How do I keep my grow tent warm in winter?

Q: How do I keep my grow tent warm in winter?

Keep a grow tent warm in winter by keeping the light's own heat in: slow the exhaust on a thermostat, insulate the tent and floor, run the lights during the coldest hours, and only then add a thermostat-controlled heater. Hold 24–28 °C (75–82 °F) by day in veg, 22–26 °C (72–79 °F) in flower, nights 3–6 °C (5–10 °F) cooler, and the root zone at 18–22 °C (64–72 °F).

Short answer

Keep a grow tent warm in winter by keeping the light's heat in before buying any: slow the exhaust on a thermostat, insulate, run the lights in the coldest hours, then add a thermostat-controlled heater. Hold 24–28 °C (75–82 °F) by day in veg, 22–26 °C (72–79 °F) in flower, nights 3–6 °C (5–10 °F) cooler. Grower practice.

Why

Cold leaves photosynthesise slowly; the cannabis optimum is around 25–30 °C leaf temperature under strong light ^[1]. A cold tent wastes the light.

Cold roots take up water and nutrients slowly. Hydroponic root zones belong at 18–22 °C (64–72 °F) ^[2], and pots on a winter floor can sit far below air temperature.

Big lights-off drops push humidity up. Cold, damp nights in flower are when bud rot starts ^[4].

Heaters are the biggest single electrical load in many home grows, and overloaded outlets and cords are a leading cause of home electrical fires ^[3].

The numbers

Item	Target
Veg, lights on	24–28 °C (75–82 °F)
Flower, lights on	22–26 °C (72–79 °F)
Night drop	3–6 °C (5–10 °F)
Root zone / reservoir	18–22 °C (64–72 °F)
Too cold	canopy below 15 °C (59 °F)
Heater on a 15 A, 120 V circuit	keep the whole circuit under 1,440 W continuous

What to do

Put the exhaust on a thermostat or speed controller so it slows when the tent is cool.
Run the lights through the coldest hours, usually overnight.
Lift pots onto foam board or a pallet; insulate under a reservoir.
Wrap the tent in a blanket or foam panels, keeping intakes clear.
Add an enclosed, thermostat-controlled electric heater with tip-over and overheat cut-offs, outside or low in the tent.
Plug the heater straight into a wall outlet and add up the circuit's load.

Safety: never run open-element or combustion heaters near the tent, never through a thin extension cord, and keep a working smoke alarm nearby ^[3].

Common mistakes

Heating the tent while the exhaust runs flat out.
A heater with no thermostat.
Heater and lights on the same overloaded circuit.
Warm air, cold pots on concrete.
Daylight leaking into a garage during the dark period.

Follow-up questions

What heater is safe for a grow tent?

The safest grow-tent heater is an enclosed, thermostat-controlled electric unit such as an oil-filled radiator or a ceramic heater with tip-over and overheat cut-offs, placed outside or low in the tent away from water and fabric. Never use an open-element heater near plastic or water, never use a combustion heater inside, and plug it straight into the wall, not a shared power strip.

Should my lights be on at night in winter?

Yes, in a cold room run the grow lights during the coldest hours, usually overnight, so their heat lands when the tent most needs it. The dark period then falls in the warmer daytime. Keep the dark period truly dark: a garage or basement with daylight leaks will interrupt flowering in photoperiod plants.

How do I keep the roots warm in a cold grow room?

Keep roots warm in a cold grow room by lifting pots off concrete onto foam board or a wooden pallet, and by watering with water at about 20 °C (68 °F). In hydro, use an aquarium heater in the reservoir set to 18–20 °C (64–68 °F). Cold roots slow water and nutrient uptake even when the air is warm, and the target is 18–22 °C.

Sources

Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299–306. doi.org/10.1007/s12298-008-0027-x

e-GRO / Colorado State University Extension, Root disease problems and management in hydroponic systems (root-zone 68–72 °F). planttalk.colostate.edu/topics/miscellaneous/2043-root-disea

US Fire Administration. Electrical fire safety (overloaded outlets, extension cords, smoke alarms). usfa.fema.gov/prevention/home-fires/prevent-fires/electrical

Utah State University Extension. Hemp bud rot (IPM fact sheet). extension.usu.edu/planthealth/ipm/notes_ag/hemp-bud-rot.php