Short answer
Cannabis is too cold when canopy air stays below about 15 °C (59 °F). Lights-on air below about 20 °C (68 °F) already costs growth, and the root zone should not fall below 18 °C (64 °F). Late-flower nights of 16–20 °C (61–68 °F) are normal. Supported for the photosynthesis response; the limits are Grower practice.
Why
Photosynthesis in cannabis peaks around 25–30 °C (77–86 °F) leaf temperature under strong light and falls steadily as leaves cool [1]. A cold day wastes the light you are paying for.
Cold roots take up water and nutrients slowly, so a plant in a cold pot or reservoir sulks even when the air is warm. Extension guidance puts hydroponic root zones at 18–22 °C [2].
Young plants are least tolerant. Hemp seed germination slows sharply at the cool end of its range [3], and seedlings and clones stall in cold rooms.
In flower, cold nights push relative humidity up and surfaces towards the dew point. Wet, cool buds are where Botrytis starts [4].
The numbers
| Situation | Fine | Too cold |
|---|---|---|
| Veg, lights on | 24–28 °C (75–82 °F) | below 20 °C (68 °F) |
| Flower, lights on | 22–26 °C (72–79 °F) | below 20 °C (68 °F) |
| Late-flower night | 16–20 °C (61–68 °F) | below 15 °C (59 °F) |
| Seedlings and clones | 23–27 °C (73–81 °F) | below 20 °C (68 °F) |
| Root zone / reservoir | 18–22 °C (64–72 °F) | below 18 °C (64 °F) |
| Any stage | frost, 0 °C (32 °F), kills soft tissue |
What to do
- Measure at canopy height and in the pot, not on the floor or ceiling.
- Log 48 hours; the coldest point is usually the end of the dark period.
- Lift pots off cold floors onto insulation.
- Keep the lights-off drop to 3–6 °C (5–10 °F).
- Add a thermostat-controlled heater if the room still runs cold (see keeping a tent warm).
Common mistakes
- Reading the room thermostat instead of the canopy.
- Ignoring cold pots on a concrete floor.
- Chilling nights hard to "boost terpenes": colour only, in cultivars that carry it.
- Cold nights in late flower with no dehumidifier running.
Related
- Full article: Temperature by stage
- How do I keep my grow tent warm in winter?
- How do I lower the humidity in my grow tent?
- Terms: cold nights · dew point
