There are three: the cold house; the warm house; the hothouse. The first is a glorified frame so far as cultural possibilities are concerned, though the range is greater, and in the coldest weather frost can be kept at bay by slight heat, say 50 degrees F., from a motor radiator lamp, which burns very slowly but maintains a regular heating capacity. The warm house is one which can be heated all the year round by pipes from some heating apparatus or other, of which there are various types using different heating mediums. The temperature should never fall below 6o°. The hothouse has air always in a state of warm or hot moisture, varying in degree according to what is grown. In construction each follows the same method, a foundation, brick or wooden, preferably cedar, walls of say one-third the height of the whole surmounted by four glazed sides, with windows mostly opening horizontally, and an apex glazed top, also with ventilation. The staging runs along each side and the back and is slatted, not solid boarding. The cold and warmcan also be of the lean-to type, which may be described roughly as half a , because it is built against a wall or the side of the dwelling house, but otherwise follows the same principles of construction. Iron-framed greenhouses are also favoured. Greenhouses built in aluminium alloy are ideal for the amateur and fully justify the rather higher initial expense compared with steel, wood etc. They give the maximum light and the glazing is usually cushioned in a plastic such as polyvinyl chloride to eliminate any risk of breakages. They require neither painting nor maintenance and are highly resistant to corrosion. Any dirt or dust which collects can be easily washed off. There are various means of heating. The traditional boiler is still popular but is losing ground to electrical heating — see following paragraphs. Self-contained oil heaters are useful for keeping out frost in a cold house but as a permanent method of heating cannot be recommended without reservations. The fumes given off by burning oil damage most plants, although if the heater is kept absolutely clean, I.e. the wick is trimmed regularly and during really cold weather when more heat is required, rather more ventilation is given than with other types of heating, the risk of injury is lessened to a certain extent.
Despite the high cost per unit of electricity, this method has considerable advantages over other forms of heating. Electricity is, of course, clean, and there are no storage problems as with solid fuel etc. If the electrical installation is thermostatically controlled, the desired temperature can always be maintained, irrespective of sudden weather changes. You can safely go away for a long week-end with the assurance that nothing will go wrong and your plants will be perfectly happy. Tubular heaters attached to brackets round the greenhouse walls are excellent as they take up relatively little room. Note that a greenhouse which is more or less portable with no permanent base does not justify any increase in the rateable value of your property. A greenhouse fixed to a brick or other permanent base definitely increases the rateable value.
Appropriate plants for each class of greenhouse are given throughout, while for starting plants, either vegetables or, the cold or warm greenhouse provides scope for intensive cultivation on lines given under INTENSIVE CULTURE and as recommended for numerous individual plants. Scrupulous cleanliness must be the rule to prevent invasion of insect pests, fungi, etc. and precautions must always be taken against damping-off, .
The interior and staging should be lime-washed annually and means provided to shade the glass to prevent the sun from making the interior air too hot. Various proprietary preparations are available or a fairly persistant shading may be made by working fresh slaked lime into a thin cream by adding skimmed milk. Dilute with water to the required consistency. The use of sterilised soil is recommended, especially for theboxes. John Innes and composts are suitable for most plants.
In the summer the greenhouse, if not bare, is sometimes lacking in brightness. Sown early, say in February in box and transferred toin due time, a very few of this or that remedy the drabness by a pleasing and varied summer at little cost or trouble. The Swan River daisy (brachycome), kingfisher daisy (felicia), nemesia and petunia are some of the most suitable to raise from in February in a slightly-heated greenhouse or in March if no artificial warmth is available.
During the winter also there is a break in brightness, especially where heating facilities are not readily obtainable or it is difficult to keep heat going. Here, too, even an unheated greenhouse can be kept bright with flowers in winter and early spring. Reliance must be placed chiefly on hardy plants, but a few others may be included in the collection, for instance, the perpetual flowering carnation,malacoides, stellata and Indian azalea. Certain hardy plants may be lifted from the garden in November or December and potted for the cold greenhouse — wallflower, polyanthus, pansy, , violet, primrose, forget-me-not, and even and other early bulbs. Keeping the ventilators closed for a few days will help the plants to become established, but the greenhouse ought to be ventilated freely in mild weather. Owners of frostproof greenhouses should seeds of a quick-growing cabbage in February for early planting out. They will be ready to cut in late summer and thus help to maintain an almost unbroken supply of this vegetable.
A supply of winter salads is obtained by taking up andlettuces and planting them in the cold greenhouse. Another way of making use of an unheated greenhouse in winter is to sow seeds of some of the hardy , e.g. larkspur, love-in-a-mist, godetia, annual and clarkia. The seeds are sown on a bed of soil, the being planted out of doors in spring.