The winter months are feeling ever closer and so once again our attention turns to our greenhouses and trying to prolong the growing season. But now more than ever, budgets are tight, so how do you heat or at least warm a greenhouse without spending a small fortune?
1. Bubble Wrap Insulation
Before we even start to think about other methods of heating our greenhouses it is essential that they are well insulated first, otherwise, we are just throwing time and money away trying to keep them heated.
The good news is this is a lot easier and cheaper to do than you might think. All we need is a roll of bubble wrap and some sticky tape.
You want to wrap the entirety of the inside of your greenhouse in bubble wrap, including the roof. If possible keep the bubble wrap from touching the glass of your greenhouse, this will then create a trapped pocket of air which will add even more insulation.
If you have enough bubble wrap you can even add a double layer of insulation. Bubble wrap is ideal because all of the air pockets help its insulation value and it is cheap and also still lets plenty of light through.
2. Hot Beds
As plant material breaks down to form compost it creates heat, you will have seen this if you have ever dug into the centre of a compost pile and seen the steam coming out.
We can harness this to provide a little bit of free heat in our greenhouses, this is commonly referred to as a hotbed.
Essentially you are just setting up a compost pile inside your greenhouse. Usually, it is done in a large raised bed.
Just fill a bed with layers of material that will break down over winter, lots of high nitrogen things like grass clippings really get a pile hot but you also need some brown material in there to stop it from going slimy.
If you have access to it then horse manure can be used to create the entire hotbed and that can get really nice and toasty. It is also quite common to have a layer of compost on the top of the hotbed, this can then be grown in, giving you really nice warm soil for tender plants or starting seeds in.
3. Passive Solar
This can be a really cheap, often free, way of heating your greenhouse, note that I am not talking about extravagant and complex systems here but rather really simple steps you can take to harness some of the sun’s energy.
The heat from the sun can be absorbed during the day and then radiated at night to help increase the temperature of your greenhouse. The amount of energy stored this way in a given material is called its thermal mass.
Water has a really high thermal mass, so we can exploit this to store some solar energy. The easiest way to do this is by filling up old plastic bottles with water and placing them around your greenhouse.
Old milk bottles and large bottles of pop work really well because of their size.
Painting the outside of the bottles black makes them even more efficient as they will absorb more of the heat from the sun. You can even place the bottles right next to the plants that you want to protect.
4. Solar Pool Heater
This is a more commercial way of harnessing the sun’s energy but works in the exact same way. These products are actually designed to heat swimming pools in the summer but can be repurposed and used to add a little heat to our greenhouses.
They are essentially just a pipe filled with water that loops around to maximise the surface area. The water in the pipes gets heated up in the sun and will then dissipate through the greenhouse very gradually.
5. Parafin Heater
Paraffin heaters are the most powerful of all the methods listed here but also the most expensive, beyond the purchase cost is the cost of having to buy paraffin to burn in the heater.
This again will not get your greenhouse really warm but it should raise the temperature by a few degrees and keep any frost away.
You can get 16 litres of paraffin for £38.44 (price correct on 07/10/2022) and a small paraffin heater should last 9-10 days on one tank of fuel which is 1.7 litres.
So you should get roughly 85-94 days of heat from your £38.44 which works out to between 45p and 40p per day of use.