# Reservoir or root zone cold (below 16 °C / 61 °F): likely causes and fixes

> Reservoir or root zone cold (below 16 °C / 61 °F) on cannabis is most often Cold root zone (below about 16 °C / 61 °F). How to tell them apart, the fix for each and what recovery looks like.

Also called: res too cold, cold water in reservoir, cold floor pots, water temperature low.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=roots&s=reservoir-cold

## Likely causes, most likely first

### Cold root zone (below about 16 °C / 61 °F) (Possible)

- Kind: Root zone; urgency: Deal with it this week
- Fix: Raise root-zone temperature to 18-22 °C (64-72 °F): lift pots off cold floors onto insulation, warm the room at night, use an aquarium heater in reservoirs (with a thermostat), water with room-temperature solution.
- Recovery: Growth resumes and new leaves green within a week.
- Confirm: Measure reservoir and root-zone temperature at the end of the light period: Measure the solution in the reservoir and in the root chamber or bucket, at the warmest time of day. Target 18-22 °C (64-72 °F). Above about 23-24 °C (73-75 °F), dissolved oxygen falls and Pythium risk rises; below 16 °C (61 °F) uptake slows.
- Confirm: Lift the pot and check moisture at depth: Lift the pot right after a full watering to learn its wet weight, then lift before each watering. Water when it is clearly lighter (about half the added water gone). Push a finger or a moisture probe 5 cm (2 in) down. Heavy for 3-4+ days with droop: overwatering. Very light with limp papery leaves: thirst.
- Article: [Temperature by stage: day, night and root-zone targets](https://genetixbay.com/knowledge/environment/temperature-by-stage.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.