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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.
Greenhouses need to be well ventilated to prevent dampness and mould. Image: RHS/John Trenholm
The following heating methods and energy saving tips are best suited to modest greenhouses, rather than commercial settings.
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.
The main loss 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.
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.
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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The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.