Greenhouses, when they are used for raisingand for other mainly utilitarian purposes, are usually relegated to the vegetable garden or service part, when their harmony of design is unimportant. A greenhouse devoted to the growing of beautiful flowers such as cyclamen, and gardenias, however, needs to be near the house to be thoroughly enjoyed, and in this case its design is very important.
There are three types of glasshouses—the span-roofed, the three-quarter span and the lean-to. The former is an all-round house suitable for general purposes. The house should run from north to south and should have the door at the south end and the boiler at the north. To allow tall-growing plants like carnations to be grown, the height of the sides above the staging should be never less than 2-1/2 ft. unless the house be sufficiently wide to allow of a centre staging. The span roof should not rise immediately from the brickwork and staging even if the house be required solely for growing seedlings.
Three-quarter span houses should have a south aspect, and are useful against a wall when this is not high enough to take a lean-to. Such houses are warmer than span ones, as there is no glass on the north side.
The lean-to house is the simplest and most easily constructed, and is therefore less expensive. Like the three-quarter span it should face south and the roof should have a good fall.
Greenhouse staging may be of lattice work or of pebbles or coke laid over a solid staging. Coke retains moisture whereas lattice work does not. In a span-roofed house staging is placed along both sides and, if the house is wide enough, down the centre. It is very important that the staging be wide enough to make the plants at the back easily accessible for watering and attention. The three-quarter span and lean-to types allow staging to be placed in tiers, thus greatly increasing the capacity of the house and making full use of the available space.
The ventilation of a greenhouse must receive great attention. The usual method is to have lights in the roof, in the sides also in span houses, and ventilators in the brickwork below the staging in all types. These are very important as they change the air in the lower parts of the house and help to circulate moisture.
In the summer, glasshouses should be shaded by using blinds of the chain and lath type inside the house, Chinese blinds outside the house, or by spraying the glass with whitewash or with some patent green-shaded preparation. Do not forget, too, to collect the rain-water inside the house.
Low-pressure hot-water systems are most usually used for heating glasses, although a small stove is often satisfactory in a small house. Oil is used in very large houses, while electric tubular heating, thermostatically controlled, is becoming very popular for houses of any sort.
Gas too is used, and although these and electricity stems are more expensive to run, they take care of themselves, and, needing no attention, save a deal of labour.
One house may be divided into two or more sections by using partitions, and valves in the water-pipes. This enables different temperatures to be maintained in the separate sections and makes the house much more useful.