Greenhouses: heating efficiently

The high cost of fuel and the desire to garden sustainably are concentrating gardeners’ attention on operating greenhouses with the minimum energy usage. Here is our practical advice on heating greenhouses efficiently.

Biennial bearing means a fruit tree produces a bumper crop one year, and a poor crop the next. Image: RHS

Quick facts

Suitable for Greenhouses
Timing Autumn to spring
Difficulty Easy to moderate

Suitable for...

The following heating methods and energy saving tips are best suited to modest greenhouses, rather than commercial settings.

Electric or gas heating?

Electricity may seem costly, but the ease with which it is controlled and the lack of combustion products that affect plants such as water, ethylene and carbon monoxide makes it much easier and safer to use.

The need to ventilate freely when burning gas or paraffin reduces any savings from using these fuels and these heaters are only really useful for crops grown at low temperatures.

Other sustainable sources, such as ground source heat pumps, offer the hope of more environmentally friendly options in the future. Unfortunately, for now, they are not cheap, easy to install or particularly suitable for modest greenhouses.

How to insulate a greenhouse

The main lose of heat from greenhouses is from draughts and through the structure. The following tips should help improve the efficiency of the heating you choose, and save money in the process.

  • Seal cracks, replace broken panes and ensure vents and doors fit snugly
  • Add a layer of bubble polythene to insulate the greenhouse. Unfortunately every layer of plastic cuts out a significant amount of light (about ten percent). This will impact on growth. This also applies to double-glazing, which reduces light transmission
  • Transparent mastic can seal quickly and easily, but use a flexible material otherwise panes can break when the structure flexes in the wind
  • Renewing seals on doors and ventilators can help but, sometimes, taping plastic sheeting over leaking areas is required. However, take care to leave some facilities for ventilating
  • In large greenhouses, screening off part of the greenhouse with polythene and battens to limit the area to be kept heated is possible
  • Additional internal protection from frost is possible by draping fleece over plants in cold snaps. However, covering the whole greenhouse at night with roll-down blinds or by using thermal screens suspended above head height is much more effective (but there is the drawback that they have to be deployed each evening and rolled back in the morning)
  • Choosing the right temperature can help save fuel. A minimum of 3ºC (37ºF) is sufficient to sustain many tender plants but it is bit risky if they get damp; and most greenhouses have cold corners
  • The lowest practical temperature is 7ºC (45ºF) and 10ºC (50ºF) will give greater peace of mind. However, alert gardeners can use fleece and other means to protect plants at these fuel saving temperatures

Containers of water placed in the greenhouse to give off warmth as they freeze (latent heat of freezing) are advocated but the ‘temperature lift’ they provide is negligible and by the time the water freezes the plants will already be damaged.

Problems

Damp and associated moulds and rots can be very damaging in winter, no matter how carefully plants are watered. On sunny days ventilate freely to shed surplus moisture. In fact, you may have to use a little heat to dry the house out, but it preferable to do this than lose plants.

Some crops, winter lettuce and alpines for example, need as much light as possible and will suffer if too much shade is cast by insulation. Avoid using materials such as bubble wrap where the plants are hardy enough to survive as they will benefit from the light.

Keeping patio plants over winter needs little light, but they must never freeze. Ideally they should be kept at no less than 7ºC (45ºF) as they don’t need to grow to survive. Insulation is also well worthwhile. Other plants need to grow and so a compromise is needed: installing bubble polythene to the sides or the north facing roof and gable ends. Raising seedlings and keeping a collection of orchids are examples of where compromise is practical.

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