Greenhouse heating is about keeping a greenhouse above freezing through the coldest nights, so tender plants survive winter and spring sowing can start early. A greenhouse without heat is still useful — it shelters plants from wind and rain and warms up in the sun — but on a frosty night it can get nearly as cold inside as outside. Heating changes that, but it also costs money and needs care: the goal is to heat only as much as you need, to keep the heat in, and to do it safely. This guide explains the options in general terms, without prices or invented energy figures, because costs vary by heater, fuel and supplier.
Start by deciding what you need the heat for. Keeping a greenhouse just above freezing — frost-free — is the cheapest and most common goal, and it suits overwintering tender plants, hardy seedlings and stored pots. Keeping a greenhouse warm enough for tropical plants or for growing tomatoes through winter needs far more heat and is expensive in most climates. Most home gardeners aim for frost-free, and they grow their tender crops in summer and use the greenhouse in winter for storage, overwintering and early sowing. Use the frost date calculator to check your local frost dates, and our winter frost protection guide for protecting plants outside the greenhouse.
Insulation first: keep the heat in
Before you buy a heater, insulate. Heating an uninsulated greenhouse is like heating a house with the windows open: the heat escapes as fast as you add it. Bubble wrap insulation fixed to the inside of the glass or polycarbonate is the most common home method: it lets light through, adds an insulating layer of air, and is easy to fit in autumn and remove in spring. Seal gaps around doors, vents and panels, because cold air leaking in undermines any heating. A thick layer of mulch or straw on the floor, and water containers such as large barrels painted dark, can help in a small way: water stores heat from the day and releases it slowly at night, which softens the overnight drop. This passive storage will not keep a greenhouse frost-free on its own in a hard frost, but it reduces the work a heater has to do.
Site and structure matter too. A greenhouse sheltered from cold winds by a fence, hedge or wall loses less heat than one in an exposed spot. A smaller greenhouse is cheaper to heat than a large one, so some gardeners divide a large greenhouse with a plastic curtain in winter and heat only the smaller section where the tender plants are. Double glazing or twin-wall polycarbonate insulates better than single glass, but most home greenhouses are single-glazed, and bubble wrap is the practical fix for them. Keep the greenhouse clean: dirty glass lets in less sun, and winter sun is the free heat you want most.
Heater types in general terms
Electric fan heaters are the most common choice for home greenhouses: they are easy to install, they spread warm air around, and they can be paired with a thermostat so they switch on only when the temperature falls below a set point. A thermostat is essential, because a heater without one runs constantly and wastes energy. Electric tube heaters give gentle background heat and are often fixed low down along a wall. Paraffin heaters are used in greenhouses without electricity, but they need ventilation, they produce moisture and fumes, and they must be kept away from plants and flammable materials — follow the manufacturer's instructions exactly, and never use a heater designed for outdoor use inside a greenhouse. Gas heaters also need proper ventilation and a safe gas supply, and they should be installed by a qualified person. Whatever type you use, check that it is suitable for greenhouse use, keep it clear of plants, soil and stored materials, and have it checked regularly.
Solar heating on its own is rarely enough in winter in most climates, because the days are short and the nights are long. A greenhouse can gain useful heat from the sun during the day, and thermal mass such as water barrels helps hold some of it into the night, but on a cold, cloudy spell there is little solar heat to store. Treat solar gain and thermal mass as helpers that reduce heating, not as a replacement for a heater if you need a frost-free greenhouse.
Thermostats, temperature and monitoring
A thermostat is the single most useful part of a greenhouse heating setup. Set it to the lowest temperature your plants need — just above freezing for frost-free overwintering — and let it switch the heater on and off as needed. A minimum-maximum thermometer, or a simple digital thermometer you can read from the house, tells you whether the setup is working: check it on cold mornings and after a frost, and adjust the thermostat if the greenhouse is colder than you expected. Keep a record through the first winter, so you know how the greenhouse behaves in different weather. Ventilation still matters in winter: on mild, sunny days, open a vent or door briefly to let out damp air, because a warm, damp greenhouse encourages mould and disease. Close it again before evening so the heat stays in overnight.
Safety
Heating safety deserves care. Keep heaters away from flammable materials such as straw, paper, dry leaves and stored chemicals. Keep cables tidy, dry and out of the way, and use electrical equipment that is suitable for a damp greenhouse environment. Never use a heater that is damaged, and never improvise a heating setup with a device not designed for the job. If you use a fuel-burning heater, make sure there is ventilation, keep a fire extinguisher or a bucket of sand nearby, and never leave it running unattended for long periods without checking it. Keep children away from heaters, and store fuel safely and out of reach. If you are unsure about the electrical or gas setup, ask a qualified electrician or gas engineer rather than doing it yourself. Check your local extension service if you want advice on overwintering plants in your area.
Overwintering plants in a heated greenhouse
A frost-free greenhouse is ideal for overwintering tender plants such as pelargoniums, fuchsias, citrus in pots, and young plants raised for spring. Water them sparingly in winter, because they are barely growing and wet soil in cold conditions rots roots. Remove dead leaves regularly, because they harbour mould, and space plants so air can circulate between them. Check plants weekly for pests such as whitefly and aphids, which can build up in a warm greenhouse, and use the pest identifier helper if you are unsure what you are looking at. Hardy plants such as many herbs and perennials do not need a heated greenhouse at all — they are better left outside or in an unheated cold frame, where they get the cold they need to rest properly. Our cold frames guide covers unheated protection for hardy plants.
Common mistakes
The most common greenhouse heating mistakes are heating an uninsulated greenhouse, running a heater without a thermostat, setting the thermostat higher than the plants need, forgetting ventilation so the greenhouse becomes damp and mouldy, overwatering plants in winter, and using an unsuitable or unsafe heater. Another is heating the whole greenhouse when only a small section holds tender plants. The fixes are simple: insulate first, use a thermostat, heat only to the temperature your plants need, ventilate on mild days, water sparingly in winter, and use a heater designed for greenhouse use. A little heat, well kept in, does far more than a lot of heat escaping through gaps and single glass.
Frequently asked questions
What temperature should I keep my greenhouse in winter?
For most home gardeners, just above freezing is enough — a frost-free greenhouse that protects tender plants and lets you start sowing early. Tropical plants need much more warmth, and that is expensive to maintain through winter in most climates. Set the thermostat to the lowest temperature your plants need, and no higher.
Is bubble wrap insulation worth it for a greenhouse?
Yes, for most home greenhouses. Bubble wrap fixed inside the glass or polycarbonate adds an insulating layer, still lets light through, and is easy to fit in autumn and remove in spring. Combined with sealing gaps around doors and vents, it makes a noticeable difference to how much heating you need.
What is the cheapest way to heat a greenhouse?
There is no single cheapest option, because costs depend on your heater, your fuel and your supplier's rates. In general, the cheapest heat is the heat you do not lose: insulate well, seal gaps, heat only a small section if that is all you need, and use a thermostat so the heater runs only when the temperature falls. Check the label on any heater you buy and your supplier's rates before deciding.
Can I heat a greenhouse without electricity?
Yes, with a fuel-burning heater such as a paraffin heater, but it needs ventilation, it produces moisture and fumes, and it must be used exactly as the manufacturer instructs. Thermal mass such as water barrels and good insulation help, but they will not keep a greenhouse frost-free on their own in a hard frost.
Do I need to ventilate a heated greenhouse in winter?
Yes, on mild, sunny days. A warm, damp greenhouse encourages mould and disease, so open a vent or door briefly to let out damp air, then close it again before evening so the heat stays in overnight. Ventilation and heating work together: heat keeps the plants frost-free, and fresh air keeps them healthy.