How to Mulch When Growing Vegetable Crops
Mulching is one of the most ancient horticultural techniques, utilized over the centuries by such highly skilled gardeners as the Arabs and Chinese. It was practised in England long ago: there are thirteenth-century records in Norwich of labourers paid for ‘thatching’ fields with straw.
While mulching has enabled crops to be grown in what would normally be considered impossibly hostile conditions, in places with exceptionally low rainfall, and on rocky, barren or salty soils, it also enables better crops to be grown under everyday conditions in ordinary gardens.
What Exactly is Mulching?
It is simply covering thesurface with a protective layer, a barrier between the soil and the air. Imitating nature, you might say, for in nature you rarely see bare ground. It is constantly being ‘mulched’, either by falling leaves and decaying plant debris, or by a living mulch of plant growth.
The main purpose of artificial mulching is to conserve moisture in the soil by preventing or reducing evaporation. Vegetables, especially leafy vegetables, are thirsty crops, and in most years most vegetables would give higher yields if they could have more water. Although we think of Britain as having a high rainfall, a tremendous proportion of it is lost through evaporation.
In the average summer the top few inches of the soil dry out through the effects of wind and sun. Not only does this deplete the reserves of moisture in the soil, but plant roots can only extract the nutrients they need — most of which are in the surface layers of the soil — from soil which is moist. When the soil becomes dry they are effectively deprived of both water and nutrients. Mulching prevents this happening, so growth can continue unchecked. It also, of course, prevents soil erosion, and is a very effective means of controlling.
What to Use and When
The material used for mulching can be organic — seaweed, peat, or gardenfor example; an inert substance such as sand, gravel, stones or clinker; or manufactured material such as plastic film. In practice it is largely a question of what can be obtained without undue cost or effort.
On the whole, organic materials are best for mulching vegetables: when their mulching role is over, they can be dug into the soil to increase its humus content. Among suitable candidates are garden compost, spent mushroom compost (probably one of the very best mulching materials), seaweed (also excellent), very well rotted farmyard manure, debris from old haystacks, old reed straw and old hay, deep litter from poultry farms, spent hops, leafmould, bracken and lawn mowings. In short, you can use anything derived from living matter which will rot.
A cautionary note on lawn mowings. If fresh lawn mowings are put on in too thick a layer they tend to heat up when they start to rot, forming a thick unpleasant mass. Either let them dry out for a day or two, turning them to help the process, or put them on in layers of a couple of inches or so at a time.
Wood derivatives, such as sawdust, and pulverized or shredded bark, should also be used with caution in vegetable gardens. They break down slowly, robbing the soil of nitrogen in the process.
An important factor in choosing a mulch is texture. The ideal mulch should be slightly loose when settled, so that rain and air can penetrate through to the soil. Peat, though excellent for controlling weeds, is not very suitable for mulching vegetables since it absorbs rain on the surface and re-evaporates it, with the result that the soil remains dry beneath.
In Britain, sand, gravel and stones are rarely used for mulching, though there is scope for doing so on a small scale, for example in greenhouses. Moisture evaporates very rapidly from greenhouse soil during the day, and a mulch of sand or stone, 8cm (3in) deep around plants, can be invaluable for conserving moisture. Moreover, the heat absorbed by the mulch during the day is thrown out at night, so raising the greenhouse temperature a degree or so.
The important thing to remember about mulching is that it preserves the status quo. So never mulch when the soil is very dry, very wet or very cold. It will remain that way. The best time to mulch is in spring or early summer, when the soil has warmed up but is still moist.
It is easiest to mulch when. Brassicas, , pumpkins, tomatoes, lettuces and beans, for example, can be given a good watering when planted, and then mulched. In many cases no further watering will be necessary.
For crops sown in situ it is best to delay mulching until theare through the soil and at least a couple of inches high. Otherwise they may be smothered by the mulch, especially if birds come and scratch about in it, as sometimes happens.