The Warm Greenhouse
Having sung the praises of the cool, which offers so much for a relatively modest heating cost, let us look now at the warm greenhouse in which a minimum temperature of 10 to 13° C. (50 to 55° F.) is maintained during the winter months. Obviously, such a greenhouse allows one to be even more ambitious and to grow many more plants than would otherwise be possible. A warm greenhouse is appreciated more during the winter and early spring than at any other time of year.
Plants like the winter-flowering begonias, dracaenas, poinsettias and cyclamen can all be grown quite easily in temperatures ranging from 10 to 16° C. (50 to 60° F.). Some ferns are also suitable, for example Nephrolepis exaltaia, the Ladder Fern. Bulbs, too, like pre-cooledand narcissi, and specially prepared , can be brought into flower much earlier under these conditions.
This interest in terms of plants is very well worth having, and there are other advantages of a warm greenhouse which I shall refer to a little later. But one must be realistic and look at the obverse side of the coin before embarking on this form of, for instance, the cost of heating such a structure. If electricity is the source of heat, raising the minimum temperature from 4° C. (40 °F.) to 10°C. (50°F.) the difference between the cool and the warm greenhouse, can quite easily double the heating bill, also. Although can be raised quite easily in a warm greenhouse they do need a second, half-way stage before planting out and this makes a garden frame far more necessary than is the case with a cool greenhouse. Bedding plants that have been raised in a cool greenhouse can be stood beside the house in the open to completely harden off before planting out.
On the credit side it is much easier to maintain plants during the winter months in a warm greenhouse without the likelihood oftaking its toll also. Watering need not be done with the same care in winter time as in houses with less heating or none at all. A warm propagating frame such as I have suggested for the cool and unheated greenhouses is unnecessary in this kind of greenhouse.
Plants grown in a warm greenhouse need more or less the same treatment as those in a cool greenhouse during spring and summer, e.g. free, shading (particularly where such plants as begonias are grown), and plenty of moisture in the atmosphere.
It is not essential to give melons warm greenhouse conditions but they are easier to grow in this way, provided a warm. Humid atmosphere is maintained, and they will mature much earlier.in pots can be carrying ripe fruit in late March and early April whereas in a cool greenhouse it would be late April or early May before the first fruits begin to ripen. The same applies to a trained or tree grown on the wall of a lean-to greenhouse.
It is almost impossible to grow codiaeums (crotons) of any size or value in a cool greenhouse but these highly decorativecan be grown most successfully in a warm house. Other plants which can be grown well in the warm greenhouse are allamanda, the greenhouse climber with lovely yellow flowers. Streptocarpus, and the trailing columnea which is ideal for a hanging basket. Exotic orchids such as cattleyas and paphiopedilums, can be quite easily grown in a warm greenhouse, provided they are given the correct compost and sufficient moisture during their growing season.
A mist propagation unit is a very useful aid in the warm greenhouse with soil-warming cables maintaining the temperature of the bed beneath theat 18 to 21°C. (65 to 70°F.) This fascinating item of equipment has been with us for a good many years now but not so long that one cannot remember how much more difficult many plants were to increase from before its introduction.
To sum up then, a warm greenhouse is going to cost you more to run than a cool greenhouse but it will be a considerably easier proposition to provide the right growing conditions for a good many plants. In a very real way, too, it can be looked on as a method of extending one’shorizons. It would be quite a good idea to start with a cool greenhouse and move on to the warmer conditions when you feel you are ready to make the change.