Unheated mini-greenhouses are widely used to grow a crop of tomatoes during spring and summer, and then for growing late-flowering chrysanthemums in autumn, when the tomato plants have been discarded. For the gardener who is interested in obtaining the finest blooms, an unheated house is invaluable for growing a wide range of half-hardy shrubs, annuals, lilies, gladioli and many other bulbs.
All out-of-season vegetables normally grown under frames or cloches, such as lettuces, carrots, radishes, potatoes and French beans, can be grown just as well in an unheated greenhouse.
Cool house plants that live for many years include pelargoniums fuchsias, begonias, orchids and perpetual-flowering carnations.
A cool mini-greenhouse is particularly useful for growing flowering plants from seeds or cuttings early in the year, to be planted in the garden at the beginning of summer. This is the technique known as bedding, used widely in public parks. Zonal pelargoniums (geraniums) and the fibrous-rooted begonias are two of the most popular bedding plants. The first decision when
buying a mini-greenhouse, frame or cloche, is whether to choose glass or plastic.
The main advantage of glass For greenhouses is that short-wave radiations from the sun pass through it easily, but once they are converted into long-wave heat radiations the glass becomes a barrier, and so the heat is retained inside the greenhouse. Plastic takes in the rays but does not trap them, so that when the sun goes in the house cools down again very quickly.
Horticultural grades of polythene are available for greenhouses, and these last longer than ordinary plastic sheeting. Corrugated plastic should last about five years and is quite cheap to buy. A recently introduced acrylic plastic is very similar to glass, but is expensive.
Plastic scratches more easily than glass and the scratches become ingrained with dirt.
Condensation can also be a problem because whereas on glass the moisture forms a film, on plastic it collects, obscuring the light and causing drips. However, for cloches and frames plastic is cheaper, easier to erect and gives excellent results where only temporary protection is needed.
Where to site the mini-greenhouse
A mini-greenhouse should be given an open position where it receives all the light available. Never site a mini- greenhouse next to trees. They cast shade, dirty the glass and can cause damage with falling branches. Rising ground on the north or east side can shelter a site from cold winds.
It is convenient to have the mini-green-house near the house, for easy access and to provide electricity and water.