Ideally, a greenhouse — or the soil in a greenhouse — should be 37°F or warmer to support a garden. While it’s more difficult to reach this temperature during the coldest months of the year, it’s not impossible. Insulating your greenhouse and installing a heater are not the only heating methods available to you. Continue reading to learn more about how to heat a greenhouse for winter gardening so you can have fresh veggies all year round.
1. Increase insulation
Insulation passively heats a greenhouse; rather than adding more heat, increasing insulation minimizes heat loss. While you can insulate the whole greenhouse structure itself, you can also focus on insulating the soil or individual plants. As long as the plants are at the right temperature, you can forgo insulating or heating the rest of the greenhouse.
Here are some ways to insulate your greenhouse, soil, or plants:
- Use bubble wrap
- Recycle fall leaves
- Add mulch
- Cover your plants
Use bubble wrap
If you have a bunch of bubble wrap lying around, you can use it to insulate your greenhouse. Simply wrap the inside walls of the greenhouse in bubble wrap. So, the next time your Amazon package comes with a questionable amount of bubble wrap, keep it around for the winter months.
Recycle fall leaves
Don’t have bubble wrap? No problem! There’s a cheap — even free — alternative homeowners like you can use: fallen leaves. Bags of fall leaves can be reused as insulation by pilling them up against your greenhouse walls.
Among the many benefits of mulch is the insulation it provides for your soil. Mulch helps keep the soil (and your plants) warm and toasty throughout the wintertime.
Cover your plants
Covering your plants is one way to protect them from a freeze. Greenhouse plants can be covered in horticultural fleece (also known as gardening fleece), row covers (typically made of clear plastic), or cloches. The latter can be recycled drink bottles, old milk containers, or even mini greenhouses (hotboxes).
2. Add thermal mass
Thermal mass is the ability of a material to catch, store, and release heat energy, such as the heat from the sun. Increasing your greenhouse’s thermal mass is as simple as adding more materials with a high thermal mass. These materials are heat sinks that trap the sun’s heat in the morning and release it throughout the night.
Water, soil, clay, stone, bricks, and ceramics are some materials with high thermal mass that homeowners can easily find. Here are some ways to integrate them into your greenhouse:
- Place barrels or tanks of water, preferably black ones for maximum heat absorption, in the corners and along the north wall of the greenhouse. If you don’t have much space, even small jugs of water can help.
- Add brick or stone paths and edging. This not only adds thermal mass but also adds style.
- Store extra bags of soil inside your greenhouse.
3. Make compost
Decaying organic material releases heat as it breaks down. So, making compost inside can help keep your greenhouse warm in the winter; as a bonus, you’ll also get black gold to use in your garden. You can simply dig a trench and make a compost pile, but making a hotbed is a more useful and more aesthetically pleasing option.
Used to start seedlings in the winter, a hotbed is a raised garden bed that’s filled mostly with composting materials topped with a growing medium (such as soil). Aim for a hotbed at least 20 to 30 cm deep and maintain a 3:1 ratio of compost to soil. You’ll also need a good ratio of greens to browns. You can cover your hotbox with a cloche, row cover, or even an old window pane if you want even more heat.
If you don’t want to make anything, you can simply use a compost bin. We have a list of the best compost bins that you can check out for ideas.
4. Add a heating system
Installing a heating system is the most sure-fire way to heat a greenhouse. There are many sources of fuel, such as wood, coal, natural gas, propane, kerosene, electricity, and renewable energy (including solar energy).
They can be used to power these types of heating systems:
- Electric room heater
- Hot water heating system
- Unit heater
- Radiant heater
- Ground-to-air heating system
- DIY small heater
Just make sure that the heat isn’t stuck in one place; you’ll need airflow to move the heat around. If you don’t, moisture might build up and invite mold into your greenhouse.
Some heating systems are better installed before you set up your greenhouse. Otherwise, you may need to tear up some parts of the structure to accommodate elements of the heating system.
If you’d like some suggestions on what greenhouse heater to buy, you can check out our list of the best greenhouse heaters available on the market right now.
Electric room heater
As their name suggests, electric room heaters are powered by electricity. Some of them can be rather fancy and energy-efficient. For example, some electric heaters come with built-in thermostats and can be programmed to work only when the temperature drops below a certain point. A few even come with built-in fans.
Compared to gas heaters, electric heaters are one of the better heating systems, as they don’t produce fumes. As such, they don’t need venting.
Hot water heating system
Hot water systems consist of a boiler and pipes. They warm up greenhouses by boiling hot water and pumping it through a system of pipes throughout the structure. They can also pump steam instead of hot water, similar to how 15th-century Korean greenhouses were heated.
One option you can explore is using solar water heaters instead of a traditional boiler. That way, this type of system won’t need venting.
A unit heater is a standalone heating system which doesn’t have any ducts. Unit heaters can be vented or unvented and powered by a variety of fuel sources. Used in residential and commercial greenhouses alike, they gradually exchange cold greenhouse hair for warm air.
You will need to be careful when using one of these, though, as they need oxygen to function. Your plants need oxygen, too, and you don’t want the heater to leech all of it from your plants.
Unlike a unit heater, radiant heating systems warm greenhouses by heat radiation. That means they only heat the surrounding area without blowing air around. Radiant heaters are quieter than their convection counterparts, as they don’t need fans.
However, they have their downsides. Radiant heaters are best at heating smaller spaces, so you’ll need to be wise about where you put them; place them near the plants that need heat the most. They’re also more expensive.
Wood-burning stoves are a heating system worth trying if you have a steady source of wood for fuel. They can save you a lot of money in the long run if this is the case. However, you have to set these up carefully. Wood stoves are a fire hazard, and they absolutely need venting.
It’s recommended to buy a thermometer if you choose to get this heating system to keep track of how hot your greenhouse is.
Ground-to-air heating system
In a ground-to-air or geothermal heating system, air is heated underground by the soil. Aside from having thermal energy itself, the soil absorbs a lot of heat from the sun because of its high thermal mass. So, you can heat air in underground pipes to pump back into the greenhouse once the cold night sets in.
DIY small heater
What heater can you use in a small greenhouse? You can actually create your own mini space heater using candles and a few terracotta or ceramic pots. Here’s a tutorial on how to make your own candle-powered heater by Gardening for Beginners:
5. Raise animals
Have you ever wanted to supplement your garden veggies with some fresh eggs? If you’ve thought of raising chickens, then consider raising them in your greenhouse or in a coop right beside it. Warm-blooded animals produce body heat, which helps heat up a greenhouse.
FAQs about heating a greenhouse
Can a greenhouse be too warm?
Yes, it can. 90°F is uncomfortable not just for you but also for your plants. Aim for a temperature of at least 37°F to support tender plants, but the ideal temperature is around 45°F to 50°F for optimal growth. Unheated greenhouses can still extend the growing season, but most likely won’t be enough to support year-round gardening in a cold climate.
Disregard this if you’re trying to raise tropical plants or other exotic plants that need a specific temperature to thrive. In that case, you’ll need to heat your greenhouse according to the recommended growing conditions for your plants.
Where should you build your greenhouse for the best winter sun?
Whether you’re contracting someone to build you a greenhouse or building a DIY greenhouse, you’ll want to build it so it’s facing south. In the northern hemisphere, the winter sun stays on the southern side throughout the day. Maximizing how much direct sunlight your greenhouse gets in the winter can reduce the need for heating.
How much does it cost to build a greenhouse?
On average, a greenhouse costs around $11,000, including insulation and heating. That’s if you hire a contractor to install it for you. If you want to save money, you can try building your own greenhouse. You can also set up one of the best greenhouse kits instead of starting completely from scratch. These kits can be pricey, but they’re still cheaper than hiring a contractor.
Is your garden ready for the winter?
Preparing your garden for the winter – whether it’s outside or inside a greenhouse – takes time and effort that you might not have. Instead of fussing over your flowers and other foliage, why not hire a professional gardener to take care of them for you? Lawn Love can connect you with pros in your area who can take care of all your gardening, lawn care, and landscaping needs. Hire a gardener through Lawn Love today, or just get started with a quick, free, and easy quote.