Heating a Greenhouse in Winter: Costs and Cheaper Options (UK)

A heated greenhouse sounds like a luxury, and for most vegetable growers it is one you can skip. But if you want to keep tender plants alive until spring, a little heat on the coldest nights is the difference between a pelargonium that starts into growth in March and a blackened stump.

The good news is that you rarely need much heat, and the cheapest heat is the heat you don't lose. This guide works through it in the order that saves money: decide whether you need heat at all, insulate, measure, then choose and run a heater, with a calculator to show what it costs at today's electricity price. It's part of our cluster on protecting plants from frost.

Do you need to heat it at all?

Start with what's going to be in the greenhouse over winter. Heat is for plants that can't survive being frozen, the ones gardeners call tender. If you're not sure which yours are, our guide to hardy and tender plants explains the RHS hardiness ratings and what each means for your garden.

An unheated greenhouse is still a busy place in winter. Hardy winter lettuce, mustards and other salad leaves crop through the cold months, broad beans and garlic can be started off in modules, and Mediterranean herbs in pots are kept dry. None of those need heat. A sheet of fleece draped over them on the hardest nights is enough, and our guide to an unheated greenhouse through the year has the full plan.

Heat earns its keep when you want to bring tender perennials through to spring: pelargoniums, fuchsias, dahlias you're keeping in pots, citrus, or the chilli plants from overwintering chillies. Our guide to overwintering tender plants covers which of them really need a frost-free greenhouse and which will sit happily in a cool room or a dry shed instead. If you only have a few, a cool spare bedroom or a bright porch is often cheaper than heating a whole greenhouse for them.

Insulate first: keep the heat you already have

Glass lets light in beautifully, and it lets heat straight back out on a clear night. A greenhouse cools fastest on exactly the nights frost is most likely: clear skies and still air. So before you buy a heater, slow that loss down.

Line the glass with bubble insulation. Greenhouse bubble insulation is a tougher, UV-resistant version of the bubble wrap you'd use for parcels. Fixed to the inside of the frame, it traps a layer of still air against the glass. Put it up in early autumn while the glass is dry and the days are mild, so the clips hold and you're not fighting condensation. It does take a little light, so if you're raising seedlings in late winter you may want to take some of it down on the sunny side.

Stop the draughts. Check the door closes properly, the vents shut fully, and no panes are cracked or missing. A single broken pane lets cold air pour in all night.

Use the free heat. A water butt or a few large containers of water inside the greenhouse soak up warmth during the day and give it back slowly at night. A bed of soil does the same. None of this replaces a heater on a hard night, but it all helps.

Heat only the part you use. If your tender plants fit at one end, hang a curtain of bubble insulation across the greenhouse to make a smaller "room" around them. You're then heating a fraction of the space.

Measure: a max/min thermometer

Most people guess how cold their greenhouse gets, and most guesses are wrong in one direction or the other. A max/min thermometer records the highest and lowest temperature since you last reset it. Hang it at plant height, out of direct sun, and check and reset it each morning.

A week or two of readings tells you a lot. You'll see how far below the outside temperature your greenhouse falls (or doesn't), whether your insulation is working, and how many nights a winter it actually dips below freezing. That last number is what drives your heating bill, and it's the one to put into the calculator below instead of a guess.

Pair it with the frost date checker, which shows when the first frost typically reaches your region and when the last one is usually behind you. Those are typical dates, not a forecast, so keep an eye on the Met Office forecast on cold, clear evenings too.

Choosing a heater

There are three common kinds for a home greenhouse. Whichever you choose, follow the maker's instructions for use in a damp space, and have any mains electricity in a greenhouse installed by a qualified electrician with proper outdoor-rated sockets and RCD protection.

Electric fan heater. The most popular choice where there's power. A fan heater blows warm air, which keeps it moving around the greenhouse and helps stop cold, damp corners where mould sets in. Choose one made for greenhouses with a built-in thermostat, so it switches itself off once the air is warm enough. The heat is dry and there are no fumes.

Tube heaters. Low-powered electric tubes mounted near the floor. They give a gentle, even background warmth and suit a small greenhouse or a curtained-off section. On their own they may not keep up on the very coldest nights in a larger house, and they need a separate thermostat to control them.

Paraffin heaters. Useful where there's no electricity, and they need no wiring. But they burn fuel inside the greenhouse, so they give off fumes and a lot of water vapour, which adds to winter damp. They need a little ventilation, a clean, well-trimmed wick and careful refuelling, and there's no thermostat: they run until you turn them off. Many growers now choose electric for those reasons.

Set the thermostat low

The aim over winter is to keep tender plants alive, not growing. For most of them that means keeping the greenhouse just above freezing. Our greenhouse growing guide describes a "frost-free" greenhouse as one kept at around 3–5°C. Set your thermostat at the low end of what your plants need, check it against the max/min thermometer, and resist the urge to keep things cosy: every extra degree means more hours of running.

Keep the plants on the dry side while it's cold. Damp, cold compost is more dangerous to most overwintering plants than a degree or two of chill, and a dry atmosphere helps keep grey mould away.

What it costs to run

Electricity is sold by the kilowatt-hour (kWh): one kilowatt of power used for one hour. So the running cost of an electric heater is a simple sum:

watts ÷ 1,000 × hours it runs × price per kWh

Here's one hour of a 2 kW heater at the Ofgem price cap for 1 October to 31 December 2026, which is 26.32p per kWh (GB average, Direct Debit):

• 2,000 W ÷ 1,000 = 2 kW
• 2 kW × 1 hour = 2 kWh
• 2 kWh × 26.32p = 52.64p, so about 53p an hour at full power

The important word is hours. A heater on a thermostat doesn't run all night: it cuts in when the air falls to the set temperature and cuts out again once it's warm enough. On a mild night it may hardly run at all; on a hard frost it may run for much of the night. That's why the max/min thermometer matters, and why insulation pays for itself in hours saved. If you want an exact figure, a plug-in energy monitor between the socket and the heater shows how many kWh it used overnight.

Try your own numbers below. It starts at the Ofgem cap rate, but your own tariff is on your bill, so put that in if it's different. The cap changes again on 1 January 2027.

The same calculator is on its own page as the greenhouse heater cost calculator if you want to bookmark it.

Ways to keep the bill down

• Insulate, and fix gaps, before the first cold night.
• Heat a smaller space by curtaining off one end, or move tender plants together onto one bench.
• Set the thermostat just above freezing and check it really is cutting out when it should.
• Only heat when you need to. Many winters bring long mild spells; a heater on a thermostat looks after this itself.
• Move some plants indoors instead. A handful of pelargonium cuttings on a cool windowsill can replace a whole greenhouse of heat.
• Keep fleece handy for the plants that don't need heat, so the heater isn't working to protect them too. Our frost protection guide explains fleece, cloches and cold frames.

What you'll need

If you do heat, the two things that make the biggest difference are a lining that keeps the warmth in and a heater with its own thermostat.

Before the first frost, work through our list of jobs to do before the first frost, and check when it typically arrives in your area with the frost date checker.