Greenhouse Heating and Lighting

An unheated greenhouse is certainly better than none, but with extra heat and light, your options are greater. You can start dormant plants into growth early, and give seeds and cuttings a head start, too.

The cost of heating and lighting a greenhouse may seem off-putting at first, but modern, energy-saving technology means that nothing need be wasted, and even a little extra heat and light go a long way. Also, you don’t have to heat the whole greenhouse. Merely heating a small area will provide the conditions for growing a few chosen plants.

How much heat?

Most people heat their greenhouses from autumn to spring, and rely on the sun’s warmth the rest of the year. A ‘cool’ greenhouse, kept at a minimum of 45°C (40°F), is economical, can produce flowering plants year round, and is safe for overwintering Fuchsias, Pelargoniums and other nearly hardy patio plants. An ‘intermediate’ greenhouse, with a minimum of 10°C (50°F), is fine for semitropical plants. In a ‘warm’ house, at 15°C (60°F), and a ‘stove’ house, at 21°C (70°F), tropical plants can flourish, but it costs twice as much to heat a greenhouse to 10°C (50°F) as 45°C (40°F), even more in the higher ranges! You can divide a large greenhouse with a sliding glass partition, and keep each area at a different temperature.

Types of heating

There are four main types of heating. Before deciding, consider installation and running costs, availability, safety and the amount of attention needed. The manufacturer or supplier will calculate the exact size of the heater you need, on the basis of the size and construction materials of your greenhouse, the lowest winter temperature expected outside and the minimum temperature required inside.

Paraffin gives off heatparaffin-greenhouse-heater when the wick is lit. Paraffin is cheaper than electricity, and stoves come in many sizes, including inexpensive portable ones. Larger models can be connected to pipes or ducts, to distribute heat effectively. Some models have thermostat controls. Paraffin gives off carbon dioxide„ which benefits plants, and fumes, which can be unpleasant. (Blue flame burners give off less fumes than yellow flame burners.) Paraffin stoves need frequent attention, cleaning and wick trimming. They produce water vapour, which causes condensation, so ventilation is needed at all times. Small stoves can easily be knocked over so ensure that they are properly secure; lock the greenhouse if you are not in there.An electric heater for the greenhouse is expensive but very efficient and effective. Paraffin heaters are cheaper than electricity but ventilation is needed for the fumes.

Gas is either natural, or bottled butane or propane. All operate thermostatically controlled warm air cabinets, which send out heat from the top. Natural gas, if available, is cheaper than cylinders, but has to be professionally installed and piped to the greenhouse. Both give off carbon dioxide and need little attention, but condensation and fumes can be a problem.

Electricity is ideal, but also the most expensive. It is clean, needs no ventilation, and there are electric-greenhouse-heateraccurate timing and temperature control systems available. Electricity is most cost effective at low settings, up to 7°C (45°F). There are several types of electrically run heaters.

Conserving heat

  • Make sure there are no gaps in the greenhouse structure and that all doors and vents are draught-free. Use draught-excluding strippers if necessary.
  • Use thermostatically controlled heating.
  • In cold weather, line the greenhouse with bubble plastic or with thin, transparent polythene to make an insulation layer of still air 15-25mm ( ½–1 inches) thick between the glass and plastic.
  • Portable fan heaters are useful in a small space. They blow out warm air, circulating it in the greenhouse.
  • Banks of hollow tubes, containing heating elements, can be installed along the sides or under the staging.
  • Convection heaters – cabinets with heating elements inside –are useful for large greenhouses. Cool air is drawn in from the bottom, heated and blown out from the top.
  • Air warming cables which are fitted to the walls, are ideal for a small or mini-greenhouse, if you only want to keep it frost-free.

Soil-warming cables are buried in a soil bed, or in sand on greenhouse staging, where they provide ‘bottom heat’ for cuttings. The heat is local, though, and you need another heater to warm the air. Air and soil-warming cables are cheaper to run than other forms of electrical heaters, but make sure the cable system is supplied with a thermostatic control.

Hot water pipes, fixed to greenhouse walls, are fed from an outside boiler fuelled by oil, natural gas or solid fuel. Solid fuel is economical, especially for maintaining high temperatures, but needs daily stoking and ash clearing. Oil and gas boilers require little attention, but are more expensive. Though thermostatically controlled, the response to temperature changes is slower than other forms of heating. There is no condensation.

Artificial Greenhouse Lighting

Extra light is most useful to plants in winter and early spring when natural light levels are low. Combined with extra heat, it helps bring on seedlings and cuttings, and starts resting plants into growth and flower.

Fluorescent light

This is the most popular form of greenhouse lighting. Banks of tubes are mounted 15-60cm (6-24 inches) above the staging, where young plants are coming on. Use tubes designed for horticultural use, with good light spectrums.

If you are making your own fitting, mount 2 x 40 watt tubes under a reflector, with a pulley, to raise or lower the light. Aim for 15-20 watts per square foot of growing area.

Mercury vapour lightgreenhouse-lighting

This is excellent for plants, and little energy is wasted in heat. Large lamps are effective over 2.75 sq m (30 sq ft) but are expensive to run, since they have a wattage of 400, and need special control gear. More economical are small, 125 watt mercury lamps, effective over .6 sq m (7 sq ft).

Incandescent light

This is the ‘normal’ light used in domestic lightbulbs. Most of its energy is given off as heat, so plants close enough to benefit are liable to be scorched. Incandescent floodlights are better, because the light is concentrated by built-in reflectors.

House plants for a cool greenhouse

A greenhouse with artificial heating and lighting can provide a year-round supply of flowering house plants, and somewhere to overwinter tender patio plants.

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