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How to heat a greenhouse and everything you need to consider
Last Updated on January 21, 2020 by John
A greenhouse can provide an affordable and sustainable way to maintain a garden, so long as you heat it efficiently. It offers a great way to protect more tender plants you might have growing in pots and is also a great way for protecting any cutting you may have taken. What most people don’t realise is you can heat a green more affordable than you might think as you often don’t need to keep it warm, at a minimum you can have your greenhouse frost free at around 3C (37F) which can easily be achieved with good insulation and a small heater for most small greenhouses. If you have more tender plants such as geraniums then you might need to consider increasing this to 10C (50F).
Greenhouse heating systems to consider
There are different forms of heating systems from which to choose, namely electric or gas types of greenhouse heaters are the best as they usually incorporate thermostats to control the temperature more accurately.
Electric greenhouse heaters
Electricity might seem more expensive by comparison, but it also allows for better control over precise temperatures and it is safer for plants because it does not produce combustion by-products such as carbon monoxide or ethylene.
Gas greenhouse heaters
Gas heating is the cheapest greenhouse heater to run in theory, but with this, you also need to freely ventilate which may even out any savings you might otherwise achieve. Realistically, gas is only best for those growing low-temperature plants or where you are unable to use an electric greenhouse heater because you don’t have access to a plug socket.
Paraffin greenhouse heaters
The final type of greenhouse heater is those that run on paraffin, these are perfect for keeping a greenhouse frost-free when used with a thermometer to keep an eye on how low the temperature drops at night, that way you can turn the heat up or down as needed. Note that paraffin heaters are designed to produce a little heat and not to heat a greenhouse to a warmer temperature.
Insulating a greenhouse to make your greenhouse super efficient with minimum heat loss
Seal any cracks or gaps in the greenhouse with tape
The biggest loss of heat you will face with your greenhouse is from the structure itself, and with draughts, by there very nature greenhouses are not very efficient at holding heat anything you can do to help is essential. To that end, in order to insulate your greenhouse, you should start by sealing any cracks in the structure with tape, replacing any broken panes, and making sure the doors/vents all fit snugly without allowing for a draught.
Use bubble wrap to cover windows for extra insulation
In terms of windows, one thing you can do is add double glazed windows but this is not always possible or simply too expensive. The second-best alternative and most popular is to apply a layer of bubbly polythene also known as bubble wrap around the windows. However, each layer of bubble wrap can reduce the light inside by as much as 10%, this is not often too much of a problem as most plants are not actively growing in winter. Its worth noting that most greenhouses are actually designed to have bubble wrap fastened to the frame and you can but special plastic greenhouse clips and washers that hold the bubble wrap in place.
Use transparent mastic to seal any small gaps
When sealing, transparent mastic which is like silicone can be used but make sure the material you use on the greenhouse is flexible, so that it doesn’t break in the wind and so that it can expand with the greenhouse as it heats up and cools.
Split large greenhouses into smaller section so you can heat a smaller area
For a larger greenhouse, you can screen off part of it with polythene to limit the area you heat only to the area you are using.
Use fleece to add an extra layer of protection to plants in a greenhouse
During a cold snap, you can protect against frost by placing fleece over the plants themselves, this alone can keep the plats a couple of degrees warmer and cover the entire greenhouse at night with thermal screens attached above or roll down blinds.
Using a thermostat
The right temperature for a greenhouse is crucial to its effectiveness. Using a thermometer choosing the right temperature for plant survival and lower heating costs can be achieved. A minimum of 3 degrees C to keep a greenhouse frost-free as already mentioned will help tender plants but does place your plants in a pickle if they get damp, and most greenhouses have at least one area where dampness is more likely. That said the best minimum temperature is 7 degrees for most semi tender plants, while 10 degrees will give your plants the best chance for survival, specially more tender cutting or plants such as citrus. Of course, all of the aforementioned methods of protecting plants can be used in conjunction with the fuel-saving lower temperature. We strongly recommend using a greenhouse heater with a thermostat as this is the most efficient way of avoiding you greenhouse falling below a certain temperature but also from wasting heat by heating it to warm.
Always use a thermometer to keep a check on minium and maximum temperatures
You can monitor the progress with a thermostat, and if you are using an electric heating system, you can even change the settings to coincide with times of the day or increase the heat during cold snaps. If you are using another heating system, using a thermometer will help you verify the daily minimum and maximum readings so that you can adjust the heater as necessary. If you use a paraffin heater, this will help you verify you are maintaining a specific minimum temperature at night.
Consider using a heated propagator for a few smaller plants or plugs
In lieu of contending with various heating sources, you can consider using a heated propagator. With this, you do not even need a full greenhouse, just a space in your home where the propagator can sit and be maintained at an even temperature.
These propagators have heating elements in the bottom, and the trays of seedlings are placed on top, then covered. The heat source plugs into an outlet but is then set at a constant temperature. With a special thermostat, you can monitor the temperature too. This takes considerably less energy because the energy used to create that greenhouse environment is limited to the smaller trays/containers rather than a larger outdoor building.
You also have the option of propagating indoors, reducing some of the extra heating costs, if you don’t have a large enough crop to span the greenhouse. These heated propagators can be used until such time as the seedlings are ready for transfer, which typically coincides with warmer weather making it easier and less costly to heat the greenhouse.