How to Winterize Your Grow Tent or Grow Room
, by Matt’s Hydroponics Team, 7 min reading time
When the heat comes on in New England, grow rooms change: dry air during the day, cold and damp at lights-off. Here’s how to keep temperature, humidity and airflow steady until spring.
When the heat comes on in New England, grow rooms change. The air in your house dries out, and a tent in a basement or garage can lose its heat fast once the lights shut off. Dry days and cold, damp nights are behind a lot of the winter trouble growers bring to our counter: slow growth, crispy leaf edges, powdery mildew and bud rot.
None of it is hard to fix. Here’s how to keep your temperature, humidity and airflow steady from the first frost until spring.
Know your numbers first
Hang a thermometer and hygrometer at canopy height, not on the floor or against the tent wall, and pick one that remembers the day’s high and low. In winter, the number that matters most is the low your room hits after the lights go off.
Under LED lights, these are solid starting targets:
| Stage | Lights on | Lights off | Humidity |
|---|---|---|---|
| Veg | 78–80°F | 70–72°F | 60–65% |
| Flower, weeks 1–3 | About 80°F | About 72°F | About 58% |
| Flower, weeks 4–6 | 76–78°F | About 70°F | 50–55% |
| Last 2–3 weeks | About 74°F | About 65°F | About 45% |
- Keep the lights-off drop to about 10°F or less. Bigger drops make humidity spike overnight.
- LED rooms run warmer than the old HPS numbers. Leaves under LEDs sit a couple of degrees cooler than the air around them, so the room has to make up the difference. If you switched from HPS to LED, expect your room to run colder this winter than it used to.
- Running CO2 with strong light? Stay at the warm end of these ranges through veg and the first six weeks of flower. Under high light and CO2, plants want the heat.
Stop the lights-off temperature crash
- Run your lights at night. Shift your schedule so the lights are on during the coldest hours. Your lights already make heat you’re paying for, so let them work while the house is coldest and put lights-off in the warmer part of the day.
- Get the tent off the cold floor. Concrete pulls heat out all night long. Set the tent on rigid foam insulation board, and keep it away from outside walls, windows and drafty doors.
- Add heat on a thermostat, not by guesswork. A heater should kick on by itself when the room hits your nighttime low and shut off once it’s back in range. The AC Infinity THERMOFORGE T3 is built for grow spaces, with its own temperature probe and triggers, and it can sit outside the tent and blow warm air in through its hose. For a whole room, an oil-filled radiator like the Hurricane Heatwave gives steady heat with no exposed heating element.
Heater safety: the part people skip
- A space heater is often the biggest single electrical load in a home grow. The Hurricane Heatwave on high is 1,500 watts, which pulls 12.5 amps on its own. On a standard 15-amp, 120-volt circuit, continuous load should stay at or under 12 amps, so a heater on high takes the whole circuit. Give it its own circuit, separate from your lights, and use a 20-amp circuit if you have one.
- Plug heaters straight into a wall outlet, never into an extension cord or power strip.
- Keep space heaters at least 3 feet from anything that can burn, tent fabric included, and follow the maker’s clearance rules. For the THERMOFORGE, AC Infinity’s manual says to set the heater outside the tent, 3 feet from walls and furniture, and run its duct in through the lowest duct port. Choose a heater with tip-over and overheat shutoff.
- Skip propane and kerosene heaters in a grow space. Burning fuel adds moisture and fumes to the room and can put carbon monoxide into your home.
- New circuits, and anything that runs on 240 volts, are a job for a licensed electrician.
Winter humidity: dry days, damp nights
Cold outdoor air holds very little water. Once your furnace warms it up, its relative humidity drops, which is why houses get so dry in winter. A tent pulling in that air can run too dry during the day, especially for clones and young plants.
At night it flips. As the room cools after lights-off, humidity climbs, and if leaves or buds cool to the dew point, water condenses right on them. Wet flowers in a cool room are how bud rot gets its start.
- Too dry during the day: run a humidifier for clones and veg, and let a controller slow your exhaust fan so you’re not constantly swapping moist air for dry air. Keep a little negative pressure so smell still leaves through the carbon filter.
- Too damp at night: set your dehumidifier to start before lights-off rather than after humidity spikes, keep fans moving air all night, and if you hand water, finish at least two hours before the lights go out.
- Late flower: keep humidity around 40–55% day and night, and don’t let it sit above about 60%.
Watch where your air comes from
- If your tent pulls intake air from outside or from an unheated room, that air can be close to freezing. Pull it from a heated space instead, and never let cold air blow straight onto your plants.
- Your exhaust fan doesn’t need to run flat out in winter. A fan controller can run exhaust by temperature and humidity so it isn’t dumping your heat all night.
- Running CO2? Winter is a good season for a sealed room. With no exhaust running, the heat from your lights stays in the room, and so does the CO2. Use it only in a sealed room with a CO2 monitor and alarm, closed off from living space.
Keep the roots warm too
- Pots sitting on cold concrete chill the root zone. Cold roots drink less, stay wet longer and are easier to overwater, and they take up phosphorus poorly, which shows as slow, purplish growth. Warm the roots before you add more feed: raise pots on pot risers or set them on foam board.
- In hydro, keep your reservoir around 65–70°F. A cold basement can pull it lower, and warmer isn’t better either, since warm water holds less oxygen.
- Starting seeds or clones? A heat mat run on a thermostat keeps the root zone warm in a cool room.
Your winter checklist
- Thermometer and hygrometer at canopy height that records the overnight low
- Lights-off drop of about 10°F or less
- Tent off the floor and away from outside walls
- Heater on a thermostat, on its own circuit, plugged into the wall and clear of anything that can burn
- Dehumidifier set to start before lights-off
- Humidifier ready for dry, heated air in clones and veg
- Exhaust on a controller, with intake pulled from a heated space
- Pots off cold floors, and the reservoir at 65–70°F
Not sure what your room needs?
Every winter room is a little different. A closet tent upstairs and a tent in an unheated garage are two different problems. Stop by our Webster or Milford store, tell us about your space, your lights and how cold it gets, and we’ll help you figure out what to change. You can call Webster at 508-461-5850 or Milford at 508-478-0710, book a 30-minute grow consultation, or ask MATT, our AI grow assistant, any time on our website.