Microclimate under a Cloche
The function of the cloche is to trap sunshine.
Always, therefore, try to have your cloches in a sunny position. Cloches cannot manufacture heat. They simply trap the sunbeams and it is therefore particularly important that they should be sited where the sun can reach them for as many hours of the day as possible. The most spectacular feat which cloches accomplish is the protection which they give against frost, particularly late spring frosts.
These late frosts invariably occur after a warm, sunny day during which the heat of the sun has warmed the soil under the cloches. At night, when the temperature falls, the air under the cloches is kept warm by the heat of the surface soil and prevented by the glass from escaping. As a result, crops under cloches will be perfectly safe, for there may be as great a difference in temperature as fifteen degrees between the air outside and inside the glass.
The glass itself will, however, fall to freezing point, and the air within half an inch or so of the glass will be very cold. It is most important therefore that leaves and flowers of crops under cloches during the period when night frosts are expected should not actually touch the glass. Always use a large enough cloche for the particular crop you are growing.
During the coldest months of the year the difference in temperature outside the cloches will be very much less than in the late spring; it may be as little as five degrees. The reason is that there is little or no sunshine during the daytime so that the soil never gets properly warmed up.
Nevertheless, cloches do protect crops growing underneath them from the heaviest frost. The foliage is kept dry, as no rain or mist can fall on it. Plants can stand up to a great deal more frost if they are dry rather than wet. Secondly, the tilth of the soil under the cloches is retained; this tilth forms an insulating layer through which frost cannot penetrate to any considerable depth. Outside the cloches the soil becomes caked hard and the ground freezes many inches down; every gardener knows that after heavy frost it is impossible to dig the soil without using a pick or crowbar. Under cloches, however, the soil can easily be moved with a trowel even after the most prolonged and severe frost.
