and covered with a light top dressing without disturbing the main soil structure. Another species that involves no digging is Russian Comfrey, which is planted as offsets and remains in a permanent bed year after year, producing three or four cuts of potash- rich foliage annually. Comfrey can use really raw sources of nitrogen, like pigeon manure or human urine, and convert it into an ideal fertiliser for potash-hungry crops like potatoes, tomatoes and soft fruit. The unique qualities of comfrey were pioneered by the charity the Henry Doubleday Research Association – now called ‘Garden Organic’ – and lots of information on comfrey cultivation and use can be found on its web site.

Summertime species

For plot holders with plenty of room, or those that fail to get the whole plot planted up with vegetables, there is a range of plants suitable for green manure use over the summer season. Mustard is the traditional Summer green manure for cottage gardeners, growing more bulk than any Summer weed and setting seed very late. It is therefore ideal for growing bulk humus in its 10 week growing cycle, to incorporate into your soil. Mustard is also useful in combating the build up of eel worms in the soil, that attack potato crops, but it is also a member of the cabbage family so a risky crop in a plot that has a known club root problem.

Other Summer green manure species include phacelia, agricultural lupins and buckwheat. All have good weed- suppressing qualities and attract bees and hover flies, which are important green fly predators. But perhaps the most interesting species is tagetes minuta, a relative of the french marigold. It is not really ‘minuta’ at all, growing often to a height of two and half metres, so is strictly more of a compost crop than a true green manure. Its interesting quality however is that its root secretions are able to suppress pernicious perennial weeds. It is particularly effective against ground elder, convolvulus and couch grass, some of the most intractable weeds that infect Bristol allotments. The seed is broadcast thinly over the plot and when the subsequent plants reach 2 metres they can achieve a complete kill of these problem weeds without the need for chemicals. It also has a useful soil-conditioning effect on heavy clays and delivers a mass of material for the compost heap.

Where to buy seeds

Most green manure seeds, including all those mentioned, are available from Garden Organic.

Riverside Garden Centre also stock a good selection.

Keith Cowling EMail
Ashley Vale Allotments Association

This article first appeared in in Bristol’s local food update, July-August 2011