| Key takeaways |
|---|
| • Start with free heat: Insulation, thermal mass, and compost trap or create warmth before you ever pay to run a heater. • Know your target: Most tender plants need the greenhouse at or above 37 degrees, just above freezing, before you add an active heater. • Match the heater to the space: Size the heater and fuel to your greenhouse, climate, and available power to avoid overspending. |
Learning how to heat a greenhouse for winter gardening comes down to one goal: keeping it at or above 37 degrees, just above freezing, so tender plants keep growing.
Reaching that temperature in the coldest months is harder, but far from impossible. Insulating your greenhouse and installing a heater are not your only options, and many methods cost little or nothing.
Below, we cover the free and low-cost ways to trap heat, the heaters worth buying, and how to pick the right one. If it all sounds like work, Lawn Love can set you up with a local gardening pro.
5 ways to heat a greenhouse for winter gardening
To heat a greenhouse, trap free heat first with insulation, thermal mass, and compost, then add an active heater only when a hard freeze demands it.
These 5 methods run from free and passive to plug-it-in active, and most greenhouses stay warmest when you combine a few of them.
1. Increase insulation
Insulation passively heats a greenhouse. Rather than adding more heat, increasing insulation minimizes heat loss.
You can insulate the whole structure, or focus on the soil or individual plants. As long as the plants stay at the right temperature, you can skip 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 a package arrives 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 piling them up against your greenhouse walls.
Cary Rivard, professor and center director for Olathe Horticulture Research and Extension Center at Kansas State University, does point out one issue using leaves.
“They can work, but bio-degradable materials like leaves can also lead to more disease pressure from botrytis and other plant pathogens. So, you’ll want to be careful about using those materials,” Rivard says.
Add mulch
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.
Don’t think you need to spend money on fancy mulch, either. You can find free mulch in your own backyard, such as fall leaves, grass clippings, or old paper products.
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).
Rivard prefers row covers over most materials, even bubble wrap.
“Bubble wrap could also be used to cover plants in the greenhouse, but you’re better off to use thick row cover,” he says. “It will let more light through and probably costs the same or less than bubble wrap.”
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 easy-to-find materials with high thermal mass.
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 winter, and as a bonus, you’ll get black gold for your garden.
The key is to continuously maintain the compost pile to generate the most heat.
“Compost can create an incredible amount of heat energy, but it’s usually short-lived,” Rivard says. “Keep in mind that once the pile has burned through all the readily available nitrogen in the decomposition process, that the energy output (aka heat production) is much lower. So, unless you are able to constantly cycle in fresh material, you probably won’t get a very reliable heating source.”
You can dig a trench and make a compost pile, but a hotbed is a more useful and attractive option.
Used to start seedlings in the winter, building a hotbed gives you a raised garden bed filled mostly with decomposing manure or compost and topped with a few inches of soil.
Aim for a base of at least 12 to 18 inches of manure or compost so it gives off steady heat.
You will also need a good balance of greens (nitrogen-rich scraps like vegetable trimmings) and browns (carbon-rich material like dry leaves or straw), which fuels the heat-producing decomposition.
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 build 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.
They can be used to power these types of heating systems:
Electric room heater
Hot water heating system
Unit heater
Radiant heater
Stove
Ground-to-air heating system
DIY small heater
Just make sure 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.
“Proper ventilation is critical any time that propane, natural gas, fuel oil, or kerosene is involved,” Rivard says. “Most heaters come with instructions on how to properly ventilate them and they should be followed very closely.”
Some heating systems are better installed before you set up your greenhouse. Otherwise, you may need to tear up parts of the structure to fit elements of the heating system.
If you’d like some suggestions on what greenhouse heater to buy, 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 can be rather fancy and energy-efficient. For example, some 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 options, 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 greenhouses by boiling water and pumping it through a system of pipes throughout the structure. They can also pump steam instead of hot water for the same warming effect.
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.
Unit heater
A unit heater is a standalone heating system that 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 air for warm air.
You will need to be careful when using one, 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.
Radiant heater
Unlike a unit heater, radiant heating systems warm greenhouses by heat radiation. That means they heat only 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 place them wisely, near the plants that need heat the most. They’re also more expensive.
Stove
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.
However, you have to set these up carefully. Wood stoves are a fire hazard, and they absolutely need venting.
It’s a good idea to buy a thermometer if you choose this heating system, so you can keep track of how hot your greenhouse gets.
Ground-to-air heating system
In a ground-to-air or climate-battery system, warm greenhouse air is pushed underground, where the soil stores its heat. The soil holds thermal energy and also soaks up the sun’s heat thanks to its high thermal mass.
So, a fan can draw warm daytime air into an insulated bed of soil or rock below the greenhouse and release it back inside once the cold night sets in.
DIY small heater
What heater can you use in a small greenhouse? You can 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, consider keeping them in your greenhouse or in a coop right beside it. Warm-blooded animals produce body heat, which helps warm up a greenhouse.
How to choose the right greenhouse heater
Picking a greenhouse heater comes down to a few simple questions, and answering them first saves you money and hassle.
Start with size. A bigger greenhouse holds more air, so it needs a heater with more output to hold a steady temperature on the coldest nights.
Next, look at how well your greenhouse holds heat. Single-layer glazing and drafty seams lose warmth fast, so a leaky structure needs more heating power than a well-insulated one.
Then factor in your climate and the plants you grow. A mild-winter grower keeping hardy greens happy needs far less heat than someone protecting tropicals through a hard freeze.
Finally, match the heater to the fuel you can actually use. Electric heaters suit small, powered greenhouses, while propane or gas models make sense for larger or off-grid setups with proper ventilation.
| Factor | What to check | Why it matters |
| Greenhouse size | Floor area and ceiling height | More air volume needs more heat output |
| Heat retention | Glazing layers and drafty seams | A leaky structure demands a stronger heater |
| Climate and plants | Coldest lows and plant hardiness | Tropicals in a hard freeze need far more heat |
| Fuel access | Power, propane, or gas on-site | Determines which heater type is practical |
FAQs
Most tender plants start to struggle once the greenhouse dips below about 37 degrees, and a hard freeze can kill them. Temperatures above 90 degrees stress plants too, so the sweet spot for most winter crops sits between roughly 45 and 55 degrees.
An unheated greenhouse traps daytime heat and buffers a few degrees, which extends the growing season and protects hardy plants. During a hard cold snap, though, it may not stay above freezing on its own. Adding thermal mass, insulation, and row covers gives cold-tender plants a much better chance.
Amish growers lean on off-grid, low-cost methods rather than electricity. That usually means passive solar design, plenty of thermal mass, compost or manure hotbeds that give off heat, and wood-burning stoves for the coldest nights. It’s the same free-first approach any home gardener can copy.
Whether you’re contracting someone or building a DIY greenhouse, position it facing south. In the northern hemisphere, the winter sun tracks across the southern sky all day. Maximizing direct sunlight in winter warms the space naturally and reduces how much you need to heat.
Is your garden ready for the winter?
Preparing your garden for winter, whether it’s outside or inside a greenhouse, takes time and effort you might not have. Instead of fussing over your flowers and foliage, let a pro handle it.
Lawn Love can connect you with local gardening pros who handle all your gardening, lawn care, and landscaping needs. Hire a gardener through Lawn Love today, or get started with a quick, free, and easy quote.
Main Photo Credit: janiecbros | Canva Pro | License
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