Quick answer: For most indoor cannabis grows, start at around 22–28°C during lights-on. Use higher relative humidity for seedlings, then reduce it through flowering: approximately 65–75% for seedlings, 55–70% during 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.
Why do these targets change as the plant grows? Seedlings have small root systems and lose moisture differently from mature plants, while dense flowers hold more humid air around the canopy and need drier conditions.
When growing cannabis in South Africa, those stage changes must also be balanced against local weather. A tent that works during a dry winter week may become hot and humid in summer, while a cold night can push relative humidity higher after the lights switch off. The sections below explain how temperature and humidity affect the plant, which starting ranges suit each stage, and how to measure and adjust the room.
1. Why Are Temperature and Humidity Important for Cannabis?
Cannabis is sensitive to environmental conditions. Poor control of temperature and humidity can slow growth, delay flowering and increase the risk of stress or disease. The aim is not to hold one perfect number every minute, but to prevent long periods of excessive heat, cold, moisture or dryness.
Temperature's impact
Temperature is a critical variable during the cannabis growing process because it influences photosynthesis, transpiration and the plant's overall metabolic rate. Conditions that remain too hot or too cold can slow growth and make other problems harder to manage.
When the ambient temperature rises above roughly 28°C, the transpiration rate increases. Water leaves the plant more quickly, forcing the roots to draw more moisture from the growing medium to maintain balance.
Over time, a plant that cannot replace this moisture may show curled, yellowing or wilting leaves. Prolonged heat can also reduce nutrient uptake and photosynthetic efficiency. Before adding cooling equipment, check whether warm air is actually leaving the room, whether ducting restricts the exhaust and whether the light is delivering more intensity than the canopy needs.
If the temperature drops below roughly 18°C for long periods, root activity and metabolism may slow considerably. Plants can use less water, while cold growing media remains wet for longer. This combination can lead to slow growth and increase the chance of root-zone problems.
Humidity's impact
Excess humidity is a common issue when growing cannabis in enclosed tents or indoor rooms. When the air remains very humid, moisture evaporates more slowly from leaves and flowers. Combined with poor circulation or condensation, this can increase the risk of powdery mildew, bud rot and other mould problems.
These problems affect plant health and can reduce the quality of the harvest. Risk depends on more than one humidity reading: the duration of the damp period, canopy density, wet surfaces and air movement all matter.
Humidity that is too low creates the opposite problem. Young plants can lose moisture faster than their developing roots replace it, leading to dry or curled leaves. Seedlings and fresh clones generally tolerate more humidity than mature flowering plants, so the target should fall gradually as roots and leaves develop.
Controlling temperature and humidity is therefore not only about keeping plants alive. It helps create steady water use, predictable growth and a lower risk of environmental stress.
2. What Are Suitable Temperature and Humidity Levels for Cannabis?
Different growth stages require different temperature and humidity settings. The following ranges are useful starting points rather than rigid guarantees. Cultivar, leaf temperature, irrigation, canopy density and airflow can all change how a plant responds.
Seedling stage
- Temperature: 20–25°C
- Humidity: 65–70% RH
Seedlings are fragile and have small root systems. Higher humidity helps reduce rapid moisture loss, while gentle airflow keeps the space from becoming stagnant.
Vegetative growth stage
- Temperature: 21–29°C
- Humidity: 40–60% RH
During vegetative growth, roots and leaves expand quickly. Humidity can be reduced gradually while air exchange increases to match the larger canopy.
Flowering stage
- Temperature: 18–27°C
- Humidity: 40–50% RH
Flowers become denser as they mature, so moisture can remain trapped between leaves and buds. Stable conditions and reliable circulation help reduce mould risk. Avoid large temperature swings that cause humidity to rise sharply after the lights switch off.
Pre-harvest stage
- Temperature: 18–24°C
- Humidity: 40–50% RH
A cooler, relatively dry room can help limit condensation around mature flowers. There is no benefit in forcing the room excessively dry if plants begin losing moisture too quickly; stability is more important than chasing the lowest reading.
Vapour pressure deficit, or VPD, can be used as a cross-check because it relates temperature to humidity. However, VPD does not replace checks for wet surfaces, mould, weak airflow or plant stress, and it becomes less reliable when leaf temperature is estimated incorrectly.
3. How to Maintain Suitable Temperature and Humidity
For growing cannabis in South Africa, the original control methods remain useful. The main improvement is to measure conditions where the plants actually experience them and to pay attention to changes during the dark period.
Use efficient grow lighting
When choosing grow lamps for plants, LED fixtures generally release less radiant heat into the canopy than older high-intensity discharge lamps. Their electrical power still becomes heat in the room, so lighting and ventilation must be planned together.
Monitor temperature and humidity at canopy height
Install a thermometer and hygrometer in shaded, moving air at canopy height. Keep the sensor away from direct lamp radiation, wet leaves, humidifier mist, the exhaust outlet and tent walls. Record the daily high and low rather than relying on the value visible when the tent is open.
Two inexpensive sensors placed side by side for an hour can reveal whether one unit reads consistently high or low. A second sensor near the intake or lower canopy may also help identify uneven conditions in a tall or crowded tent.
Use extraction and circulation fans
Extraction replaces warm, humid air, while circulation prevents still pockets around leaves. Exhaust warm air out of the room rather than back into the same enclosed space. Reduce sharp duct bends and confirm that the carbon filter is not restricting the fan more than expected.
Air should move gently across and below the canopy without blasting one plant continuously. If heat remains high, schedule the light period during cooler hours when practical and check whether the fixture can be raised or dimmed without reducing useful coverage.
Use humidifiers and dehumidifiers only when needed
If the grow space is too dry, a controlled humidifier can help during the seedling stage. Keep the mist away from leaves and sensors, and reduce humidity gradually as the root system develops.
If humidity is too high, inspect wet growing media, standing water, crowded foliage and weak extraction before relying on a dehumidifier. A dehumidifier can be useful in humid conditions, but it also adds heat that the ventilation system must remove.
Relative humidity commonly rises after the lights switch off because cooler air holds less water vapour while plants and growing media continue releasing moisture. Keep some extraction and circulation running through the dark period, and avoid unnecessary watering immediately before lights-off when the room already struggles with an overnight humidity spike.
South African indoor conditions vary by region and season. Run an empty tent through one full light and dark cycle, record the natural high and low, and use that local baseline instead of copying settings from a room with different weather and equipment.
For lighting calculations, use the cannabis grow-light calculator. This article deals with climate management and does not state a legal plant limit.
Conclusion
When growing cannabis in South Africa, temperature and humidity directly influence water use, growth and mould risk. Start with ranges suited to the plant's stage, measure in shaded air at canopy height, and check both daytime heat and the humidity rise after lights-off.
Lighting, extraction, circulation and watering time must work together. Make one adjustment at a time and use logged results to guide the next step. For hygrometers, ventilation equipment, controllers and efficient grow lights, visit TheOneGrow South Africa.
Frequently Asked Questions
What temperature and humidity should cannabis have at each stage?
Seedlings generally use warmer, more humid air, while flowering plants need progressively drier conditions and stronger circulation. Treat the ranges in this guide as starting points and adjust according to plant response.
Why does humidity rise when grow lights turn off?
The air cools and can hold less water vapour while plants and growing media continue releasing moisture. Maintain controlled extraction and circulation through the dark period.
Where should a grow-room temperature and humidity sensor be placed?
Place it at canopy height in shaded, moving air, away from direct light, wet leaves, humidifier mist, tent walls and the exhaust outlet.
Is VPD more important than relative humidity?
VPD is a useful cross-check, but it does not directly show condensation, mould or stagnant air. Use it together with temperature, relative humidity, leaf condition and surface moisture.