Heating the Greenhouse

Greenhouses are essentially all about heat. While in the summer a well-placed greenhouse will gain most of its warmth directly from the sun, in the winter artificial ways of heating the greenhouse – and insulating it from cold prevailing winds – come into their own. The range of heating systems available to the greenhouse owner is extremely wide, ranging from central heating – with water heated by gas, oil, wood or coal burners, circulating through pipes – to self-contained portable radiators, paraffin burners, fan heaters and bottled gas units.

Picking Your Heater

While a boiler and hot pipes arrangement – like a small scale central heating system – is ideal for larger greenhouses, for the typical garden setting, the most commonly seen heaters are usually powered by electricity, gas or paraffin. Both fixed and free-standing types are available in various sizes and outputs.

For clean and controllable heating, electricity is hard to beat, with a range of units commonly available which make the job of warming up the growing environment just about as simple as it can get. Most kinds have thermostats and many also offer a frost-watch facility for added protection if the temperature unexpectedly plummets.

As a good source of background heating, small low-wattage tubular heaters are ideal and can be installed either as a single layer, or in rows of two or more depending on how much heat is needed. Electric fan-heaters are probably the easiest way of getting started, just needing to be plugged in, but their benefits are not limited to their simplicity. As well as providing fairly cheap, warm air, the constantly moving air they generate can help reduce damping off and disease in the greenhouse.

Soil warming cables to be laid a few inches under the surface and warming pads which are used on the staging to warm up individual seed trays are also available and many greenhouse owners have found them very useful in bringing on their plants.

Bottled gas heaters range in size from high output space heaters suitable for the largest of greenhouses, to much smaller affairs for the typical six-by-four feet garden version.

Modern thermostatically controlled designs offer an efficient way to maintain a given temperature or to keep your greenhouse frost-free, being safe and very simple to operate with push-button ignition and built-in safety cut-outs in the event of the flame blowing out. As with any free-standing type of greenhouse heater, care must be taken in positioning them, to ensure that they cannot be easily knocked over, nothing flammable comes into contact with them and that there is adequate air-movement around them to avoid a local build-up of heat.

Although with the advent of cheap electrical units, portable paraffin heaters have become less popular than they once were, they remain a very simple and easy form of heating to install. However, if you do chose to go down this route, it is important to buy one which is designed for the purpose, as paraffin fumes can be harmful to plants – specialist greenhouse heaters having in built features to reduce the risk.

Deciding Factors

Inevitably the kinds of plants to be grown plays a very large part in selecting the size and type of heating to be used and how closely controlled it will need to be to meet any particular needs. Obviously, the larger and hotter the greenhouse, the more heating it will require, so the cost of the fuel may be a major influence on the final decision. This can be a serious issue for the growers of true “hothouse” flowers, but for the owners of cooler greenhouses simply looking to provide a little frost protection over the worst of the winter, the options are, of course, a little more open. However, it is not all a case of doom-and-gloom, since the constructive use of insulation can help boost the effectiveness of any heating system and – most importantly – reduce your fuel bills. There are many products available to insulate your greenhouse, but even something as simple as a good layer of bubble wrap on the north-facing wall or any of the sides which are exposed to prevailing winds can make a big difference.

While unheated greenhouses can be very useful, the fact that they will always be affected by cold snaps means that they can never provide a permanent home to the more tender varieties of plants. With even slightly modest heating – perhaps maintaining a temperature no lower than 7 degrees C during the worst of the winter, the range of plants which can be successfully kept increases enormously. Although no two greenhouses are seldom – if ever –the same when it comes to heating, the good news is that with the range of heaters available today, there is bound to be something suitable, whatever your needs.