Types and Position. There are four main types of. A SPAN-ROOFED HOUSE has a ridge-shaped roof, both sides of which are of equal length. The roof stands on walls which may be all masonry or wood, ‘partly or entirely glass. The height of these walls varies from about 1 ft. to 51ft.
A LEAN-TO HOUSE has a roof sloping in one direction only, the back being formed by a wall. In other respects it is similar to the span-roofed type.
A THREE-QUARTER SPAN HOUSE is midway between these two types. The roof is in the form of a ridge, but one side is only about onetquarter the length of the other, and rests against a wall like the leantto.
The fourth type is THE FORCING HOUSE Or PIT, which has a span roof coming right down to soil level. The floor is excavated about 3 ft. below soil level and borders may be built up on this to within a foot of the base of the glass. These houses retain warmth especially well and are useful for forcing crops under high temperatures. Sometimes ordinary span-roofed houses on solid masonry walls are used as forcing houses.
Where possible, span-roofed and forcing houses should have the ridge running north and south; lean-to and three-quarter span houses should face south, the ridge running east and west. Only houses intended for ferns and certain tropical foliage plants should be in an entirely shaded place. All houses should be glazed with clear, 21toz. glass.
Temperatures. These may be varied within wide limits, but roughly there are four main groupings.
Cold houses are unheated, and in winter the temperature may fall below freezing point, though the average will be in the neighbourhood of 45°. This will rise in summer to 60°, or considerably more with direct sun heat.
The cool house is artificially heated in winter to a minimum of 45°, average 50°-55°. In summer it is not heated.
The temperate house has a winter minimum of 55° and summer minimum of 60° and the hothouse has a minimum of 65° winter and 70° summer. Both may need some artificial heat even in summer.
Cold houses can be used in winter only for hardy plants, but many tender plants can be grown in them during the summer. Cool and intermediate houses are the most generally serviceable for the majority of popularplants. Hothouses are for really tender plants from tropical regions and for forcing plants into early growth or flower.