How to Heat a Greenhouse
Written by Matt W (Greenhousestores) on 23rd Jan 2020.
Greenhouses help us produce more food for a longer time - we know this already! But what if we went the extra
How to Heat a Greenhouse
Written by Matt W (Greenhousestores) on 23rd Jan 2020.
Greenhouses help us produce more food for a longer time - we know this already! But what if we went the extra step and took our greenhouse to the next level? Greenhouses alone can extend the growing season by months - without any extra heating.
Summertime means keeping your vents open in order to keep your plants happy, but winter means a dead pause in our climate unless you plant only vegetables that thrive in low temperatures.
If you like kale but are ready for something more, consider what a busy gardener you will be if you decide to invest in a heating system for your greenhouse - winter won’t mean downtime at all.
What hothouse heating methods are available to gardeners? A lot! Some are even free. Here is a short rundown of methods that might work for you. It all depends where you live, how cold it gets there, and how sheltered your greenhouse is.
Here are a few things to bear in mind:
Heat sinks absorb heat easily and release them when it’s cold. This is a good no-energy and no-cost method for small greenhouses. What works well as a heat sink?
Plastic bottles - if you fill a plastic bottle with water and paint it black, it will act as a natural heat absorbent. Put several plastic bottles around your greenhouse and they will gather all the energy they can over the course of a day, and release it back into the greenhouse. It might not be much, but it’s good enough to make a small difference if you don’t live in the arctic. In fact, it’s a very useful method for when the days are still a bit longer.
Thermal mass floor - if you opt for a concrete slab floor, it will act as a heat sink too. Some people choose not to pave the inside of their greenhouse, and that’s fine - but remember that having some concrete slabs will help you control the environment more. And while everything runs free and wild outside - a greenhouse is a controlled environment by definition!
Insulation is probably the most important aspect of heating - because if you have a “leaky” structure, you’ll be heating the great outdoors instead of the inside of your greenhouse.
One of the best ways to insulate a greenhouse is to wrap it in bubble wrap. It’s the perfect material for insulation because a) you can get it anywhere b) air pockets on bubble wrap act as the perfect insulation, and most importantly c) it’s clear and lets in light.
If you happen to have spare packaging bubble wrap lying around, that’s great - but keep in mind that since this method works so well, gardening stores now have a special horticultural grade bubble wrap. It’s stronger and will weather it through more snow, wind, rain and UV lightning than normal bubble wrap.
Important tip - clean your windows before taping/stapling on your bubble wrap. This will help keep the loss of light to a minimum.
Now that we know a bit about insulation, here are some options for heating a greenhouse. There are a few different schools of greenhouse heating depending on how big your greenhouse is and what you want to grow inside. Some gardeners need to keep the temperature just above freezing for vegetating plants, while others want to maintain their plants cosy and active over the winter.
There are the DIY heaters and heating hacks, and the more professional “plug and play” heaters. See what works best for you.
On the cheap
Candles and a clay pot will get you surprisingly far for a short while. If you have a smaller greenhouse, putting tealights in a pot will actually help keep your greenhouse warmer, but we have to say that it isn’t the best idea simply because the open flame can be dangerous, and the amount of generated heat is very limited. We wanted to include this method because it seems to be all the rage on DIY blogs and videos.
We have seen clever setups where the pot hangs on a chain - and this might seem like a useful and aesthetically pleasing idea in a greenhouse, but we have also seen these chains heat up so much that they become a fire hazard! In addition, one tea candle can generate somewhere around 30w of heat. In a greenhouse, this can add up to a mountain of tea lights fairly quickly. While this is a good idea to heat up a room that you’re in the moment or in an emergency, it doesn’t pass the test as a sustainable and inexpensive heating source.
Compost, on the other hand, is both sustainable and effective. If you have ever walked near a compost pile in the winter, you will have certainly noticed that it creates steam. In fact, a compost pile can even spontaneously combust if not properly attended!
This is a method that’s been in use for almost a century. During the 1940s people used decomposing straw to heat their greenhouses - this is probably because fuel was scarce during the war. This is a simple system of ventilation that circulates the air through pipes in the warm compost and into the greenhouse. Some technical know-how is required. Remember, that in order for your compost to start “working