Cold Frames and Heated Frames
Frames — cold and heated
With the advent of fan ventilation the greenhouse temperature can in fact be lowered to such an effective degree that acclimatization can take place in situ without recourse to the frame. But although acclimatization is the main role of a frame, it can also be used very effectively as a production unit in its own right, like a small greenhouse — especially the elevated form with hessian sides used by specialist flower growers.
Sophisticated types of frames, some of which are on rollers and capable of easy movement, can in fact achieve results commensurate in many ways with a greenhouse. In design, shape and orientation the same basic principles apply. Ventilation and watering are two problems, the former being achieved by lifting the sashes and the latter either by hand or by the use of low level spraylines. Heating can be by any of the methods discussed on Greenhouse Heating Systems, the heat loss being calculated in a similar manner to that for , although with a brick base wall. For all practical purposes the square area in metres or feet of the frame sashes multiplied by 7.94 (w/m2 °C) or 1.4 (Btu 2h°F) and ignoring the sides will give a fairly accurate figure. It is very easy to put the required amount of piping, mineral insulated cable or tubular heaters into the frame. Soil warming cables are also used at a loading of 80watt/m2 (771 – watts per sq ft), either by a mains voltage cable led in at 20-23cm (8-9 in) depth or by low voltage lines through a transformer.
Types and construction of frames
For many years the 1.8 x 1.2m (6 x 4ft) light sashes were standard items, but their weight plus their upkeep has made them a thing of the past. The Dutch light sash measuring approximately 75 x 150cm (2ft 6in x 5ft) is ideal for frame construction. This has a single sheet of 3mm (24oz) horticultural glass 1.42 x 73cm (56 x 28-3/4in) and is simple and light to handle, being virtually maintenance-free if constructed in pressure treated wood. Bases for frames can be brick, concrete or breeze block, or wood (old railway sleepers serving on a large scale). The front of the frames should be approximately 23 — 25cm (9-10in) high and the rear 30-35cm (12-14in) to give a reasonable ‘run’.
Frames can also be constructed against the base wall of a greenhouse, an ideal arrangement when the heating system is to be linked, a sun-facing orientation usually being preferable. Metal or alloy types come with directions, but there are no precise designs otherwise; many gardeners simply adapt what materials they have to the best advantage. Polythene sashes can readily be made up by using the wrap-round principle of construction in any convenient size. Wood should be preservative treated, this including both the sash spars and the groundwork. It is important to ensure that sashes are secure, otherwise they may be blown off in a high wind.
Of special interest is the range of alloy frames now available with ingenious ventilation facilities. Many types of polythene cloches have also reached an advanced stage of design. Very significant also is the perforated polythene film which can be put over a crop in spring and expands as the crop grows (floating mulches).
Whether or not a special shed is desirable for compost mixing, potting and other activities will depend on the scale of operations, but a shed is certainly very useful for the storing of pots, tools, equipment and raw material for compost making. If work is to be carried out there, a good stout, level, well-placed bench, preferably with a window adjacent, is highly desirable.
The combined shed/greenhouse, either longitudinally or centrally, has its value, but the shade factor must invariably limit the scope of the greenhouse section, which may be undesirable.
Heat conservation for greenhouses
In recent years, commercial growers in temperate zones have increasingly incorporated heat-saving methods into greenhouse design as a standard feature. These methods include the use of polycarbonate sheeting for ends and side walls — and in some cases for the roof also — and, in addition, the use of thermal screens of various materials which are automatically pulled over the crops above head height at night. Full details of these heat-conservation measures are available from commercial greenhouse manufacturers.