How to heat a greenhouse: 6 ways to keep your glasshouse warm

Knowing how to heat a greenhouse is crucial if you live somewhere with cooler temperatures or want to try your hand at growing more exotic plants that require plenty of warmth. Luckily there are lots of options to choose from.

Whatever greenhouse ideas you're going for, keeping your plants cozy throughout the winter months will help to keep them healthy and ward off dampness and disease. It will also encourage germination for any early sowings – getting them off to a good start so you can enjoy an earlier harvest. Plus, if your greenhouse is warm, you're much more likely to want to spend time in it – whether that's for garden chores or simply to relax in your own botanical haven.

We've brought together the most common ways to heat a greenhouse to help you decide on the best approach for your plot.

Learn how to heat a greenhouse and get the best results from your garden structure this year

From plug-in-and-go approaches to cheap and cheerful DIY methods, our guide will help you learn how to keep a greenhouse warm in winter.

1. Invest in an electric fan heater

'Fan heaters are very efficient at circulating heat, even in medium and large structures,' explains the team at Hartley Botanic. Their models are made from quality stainless steel and designed specifically for a greenhouse environment. 

All you need to do is plug it in, and most electric fan heaters have integrated thermostats which means you can keep close control over the temperature. And if you're short on floor space, there are plenty of styles that can be mounted to the ceiling.

These types of heaters will require an electrical outlet in your greenhouse, which will need to be installed by a professional. Once this is in place, you can add outdoor lighting to your structure too. This means you can continue to use it – either for garden jobs or for relaxing – once night falls.

2. Heat larger structures with a boiler

'For heating extremely large structures, electricity isn't necessarily required,' says the Hartley Botanic team. 'A hot water pipe system with a boiler powered by gas or oil is the norm.' This can be installed around the walls of the structure or under the floor.

'Thermostats for this system should be located at plant height near the center of the greenhouse, where it will not be influenced by drafts or sidewall cooling and protected from direct contact with sunlight and water.'

They may be more expensive to install than an electric heater, but the running costs tend to be lower.

Want to up your sustainable gardening credentials? Consider a biomass boiler, which is fuelled by wood pellets, wood chips, or other renewable materials. They can cut the running costs even further, however, installation can be pricey.

3. Create a hot bed

Want to give your seedlings a strong and early start? Try building hot beds – an approach that goes right back to the Victorian era and is more sustainable and affordable than using electricity or gas to create heat. So, how does it work? Answer: by relying on the heat created by decomposing organic material.

To make a hot bed, raised beds should be filled with organic, compostable material (such as straw or manure) and compressed down, then topped with topsoil and compost. The ratio of organic matter to growing medium should be 3:1.

You can then sow your crops. A cover over the top, such as a small cloche, will increase the heat even further. And, in a couple of months' time, once the compost has cooled and rotted down, you can dig it out and use it around your raised garden beds outdoors – bonus!

4. Try an air or ground source heat pump

Another sustainable option is to install a ground source heat pump. They do require electricity to run, but they use the warmth that is stored in the ground and transfer it into useable heat for your greenhouse. 

Meanwhile, air source pumps take heat from outdoor air, and either transfer it to air inside your greenhouse or to water in pipes to keep it warm. As ground source heat pumps require excavation costs, air source heat pumps are less expensive to set up.

However, as the RHS says, neither are cheap, easy to install, or particularly suitable for modest greenhouses – although this may change in the future.

5. Keep it insulated

Whether you've opted for one of the best mini greenhouses or a larger design, one of the most effective ways to keep it warm is by reducing heat loss – whether you're using a supplementary heat source or not.

Any cracks along windows or doors should be checked for and sealed. Ensure that you can still open them to provide ventilation.

You can then line your greenhouse with horticultural bubble wrap to keep the cold out. Be sure to clean the glass first, and opt for a type with large bubbles so that plenty of sunlight still comes through. Bear in mind that some crops will suffer with less light: avoid using insulation around more cold-hardy winter lettuces and alpines, for instance, as suggests the RHS.

For very cold nights, adding a layer of horticultural fleece over your tender plants and seedlings can help them survive.

There are more tips on how to protect plants from frost in our practical guide.

6. Try passive solar heating

Utilizing the sun to keep your greenhouse feeling toasty is another eco-friendly approach. 

One way is to have a hydronic heating system installed which uses solar panels. But, there is another option that is more budget-friendly: using recycled plastic bottles.

All you have to do is paint them black, fill them with water, and line the edges of your greenhouse shelving. They will absorb the heat from the sun in the day (make sure you position them in direct sunlight) and then naturally release it throughout the night.

Looking for more cheap garden ideas? Our guide has you covered.

Do you need to heat your entire greenhouse?

Whether you need to heat your entire greenhouse or not really depends on what you are planning to grow, and how cold your region gets. 

If you have a mix of tender and hardy plants, partition off the more vulnerable ones with a layer of bubble wrap or polythene, for instance. That way, you can concentrate your heating methods into a smaller space, which will up the efficiency and save on costs.

Don't forget about heated propagators, too. If you are simply looking to give seedlings a healthy start (perhaps chosen from our list of what to plant in a greenhouse), then these can be an easy-to-use option.

How warm should a greenhouse be?

Of course, as well as knowing how to heat a greenhouse, you will also need to know how warm a greenhouse should actually be. 

Well, according to the RHS, a minimum temperature of 37ºF (3ºC) is sufficient to sustain many tender plants. However, it can still create risks if they get damp. Therefore, aim for 45ºF (7ºC), or 50ºF (10ºC) for greater peace of mind.

Don't forget that there is such a thing as a too-hot greenhouse. 90°F (32°C) is generally considered too warm, whereby you will need to take steps to ventilate the space and provide shade to prevent plants from harm.

Checking your greenhouse daily with a thermometer will ensure you can take action in lowering or increasing the temperatures if needed. Heating systems with integrated thermostats can make this easier.