How to Keep Your Grow Tent Warm in Winter

A grow tent that held a perfect 25°C all summer can fall to 13°C on a winter night. The lamp has been doing the heating all along, and the moment it switches off, the cold room, the cold floor and the cold air coming in through the intake take over. Most of the fix costs nothing: change when the lights run, get the tent off the floor and stop throwing warm air away. Only then add a heater, sized small and switched by a thermostat, so you pay for the few degrees you actually need.

Why a grow tent goes cold in winter

In summer the problem is heat, as covered in our summer heat guide. In winter the same tent works against you in four ways at once.

• The light is the heater. Your lamp is the biggest source of heat in the tent. While it is on, the tent runs several degrees above the room. When it switches off, that heat source disappears and the tent falls towards the temperature of the room around it. Modern LEDs run cooler than the old HPS lamps, so the gap between lights-on and lights-off is smaller, but the room itself is now the problem.
• The room is colder. A tent in a spare bedroom might sit in 17°C air. The same tent in a garage, shed or loft can be in air close to freezing on a January night.
• The intake brings in cold air. Every cubic metre the extractor pulls out is replaced by air from wherever the intake draws from. If that is a cold loft or an outside wall, you are heating the outdoors.
• The floor drains heat from the pots. Concrete and uninsulated floors stay cold. Pots standing directly on them can run several degrees cooler than the air around the leaves, and cold roots take up water and nutrients slowly.

The temperatures to aim for

These are working ranges for the air at plant height, not lab figures. The lights-on column matches our summer guide; the column that matters in winter is the one on the right.

Two rules matter more than the exact figures. First, keep the difference between day and night to roughly 5-8°C; a tent that swings from 27°C to 12°C every day spends hours outside the range in both directions. Second, avoid sudden drops. A gradual fall over an hour is far kinder than a cold blast the moment the lights go out, which is also when condensation forms.

You cannot manage what you have not measured. Put a thermometer that records the overnight minimum at canopy height for a few nights before you buy anything. Plenty of growers find their tent only drops a couple of degrees, and the free fixes are all they need.

The free fixes, in order

Work through these before you spend money on heat. Each one either keeps heat you are already paying for or stops cold air getting in.

1. Run the lights through the night

Your lamp produces heat whether you want it or not, so make it work during the coldest hours. Switching the lights-on period to run overnight, for example 8pm to 8am, puts the lamp's heat into the tent when the room is at its coldest, and leaves the lights-off period in the warmer part of the day. In summer the same trick is used to dodge afternoon heat; in winter it does the opposite job. If you are on a tariff with cheaper night-time electricity, it may also cost less. Use a proper timer or contactor if your lights draw a heavy load, as set out in our timers and contactors guide.

2. Get the tent and pots off the cold floor

Stand the tent on a sheet of rigid insulation board, an old rug or a few layers of carpet underlay, and raise pots off the tent floor on a board or upturned trays. Keep the tent a hand's width away from outside walls and windows, which radiate cold. If you have the choice, move the tent from the garage or loft into the warmest room you can spare: no heater is as cheap as a warmer starting point.

3. Slow the extractor

An extractor sized for a July heatwave is moving far more air than the tent needs in January, and every litre it pulls out takes warmth with it. Turn it down with a speed controller so it runs slowly when the tent is cool. A thermostatic fan controller does this automatically, easing off as the temperature falls and speeding up only when the lights push it up. Do not switch the extractor off altogether for long periods: it also removes moisture, and a tent with no air change at night quickly becomes a mould risk, as our mildew and grey mould guide explains. Duct runs and fan speed also affect noise; our ducting and noise guide covers that side.

4. Warm the air coming in

Whatever the extractor removes is replaced through the intake. If your passive vents or intake fan draw from a cold loft, an outside wall or a garage door, move the intake so it draws from the warmest air available, usually the room the tent stands in. Close any intake flaps you do not need, and check the zips are fully closed. A slower extractor means a smaller intake is enough; see our fan and filter sizing guide for how intake and extraction relate.

5. Then add heat, small and on a thermostat

If the tent still falls below your target after the first four steps, now is the time for a heater. The goal is a small, steady top-up controlled by a thermostat, not a big heater running flat out.

Choosing a heater

There are four kinds worth considering for a grow space. The right one depends on how cold the room is and how big the space is, not on how cold the plants look.

Tube heaters: the default for a tent in a house

A tube heater is a sealed metal tube with a low-wattage element inside. It gives off gentle radiant warmth rather than a blast of hot air, so it does not create hot spots or dry the air quickly. Running costs are low because the wattage is low: a 120W tube running for 12 hours uses 1.44kWh, against 24kWh for a 2kW fan heater running flat out for the same time. That low output is also the limit. A tube heater lifts a space by a few degrees; it will not turn a 2°C garage into a 20°C grow room. Some models include a built-in thermostat, others need an external one.

Oil-filled radiators: quiet heat for rooms

Oil-filled radiators warm the air through a set of fins and keep radiating after the element switches off, which smooths out temperature swings. They are silent, which matters if the tent is in a bedroom. At 1-2.5kW they suit grow rooms and large tents better than a small tent, and they need floor space and clearance around them.

Fan heaters: for genuinely cold spaces

A ceramic fan heater moves a lot of warm air quickly, which is what a cold garage or loft needs. The danger is overshoot: a 2kW heater in a 1.2m tent can push it past 30°C within minutes. Use a fan heater on its low setting, point it at the floor or across the tent rather than at the plants, and always switch it with a thermostat. Choose one designed for damp spaces, with a thermal cut-out.

Smart tent heaters

Some heaters are built for grow tents, with a controller that holds a set temperature or VPD target and connects to an app. They cost more, but if you already run a smart controller, one system can manage heat, extraction and humidity together. Our environment controllers buying guide compares the options.

Shop grow room heating

Thermostats and fans: stop them fighting

A heater without a thermostat is either on or off, and either way the tent ends up at the wrong temperature. A plug-through thermostat sits between the heater and the wall socket and switches the heater on only when the probe reads below your set point.

• Put the probe at canopy height, shaded from the lamp and well away from the heater's own airflow. A probe beside the heater switches it off too early; a probe in direct light reads high.
• Check the thermostat's load rating against the heater's wattage. A controller rated for 3,500W or more handles a 2kW fan heater with room to spare; a small propagator thermostat does not.
• Use separate day and night settings if your controller allows it. A day and night controller holds one target while the lights are on and a lower one when they are off, which is exactly the pattern in the table above.
• Set the heater below the fan. If your extractor speeds up at 26°C and your heater switches on at 25°C, the two will fight all night and you will pay for both. Leave a gap of 3-4°C: for example, heater on below 19°C at night, extractor ramping up above 24°C.

If you run a smart controller, much of this is built in. Our humidity guide explains how temperature and humidity targets work together as VPD.

Heater safety in a grow space

A grow tent is a fabric box full of water, electrics and warm air, so heaters need more care than in a living room.

• Electric only. Never use a gas or paraffin heater in a closed tent or small grow room. They release a lot of water vapour, use up oxygen and can produce carbon monoxide.
• Clearance. Stand the heater on the floor, upright and stable, well clear of the tent walls, ducting, cables and anything that could fall on it. Follow the clearance distances in the maker's instructions, and never cover a heater or hang anything to dry on it.
• Built for damp spaces. Choose a heater with a thermal cut-out and, ideally, an IP rating for damp conditions. Keep it away from where water can splash or drip.
• Mind the socket. A UK 13A plug and socket supplies about 3kW in total. A 2kW heater on the same extension lead as your lights and fans can overload it. Plug the heater into its own wall socket, never daisy-chain extension leads, and check leads are rated for the load.
• Not at the plants. Point warm air across the floor or into open space, never directly at leaves, which will dry and scorch.

Keep the roots warm

Air temperature is only half the story. A pot sitting on a cold floor can hold the roots several degrees below the air, and cold roots slow everything: water uptake, nutrient uptake and growth. A cheap infrared thermometer pointed at the side of a pot or the surface of the medium shows you quickly whether the roots are colder than the air.

• Soil and coco: raise pots off the floor and insulate under them. Water with water at room temperature, not straight from a cold outside tap or butt.
• Seedlings and cuttings: use a heat mat under the tray or propagator, ideally with a thermostat, rather than heating the whole tent to propagation temperatures. Our propagator guide covers bottom heat in detail.
• Hydroponics: an unheated tank in a cold room can fall well below the 18-22°C roots want. A submersible thermostatic heater sized to the tank holds it steady. Our root zone temperature guide explains why that range matters and how it links to dissolved oxygen.

Humidity and condensation at night

Winter brings a humidity problem that catches many growers out. Cooler air holds less water vapour, so when the tent cools at lights-off, the same moisture becomes a higher relative humidity. A tent at 55% with the lights on can climb past 80% an hour after they go off, and if any surface is colder than the air, water condenses on it: on the tent walls, the ducting and the leaves.

The fixes follow from everything above. Holding the night temperature steady with a thermostat-controlled heater is the most effective single step, because the RH spike comes from the temperature drop. Keeping a slow, constant air change removes the moisture the plants release overnight. A small circulation fan stops pockets of still, damp air settling in the canopy. If RH is still too high at night after that, a dehumidifier is the next step; it also gives off a little heat of its own.

Common mistakes

• Buying the biggest heater. A 2kW fan heater in a small tent overshoots, dries the air and costs far more to run than a tube heater that would have done the job.
• No thermostat. A heater left on all night, or switched by a timer, cannot respond to the weather. On a mild night it cooks the tent; on a cold one it may not be enough.
• Switching the extractor off. It keeps heat in, but humidity and stale air build up. Slow it down instead.
• Probe in the wrong place. Next to the heater, under the lamp or on the floor, the thermostat reads the wrong number and controls the tent to it.
• Heater and extractor fighting. If their set points overlap, one adds heat while the other throws it out. Leave a 3-4°C gap.
• Forgetting the floor. Warm air and cold roots is a common winter combination. Insulate under the pots before you add more air heating.
• Overloading a socket. Heaters draw a lot of current. Give a heater its own wall socket.

Quick answers

What temperature should a grow tent be at night?

For most crops aim for roughly 18-22°C with the lights off, and keep the drop from the lights-on temperature to about 5-8°C. Seedlings and cuttings like it warmer, around 20°C or more at the root zone. Growth slows noticeably once the air falls much below 15°C.

Do I need a heater in my grow tent in winter?

Not always. If the tent is in a heated room, running the lights through the night, insulating the floor and slowing the extractor is often enough. You need a heater when the tent is in a garage, shed, loft or unheated room and still drops below your night-time target after those free fixes.

What size heater does a grow tent need?

Smaller than most people think. In a tent inside a cool room, a 60-240W tube heater lifting the temperature a few degrees is usually right. Larger tents and rooms suit a 1-2kW oil-filled radiator. A 2kW fan heater is for genuinely cold spaces such as a garage or loft, and should run on a low setting under a thermostat so it does not overshoot.

Is it safe to put a heater inside a grow tent?

It can be, if you choose an enclosed electric heater with a thermal cut-out, stand it on the floor well clear of the tent walls and anything that could fall on it, follow the maker's clearance distances, and never cover it. Never use a gas or paraffin heater in a closed tent: they release moisture and can produce carbon monoxide.

Should I turn my extractor fan off in winter?

Slow it down, but do not switch it off for long. The extractor removes moisture and stale air as well as heat, so with no air change at all humidity climbs and mould follows. A fan speed controller lets you run it slowly when the tent is cool and faster only when it needs to shed heat.

Why does humidity shoot up when the lights go off?

Cooler air holds less water vapour, so when the tent cools the same amount of moisture becomes a higher relative humidity, even though nothing has been added. In winter the night-time drop is bigger, so the spike is bigger. Holding the night temperature up with a thermostat-controlled heater is one of the most effective ways to stop it.

How do I keep the roots warm in winter?

Get pots off a cold floor with a board, rug or insulation sheet, use a heat mat under seed trays and propagators, and use a thermostatic aquarium-style heater in hydroponic tanks. Aim for 18-22°C in the root zone, the same target as our root zone temperature guide.

Is this the same for tomatoes, chillies and houseplants?

Yes. Overwintering chillies, starting tomatoes early under lights or keeping houseplants and seedlings growing through a cold spell all come down to the same things: a steady night temperature, warm roots, controlled humidity and a heater on a thermostat. Nothing here depends on what you are growing.