To provide initial control over the environment, place a box with a lid of glass on ordinary soil. This cold-frame environment helps to increase soil temperatures, reduce temperature fluctuation, maintain humidity and allow light penetration, and it can be used for the propagation of a wide range of hardy plants. Its main disadvantage, which is shared with all enclosed environments, is that air temperatures build up when conditions are sunny. This necessitates either airing the frame to reduce thetemperature, and thereby losing humidity, or shading the glass to cut down the light input, and so reducing photosynthesis.
There are many plastics substitutes used in place of glass, but because of their heat/light transmission characteristics they are less satisfactory in the late autumn to spring period as they do not conserve heat so effectively as glass.
The most manageable cold frame to construct is made with "Dutch lights", which are single panes of glass held in separate wooden frames 4 ft 11 in long by 2 ft in wide. These can be laid side by side across a base frame with a distance between backboard and front-board of 4ft 9 in. For propagation the backboard is best made at a height of about 12 in and the frontboard at 9 in. The slope of the roof should be pitched in a southerly direction. The cold frame can be made more reliable by improved sealing of any cracks in the structure and by double glazing with two layers of "lights"—the lid of a cold frame being called a "light".
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