# How cold is too cold for cannabis?

> Cannabis gets too cold when air at the canopy stays below about 15 °C (59 °F): growth, water use and ripening slow sharply, and humid cold nights invite bud rot. Lights-on air below about 20 °C (68 °F) already costs growth. Late-flower nights of 16–20 °C (61–68 °F) are fine. Keep the root zone at 18–22 °C (64–72 °F); cold roots stall a plant even when the air is warm.

- Canonical: https://genetixbay.com/knowledge/answers/how-cold-is-too-cold.html
- Topic: Environment · Type: Troubleshooting
- Evidence: Supported · Status: Draft: awaiting expert review
- Full article: [Temperature by stage: day, night and root-zone targets](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)
- Also asked as: What is the minimum temperature for cannabis? | My grow room is cold: is that a problem? | Is 60 °F (15 °C) too cold for cannabis? | How cold can cannabis plants get at night?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## 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](https://genetixbay.com/knowledge/glossary/root-zone.md) should not fall below **18 °C (64 °F)**. Late-flower nights of 16–20 °C (61–68 °F) are normal. *Supported* for the [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) response; the limits are *Grower practice*.

## Why

Photosynthesis in cannabis peaks around 25–30 °C (77–86 °F) [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) 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](https://genetixbay.com/knowledge/glossary/hemp.md) seed [germination](https://genetixbay.com/knowledge/glossary/germination.md) slows sharply at the cool end of its range [3], and [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) and clones stall in cold rooms.

In flower, [cold nights](https://genetixbay.com/knowledge/glossary/cold-nights.md) push [relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md) up and surfaces towards the [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md). Wet, cool buds are where *[Botrytis](https://genetixbay.com/knowledge/glossary/botrytis.md)* 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

1. Measure at canopy height and in the pot, not on the floor or ceiling.
2. Log 48 hours; the coldest point is usually the end of the dark period.
3. Lift pots off cold floors onto insulation.
4. Keep the lights-off drop to 3–6 °C (5–10 °F).
5. Add a thermostat-controlled heater if the room still runs cold (see [keeping a tent warm](https://genetixbay.com/knowledge/answers/how-to-keep-a-grow-tent-warm.md)).

## Common mistakes

- Reading the room thermostat instead of the canopy.
- Ignoring cold pots on a concrete floor.
- Chilling nights hard to "boost [terpenes](https://genetixbay.com/knowledge/glossary/terpene.md)": colour only, in cultivars that carry it.
- Cold nights in late flower with no [dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md) running.

## Related

- Full article: [Temperature by stage](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)
- [How do I keep my grow tent warm in winter?](https://genetixbay.com/knowledge/answers/how-to-keep-a-grow-tent-warm.md)
- [How do I lower the humidity in my grow tent?](https://genetixbay.com/knowledge/answers/how-to-lower-humidity-in-a-grow-tent.md)
- Terms: [cold nights](https://genetixbay.com/knowledge/glossary/cold-nights.md) · [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md)

## Follow-up questions

**Is 60 °F (15.5 °C) too cold for cannabis?** 60 °F (15.5 °C) is at the cold limit for cannabis. As a night temperature in late flower it is survivable but slows ripening and pushes humidity up towards the dew point, which invites bud rot. As a daytime temperature it is too cold: photosynthesis is far below its best, which sits around 25–30 °C leaf temperature. Aim for nights of 16–20 °C (61–68 °F) in late flower.

**Will a cold night kill my plant?** A single cold night rarely kills an established cannabis plant unless it freezes. Above 0 °C (32 °F) the usual damage is slowed growth, purple or droopy leaves in sensitive cultivars and, in flower, condensation that starts bud rot. Seedlings and clones are far less tolerant. Frost kills soft tissue outright, so protect outdoor plants below about 2–4 °C (36–39 °F).

**Do cold nights make buds purple?** Cold nights can bring out purple colour in cannabis, but only in cultivars that carry the genes for anthocyanin pigments; colour is mostly genetic. Cold does not add terpenes, flavour or resin, which is a myth unless peer-reviewed data shows otherwise. Big cold nights also slow ripening and raise bud-rot risk, so treat purple as a trade-off, not a technique.

## Sources

1. 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. https://doi.org/10.1007/s12298-008-0027-x
2. e-GRO / Colorado State University Extension, Root disease problems and management in hydroponic systems (root-zone 68–72 °F). https://planttalk.colostate.edu/topics/miscellaneous/2043-root-disease-problems-management-hydroponic-systems/
3. Geneve RL et al., 2022. Temperature limits for seed germination in industrial hemp (Cannabis sativa L.). Crops 2(4):415–427. https://doi.org/10.3390/crops2040029
4. Utah State University Extension. Hemp bud rot (IPM fact sheet). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-bud-rot.php