Quick answer: To keep a grow tent warm in a UK winter, first stop heat escaping: insulate the room or the floor under the tent, slow the extractor fan at night, and run your light during the coldest hours. Then add a heater that is sized for the space and controlled by a thermostat, so it only switches on when needed. Oil-filled radiators give gentle, quiet heat; fan heaters warm a space quickly. Always measure at plant height with a thermometer and hygrometer.
Why winter is hard on indoor plants
Once October arrives, garages, lofts, sheds and spare rooms can get very cold overnight. Your light is a heat source, so the tent may be warm while it is on, then drop sharply as soon as it switches off. Big swings between day and night temperatures slow growth, can stall flowering and fruit set, and cold, damp air encourages mould.
The goal is not just "warmer" but steadier. A stable temperature and humidity will do more for your tomatoes, chillies or herbs than a tent that bounces between hot and cold.
Step 1: Measure before you buy anything
Put a thermometer and hygrometer in the tent at canopy height, not on the floor or right under the light. The Garden King hygrometer with probe lets you put the probe where the plants are and read it without opening the tent. Check the readings at the end of the lights-off period, as that is usually the coldest point.
Once you know how cold it gets and for how long, you can choose the right fix.
Step 2: Keep the heat you already have
Insulate
Cold floors are a common problem in garages and outbuildings. Lifting the tent off bare concrete with insulation boards or a thick mat underneath helps a lot. If the tent is in an outbuilding, draught-proofing doors and windows will cut how hard any heater has to work.
Slow the extractor at night
Your extractor fan is designed to pull warm air out. That is great in summer, but in winter it can drag all the heat out of the tent during lights-off. A fan speed controller or an EC fan with a controller lets you turn it down at night while still keeping some air moving. Do not switch extraction off completely, as stale, humid air leads to mould.
Flip your light schedule
Run your light at night, when the room is coldest, and let the dark period fall in the warmer daytime. This uses the light's heat when you need it most and is often the cheapest fix of all. A heavy-duty lighting timer makes this easy.
Draw intake air from a warmer space
If the tent pulls air in from a freezing garage, the incoming air cools everything. Where you can, take intake air from a warmer room or the house side of the building.
Step 3: Choose the right grow tent heater
Oil-filled radiators
Oil-filled radiators give gentle, even convection heat, run silently with no fan and are less likely to dry the air out. They take a while to warm up but hold heat well. We stock the Grow Gadgets 1000W oil-filled radiator for small to medium spaces and the 2000W version for larger rooms. In a small tent, many growers place the radiator in the room just outside the tent and draw warm intake air in, rather than squeezing it inside next to plants.
Fan heaters
Fan heaters warm a space quickly and push the warm air around, which helps avoid cold spots. The Grow Gadgets 2KW greenhouse heater has fan-only, 1000W and 2000W settings, a built-in thermostat, IP24 splash resistance and an automatic thermal cut-out. The 3KW model offers fan-only, 1500W and 3000W settings for bigger rooms or outbuildings. Point fan heaters away from plants, as a direct blast of hot, dry air can scorch leaves.
How big a heater do I need?
That depends on the size of the space, how well it is insulated and how cold it gets outside, so there is no single answer. As a rule, a single tent in an insulated spare room needs far less than a whole uninsulated garage. Start with the smaller option if your space is reasonably sheltered, as a heater that is far too big will overshoot and cause the temperature swings you are trying to avoid. Bring us your measurements and we can help you choose.
Step 4: Put the heater on a thermostat
A heater without control will either run constantly or not enough. A plug-in thermostat switches it on and off to hold the temperature you set. The Grow Gadgets thermostat controller works from -9°C to 50°C, handles loads up to 6000W, has a detachable probe and remembers its settings after a power cut. Place the probe at plant height, away from the heater itself, so it reads what the plants feel.
Always check that a controller is rated for the heater's wattage, and never run a heater from an overloaded extension lead.
Watch humidity too
Heating changes humidity. Fan heaters in particular can dry the air, which young plants and cuttings dislike. On the other hand, a cold, still tent at night can become damp. Keep an eye on your hygrometer. A Grow Bitz humidifier can help if the air gets too dry, and gentle air movement from a clip fan helps prevent damp pockets.
Safety checklist
- Keep heaters on a stable, flat surface, clear of tent walls, cables and water.
- Follow the manufacturer's clearance distances.
- Use heaters with overheat protection and a thermal cut-out.
- Plug heaters into a wall socket or a properly rated controller, not a daisy chain of adaptors.
- Keep water reservoirs and spray bottles well away from electrics.
FAQs
What temperature should my grow tent be in winter?
It depends on what you grow, so check the needs of your crop. Most common indoor crops such as tomatoes, chillies, peppers and herbs are happier with a steady, moderate temperature than with big swings between day and night. Focus on keeping the lights-off temperature from dropping too far.
Can I put a heater inside my grow tent?
You can, as long as it fits safely with proper clearance from the walls and plants, and is controlled by a thermostat. In small tents it is often easier and safer to heat the room and draw that warm air into the tent.
Is an oil-filled radiator or a fan heater better?
Oil-filled radiators are quiet, gentle and less drying; fan heaters are quicker and better at spreading heat around. Many growers use an oil-filled radiator for steady background warmth and a thermostat to keep it in check.
Will an LED keep my tent warm enough?
LEDs give off much less heat than old HPS lamps, which is great in summer but means many growers who switch to LED need a little extra heat in winter, especially during lights-off.
Get your tent ready for winter
See our full heating and climate control range, or come and talk to us at Unit 2 BlackLake, West Bromwich, B70 0PJ. You can also call 0121 553 7872 or WhatsApp us for advice. Free UK delivery over £99, and we price match.