The idea of green manuring is simple. We grow crops on the soil when it is not otherwise employed to later return them to enrich the soil. This costs very little except the seed, which can be self saved, and the effort brings many benefits. Sunlight energy and water is turned into biomass, the soil is protected from erosion and weeds are excluded. The only minor hitch is that green manuring has been developed mostly by farmers rather than by gardeners. So we have been recommended to use such delights as Hungarian grazing rye, tares and vetches, and clovers. Easy enough to incorporate if you have a forty horsepower tractor and a three furrow plough but a bit inappropriate and far too tough a job for the gardener!
These traditional green manures are rather fibrous, and prone to regrowth, thus they are difficult to dig in during spring when we want the soil And they take many weeks to decay before the soil is ready for most crops to be planted, potatoes and Jerusalem artichokes excepted. They were chosen as they have the ability to germinate late in the autumn, are hardy enough to grow through the winter, and are fairly cheap seed, tares and vetches were originally 'weed' seeds sifted out of the grain. They may suit the farmer or even the large scale horticulturist with power equipment but I reckoned there ought to be better options for the gardener.
First let's deal with the green manure's incorporation in the soil; digging is too much work. It has to be done too early in the year thus losing much of the succulent nitrogen rich spring growth. And for many of the traditional manures rerowth is a serious problem unless they're deeply buried. Their stems are tough and hoeing or cutting them off is not easy, neither is pulling them once they have been through winter. Of course they are all easily dispatched whilst small and succulent but not after they have grown older..
I find green manures can be killed off most efficiently and incorporated in situ with minimum effort by using a light excluding sheet of opaque plastic. If this is put in place then the leaves yellow, rot and are eaten by the soil life within a few weeks and turn into fertility. The roots expire in the effort of making more leaves -unless a leaf finds some light. This method works exceedingly well for little effort but has a few drawbacks; on my raised beds it shoots off rain and it encourages slugs. The rain loss is rarely a problem and the slugs are easily dispatched by removing the sheet early one morning and leaving them to the birds.
The plastic sheet can remain in place for some crops such as sweet corn and courgettes but for many it is best removed. Thus the sheet has to have totally killed, if not incorporated, all the green manure by the time the soil is required for sowing or planting crops. Rye grass is very fibrous and takes a little too long for convenience, and well established tares, vetches and clovers likewise and these regrow vigorously if uncovered too soon. So I have looked for alternatives that are quicker and easier to incorporate under plastic. (Of course cardboard, carpet or even layers of newspaper can be used instead.)
Ideally a green manure for winter use must germinate in the cool conditions of autumn, and preferably as late as possible. A multitude of plants could serve as green manures in the summer months but then the soil is rarely free of crops for long enough to be improved Obviously a good green manure needs to be hardy, vigorous and quick enough growing to exclude weeds. For economy it must set seed that's easy to save and it ought not be prone to any common problems. It also ought not be closely related or share problems with crop plants thus mustard, rocket and turnips which have their advocates are all a tad dubious as they are too close to brassicas to use regularly.
Clovers are frequently recommended, their performance varies very much with soil but they have the advantage of being legumes and fixing nitrogen. Unfortunately they are really a bit too slow growing to exclude weeds and to fit into a single winter of growth. Clovers give a much better return if they are left for longer so they are hard to fit into the annual vegetable rotation unless sown very densely and this is expensive. If however a bed of clover can be established and grown on over a longer period it is superb. So pathways amongst perennial crops such as raspberries and asparagus can be 'clovered' down for a year or two and then can be incorporated under plastic sheet or heavy straw mulches. (A year or two of flowering is great for the bees too.)
The Poached egg plant, Limnanthes douglassii, has long been recommended as a good attractant for beneficial insects. For that reason I grew it under my gooseberry bushes where it turned out more useful than I expected. It self sows and grows rapidly away in the autumn excluding most weeds. It seems to grow in any warm spell and if left to flower does not choke my gooseberries as they are each on a one foot leg. The Limnanthes dies down naturally in summer leaving the soil bare, after a raking, and perfect for ripening a dessert crop to perfection.
I then went on to try Limnanthes on the vegetable beds. It's brilliant you can strip it away and compost it incredibly easily leaving the soil bound with a mass of fine roots which do not regroup. Or you can cover it with an opaque plastic sheet and it rots down incredibly quickly in situ. You can even tear out patches to plant through. Shame it's not a legume.
Claytonia, or Miner's lettuce has been rampant in my cold greenhouse for years. This is unusual in that you will not get it to grow happily in warm bright places but in cool damp soils it grows like the plague. It flourishes under cloches or indoors but will also cover the soil well outdoors surviving all but the coldest winters. If pulled it leaves a fine root network holding the soil together which does not regrow and is easily incorporated under plastic sheets. It is not related to most crops and rather sadly is not a legume but it is a deliciously succulent edible salad crop at all times even when flowering. This makes it especially valuable and it is also loved by chickens!
Corn salad, Maché, Valerianella, is another unexpectedly useful green manure unrelated to most crops. I was growing a trial of salads and like Claytonia this showed a remarkable ability to grow quickly and suppress weeds. It stands the winter well and is easy to kill off though if pulled when flowering corn salad is inclined to bring the roots with some soil as well. The pale blue flowers are quite attractive and of course the leaves are edible when young and tender.
Chervil is another salad crop that can double as a green manure, it will stand moderate winters and produce a dense stand of foliage yet is easy to kill off by mulching whilst small. If left chervil gets tall with a carrot like root. Indeed this is it's misfortune -it is closely related to many of our other crops such as carrots and parsley so perhaps it shouldn't be over-used.
Several experimenters have found spinach a useful green manure but more for summer use. In cold winter areas such as mine spinach is killed by the hard frosts. In milder areas spinach may be very well employed, and again it is edible and unrelated to most crops. Spinach as a green manure is also claimed to enrich the soil in saponins (soap like compounds) which help it granulate.
Borage is reckoned to be very high in nitrates so it may prove exceedingly beneficial but I've failed to multiply enough seed and establish a big enough trial so far for a conclusive result.
Mustard is not very good at suppressing weeds but can be sown for a quick flush and hoed in. Left to stand it is killed by cold winters and it is hard to pull though it can be easily mulched out. Because it is related to the brassicas it is best avoided unless like me you want it as a crop for the condiment.
These winter green manures I've been trying are obviously improving the soil but it's early days yet. Obviously some are going to be of far greater value than others. It will be interesting to get some huge trials done and find out which green manure is the optimum to grow before each crop.