Garden Hot Beds and Heated Propagator
The most profitable way to use a garden-frame is as a hot-bed for producing early salads and vegetables while they are scarce and expensive to buy.or may be grown in it after the early salads and vegetables have been harvested, or quick-maturing turnips such as Early Snowball may be sown.
Almost any type of frame can be used, but obviously the better it is constructed the more efficient it will be. The two most popular types are the 6 by 4 ft. English light frame, and the two-light Dutch light frame. Both these cover about the same area, but Dutch lights are easier to manipulate than the heavy English lights.
Thus the use of electricity transforms the ordinary garden-frame into a miniature greenhouse. It lacks headroom, of course, but even this limitation is turned to advantage by making it very economical on fuel. The actual running costs will vary according to the temperature maintained, but an average frame will use about 1-½ units of electricity a day.
To construct a hot-bed the wire or cable is laid in the frame and covered with 6 in. of. Lay the wire so that it covers the area as evenly as possible, giving an electrical loading of 6 watts per sq. ft. To find the loading required, the area of the frame is multiplied by 6. So for a 6 by 4 ft. frame, which, of course, has an area of 24 sq. ft., the loading would be 144 watts. Thus a 150-watt soil-warming cable or transformer would be suitable.
With a loading of 6 watts per sq. ft., 10 hours of warming each night would be sufficient in the south of the British Isles, but in the north 12 hours would be required. A time-switch can be fitted which will do away with the need to switch the unit on and off each day.
Many plants can be propagated in a soil-warmed frame when the early salad crops are finished. Remove the soil down to the wires and replace it with a 2- to 3-in. layer of sharp sand which will conduct warmth to the pots and boxes of seedlings and cuttings. Most of the half-hardy annuals, and vegetable seedlings such as celery, leeks, cauliflowers and lettuce, can all be raised in this way. So can, , and a host of others. To give them an extra boost during germination, leave the current on the whole 24 hours instead of switching off each morning, and cover the frame with a mat each night to conserve the heat.
Air warming will broaden the scope of the frame and increase its usefulness considerably. Mains voltage, plastic-covered soil-warming cable, fixed round the inside of the frame on porcelain or plastic cleats, is usually used for such an installation. Copper-covered, mineral-insulated cable can also be used, and makes a neat, robust, effective installation which should give many years of trouble-free service. To conserve heat a frame must be well constructed and air-tight; during very cold weather, loosely-filled sacks of straw placed round the outside of the frame will help to keep in the heat.
In the interests of economy, air warming should be controlled by a thermostat which will switch off the current as soon as the inside of the frame reaches the required temperature. For an average frame with brick sides, or timber an inch or more in thickness, the electrical loading required will be 15 watts for every square foot of glass. For a 6 by 4 ft. frame, the loading will be 360 watts; a 300- or 350-watt soil-warming cable would be suitable.