Quick answer: For most indoor cannabis grows, begin around 22–28°C during lights-on. Use higher relative humidity for seedlings, then reduce it through flowering: roughly 65–75% for seedlings, 55–70% in vegetative growth, 45–55% in early flower and 40–50% in late flower. Measure at canopy height and adjust for cultivar, airflow and local climate.
This cannabis temperature and humidity guide is written for South African indoor conditions, where hot summers, dry Highveld winters, humid coastal air and load shedding can create very different grow-room problems.
Temperature and humidity targets by growth stage
| Stage | Lights-on temperature | Relative humidity | Practical priority |
|---|---|---|---|
| Seedling / clone | 22–26°C | 65–75% | Avoid rapid drying while keeping gentle airflow |
| Vegetative | 22–28°C | 55–70% | Build roots and leaves without stagnant, wet air |
| Early flower | 20–26°C | 45–55% | Increase air exchange as canopy density rises |
| Late flower | 18–25°C | 40–50% | Reduce condensation and mould risk |
A modest night-time drop of roughly 2–4°C is easier to manage than a large swing. Avoid treating any table as a rigid promise: leaf temperature, irrigation and genetics still matter.
Where to place the temperature and humidity sensor
- Place the main probe at canopy height, shaded from direct lamp radiation.
- Keep it away from the humidifier outlet, exhaust port and tent wall.
- Use a second low-cost meter near the intake if you need to diagnose room-to-tent differences.
- Record the daily high and low, not only the reading you see when the tent is open.
Use VPD as a cross-check—not a magic score
Vapour pressure deficit combines temperature and humidity into one indicator of drying pressure. A common progression is roughly 0.6–0.9 kPa for young plants, 0.8–1.2 kPa in vegetative growth and about 1.0–1.5 kPa through flowering. Leaf temperature can differ from air temperature, so a VPD chart is only as good as the measurements behind it.
How to lower heat in a South African grow tent
- Move the lighting period to the cooler part of the night if noise and household rules allow.
- Exhaust warm air from the room, not only from the tent into the same closed room.
- Reduce sharp duct bends and confirm that the carbon filter is not choking airflow.
- Dim or raise the fixture if PPFD is already above the crop's useful range.
- Plan a safe response to load shedding so fans and controls do not remain off in a sealed tent.
How to control high or low humidity
| Problem | Likely cause | First checks |
|---|---|---|
| Humidity spikes after lights-off | Cooling air holds less moisture | Keep extraction running, reduce wet media exposure and check night-time lows |
| Dry Highveld winter air | Low intake humidity | Use a controlled humidifier for young plants and avoid blowing dry air directly at leaves |
| Humid coastal conditions | Intake air already wet | Dehumidify the room, increase air exchange and avoid overcrowding |
| Condensation on walls | Cold surfaces plus poor circulation | Improve mixing, remove standing water and inspect exhaust capacity |
Size ventilation from tent volume
Multiply tent length × width × height to get cubic metres. For a base air exchange every one to three minutes, divide the required cubic metres per hour by 60 to estimate m³/min. Then add capacity for the carbon filter, duct length, bends and hot weather. The final choice should use the fan's rated airflow under resistance, not only a free-air number.
For lighting calculations, use our cannabis grow-light calculator. For the current private-use position, read the South Africa cannabis law guide; this article does not set a legal plant count.
Monitor and automate the climate
A canopy-height sensor shows the real conditions around the leaves, while a compatible controller can coordinate lighting and airflow. Confirm compatibility before ordering.
View the smart grow-tent controller View the temperature/humidity meter
Frequently asked questions
What humidity causes bud rot?
There is no single guaranteed threshold. Dense flowers, stagnant air, condensation and long periods of high humidity increase risk, which is why late-flower humidity and air movement deserve close monitoring.
Should the exhaust fan run at night?
Usually some air exchange is still needed because humidity often rises after lights-off. A controller can reduce speed while maintaining a safe ceiling.
Is room temperature the same as leaf temperature?
No. LED intensity, airflow and evaporative cooling can make leaf temperature different from the surrounding air.