Greenhouse Temperature control FAQs

Is it worth installing and running artificial heating?

If you are taking up greenhouse gardening seriously, some form of artificial heating is extremely worthwhile. It widens the scope of operations enormously and renders the greenhouse productive all the year round of an extensive range of food and decorative plants. Most of the ‘specialist’ plants—flowers such as carnations, orchids, chrysanthemums, and sometimes even alpines—need some degree of artificial heating for perfection.

Is a greenhouse useless without artificial heating?

Certainly not—there is a host of plants that need only weather protection and are hardy or semi-hardy as regards temperature. With the right choice of plants an unheated greenhouse can be useful and interesting all the year round—winter can be quite colourful, too. By late February the sun’s radiation will be keeping temperatures high enough for early sowings and propagation of many plants.

Will heating my greenhouse involve me in huge fuel bills?

There are three generally recognised greenhouse temperature regimes: cool, with temperatures ranging from frost-free to about TC (45°F); intermediate; with a minimum overall temperature of 12°C (54°F); and warm, with a minimum of 18°C (65°F). It must be said at once that the cost of maintaining any of these temperatures varies widely, depending (a) upon the size of the greenhouse and (b) upon the area of the country in which you live. In general, however, to maintain an average-sized home greenhouse at temperatures from frost-free to about 7°C (45°F) should not prove beyond your means, especially if you take care to conserve heat. However, to heat even a small greenhouse above this level will prove costly—even one or two degrees above would make a dramatic difference to your fuel bills. It is possible to calculate roughly the cost of heating, given the price of the fuel and the dimensions and materials of construction of the greenhouse; most suppliers of heating equipment will do this for you.

Can you list some of the more popular plants for growing in a heated greenhouse?

All the most popular pot plants can be grown in a greenhouse with a temperature minimum between frost-free and 7°C (45°F). Some typical examples are fuchsias, pelargoniums of all types, cinerarias, calceolarias, primulas, as well as innumerable tender bulbs and other storage organs, and greenhouse annuals. There are also many tender greenhouse shrubs and perennials that are easy to cultivate if the temperature is right, and these include cool-house orchids such as cymbidiums. All kinds of propagation can be fitted in, too, and bedding-plant production for the garden is unlimited—as is the growing of all the popular edible crops, many of which can be had early or out-of-season.

Which fuel would you recommend?

Nowadays the costs of the various fuels usually work out much the same in terms of the heat energy they can produce. However, some are much more convenient to use than others and are more suited to the home greenhouse. Electricity, usually regarded as expensive, is ideal—especially if used to power a fan heater; with thermostatic control there is no waste, and the atmospheric conditions created are excellent for plant health. Natural gas is also much used and can similarly be controlled by thermostat. If burned in a non-flued heater, however, it makes the air extremely damp in winter (as does a paraffin heater). Both electricity and gas need the minimum of attention, and are especially suited for greenhouses that have to be left unattended for long periods. The various forms of hot-water-pipe heating are better suited to use in larger greenhouses heated to higher temperatures.

How can I avoid wasting heat?

Make sure that all vents and doors are draught-free and that there are no gaps in the greenhouse structure. The temperature should never be higher than absolutely necessary, and as noted, some form of thermostatic control is desirable if heat waste is to be avoided. Lining the greenhouse with thin transparent polythene so as to enclose 12-25 mm (½ -I in) static air between the plastic and the glass will reduce heat loss—and your fuel bill—by about 40 per cent. It is the air layer, by the way, that forms the insulation. The lining must allow as much light to pass as possible, and should be taken down at the end of the winter.

One hears a lot of talk about solar greenhouses. What are they, and do they offer any advantage over conventional types?

The term ‘solar greenhouse’ has recently been applied rather loosely to a number of new models of various fancy shapes. In fact, a true solar greenhouse—one that can store the sun’s heat on completely sunless days—has not yet been invented. The storage of solar energy is difficult without elaborate, bulky and expensive equipment.

All glass greenhouses are ‘solar’ to the extent that glass traps the sun’s warmth. Some of this warmth can be absorbed during the day by the floor, staging, and (in the case of lean-to) rear wall, and given out overnight. There seems little point in buying one of the fancy, costly designs. In any case, in summer, keeping the temperature down is usually more of a problem that providing warmth; while in winter most conventional designs, if well-sited, will trap any of the sun’s warmth that may be available.

What is ‘bottom heat’?

This is an old term meaning heat applied from below. It is usually recommended for propagation, particularly to speed up the rooting of cuttings, but also to aid seed germination and the starting of roots or storage organs (bulbs and the like). Nowadays the heat can be conveniently supplied by electric soil-warming cables covered with moist sand, which is overlain by moist peat. The propagation containers are then plunged in the peat. However, any similar arrangement comprising a peat plunge with warmth applied from below can be employed; and where cuttings are to be rooted directly into a propagating frame, a mixture of equal parts peat and sand can be laid over the base layer of peat. Most electric or paraffin propagators available to amateur gardeners also provide bottom heat. Since most of these usually have covers there is no need for a peat plunge because adequate warmth is retained in the container as a whole.

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