You can grow just about everything in a greenhouse, providing you can give the conditions that suit the plants.
The cold (unheated) greenhouse. This has no heat as a matter of course, though some temporary heating may have to be provided in exceptionally cold weather. The simplest uses to which it can be put are those of a cold frame, but there is a lot more besides that you can do. In winter a display of hardy plants in pots can cheer up the dullest months – you can use any early flowering shrubs – they will come into flower much earlier and not be spoiled by bad weather. You can also grow early perennials – hellebores, polyanthus, lilies of the valley, doronicums and the like, hardy annuals, which will come into flower in early spring from an autumn sowing protected through the winter in the greenhouse and bulbs in pots. In all but the coldest winter you can overwinter your pelargoniums, fuchsias and other perennial bedding plants in a cold house.
Winter lettuce sown in October will be ready for cutting in April. Slightly tender peaches and vines can be grown indoors without heat. Once the temperature has reached around 45°-50°F (8°-10°C) in spring, you can make early sowings of most vegetables for planting out in pots.
You can continue to grow ornamentals throughout summer – hydrangeas, fuchsias, pelargoniums, begonias, and many more, or you may prefer to turn to vegetables like peppers, tomatoes, aubergines, cucumbers, melons, etc.
What you must bear in mind, however, is that you may not be able to grow all these things together successfully. If you have a large vine blocking most of the sunlight, you won’t be able to ripen tomatoes as well, and flowering plants needing a lot of light will suffer. The fun is experimenting to see what you can get away with.
The cool house. If you can maintain a temperature of 45°—50°F (8°—10°C) in winter by one of the methods of heating and insulation already described, you can, in addition to the plants suitable for a cold house, grow a wide range of slightly tender plants, shrubs such as Rhododendron simsii, datura, abutilon, boronia, acacia, and plumbago, bulbs like hippeastrums, Scarborough lilies, and arum lilies, perennials such as carnations and chrysanthemums, and pot plants like cyclamen, Primula malacoides and calceolarias.
In spring you can germinate the seed of tender summer vegetables and summer outdoor bedding plants, once a temperature of 60°F (15°C) can be maintained, usually from mid-March.
In summer all the plants suitable for the cold house can be grown as conditions will be identical.
In autumn you can take cuttings of half hardy perennials such as pelargoniums (geraniums) and sow seed of cyclamen and schizanthus, the poor man’s orchid. If you sow suitable varieties of lettuce in October, you should be able to cut from February, and you can also grow mustard and cress and radishes.
The intermediate house. If you want to maintain a temperature of 55°F (13°C) or more, night and day, even in the coldest days of winter, it is going to cost you money, but if you can afford it, it is worth it, because many of the houseplants which struggle to survive in difficult conditions in the home will do very well with the increased light, regular temperature, and higher humidity of this kind of environment. Dracaenas, poinsettias, marantas, ferns, coleus, the choice is endless, and also tender vegetables will appreciate this temperature. You can force strawberries, and figs grown in this heat will fruit in late spring. Bedding plants for outdoor summer display can be raised in spring.
The warm (‘hot· or ‘Stove’) house. A temperature of over 64°F (18°C) in winter and 70°-81°F (21-27°C) in summer, will give you your own touch of the tropics, because those plants which can normally survive only in a warm, light room — crotons, kalanchoes, peperomias, caladiums, etc., together with most of those suitable for the intermediate house – begonias (ornamental-leaved forms), swiss cheese plants, etc., can be grown. The increased humidity will compensate for the higher temperature.
From a practical point of view, intermediate and warm greenhouses are not feasible as they are far too costly to heat.
However, this type of environment can be created in a conservatory or sun-lounge attached to the house and used as an extra room, where you can enjoy the warmth as well as your plants.
Do not forget that an enclosed heated propagator or horticultural electric blanket will enable you to raise young plants at higher temperatures than those of the greenhouse in which it is placed, if necessary. You can also, if necessary, section off an area of a larger greenhouse to provide a different environment.