Temperature is one of the foundations of a stable indoor growing environment. It influences plant metabolism, transpiration, humidity, Vapour Pressure Deficit (VPD), root-zone activity and the amount of stress placed on a plant throughout the day and night cycle.
But temperature should never be managed in isolation. A room at 26°C and 45% relative humidity creates a very different environment from the same room at 26°C and 70% RH. Likewise, a rapid temperature drop after the lights switch off can push relative humidity sharply upwards and bring surfaces closer to the dew point.
In this complete Holland Horticulture guide, we explain how grow room temperature works, what temperature ranges are commonly used for indoor plants, why day and night temperatures matter, how lighting and ventilation affect heat, how temperature links with humidity and VPD, how to diagnose heat problems and how to create a more stable controlled environment.
Why Is Grow Room Temperature Important?
Temperature affects the speed of many biological and physical processes inside a plant and its environment.
A suitable temperature range can help support:
- Photosynthesis
- Respiration
- Transpiration
- Water use
- Nutrient movement
- Root-zone activity
- Leaf cooling
- Consistent plant development
Temperature also affects the surrounding air. As air warms, saturation vapour pressure rises. As air cools, relative humidity can increase even when the actual amount of water vapour changes very little.
This is why temperature control sits at the centre of humidity management, VPD management and ventilation strategy.
What Is A Good Grow Room Temperature?
There is no single perfect temperature for every indoor plant. Species, plant age, light intensity, root-zone conditions, humidity and leaf temperature all influence what is appropriate.
The following ranges can be used as broad horticultural starting points rather than universal targets:
| Plant Stage | General Day Temperature | General Night Temperature | Environmental Approach |
|---|---|---|---|
| Seedlings & Cuttings | Approx. 22–25°C | Approx. 19–22°C | Stable, gentle conditions |
| Established Vegetative Growth | Approx. 22–28°C | Approx. 18–23°C | Active growth with balanced RH and airflow |
| Flowering & Fruiting | Approx. 20–26°C | Approx. 17–22°C | Stable climate with controlled humidity |
Important: These are broad reference ranges, not fixed rules. Always consider the actual plant species and observe plant response.
Temperature Environment Checker
Choose a plant stage, set your day and night temperature and add your relative humidity. The tool will estimate VPD, dew point and the day-to-night temperature drop.
Your selected day temperature sits within the broad reference range used in this guide. Continue to consider humidity, VPD, leaf temperature and the size of the night-time drop.
Guidance only: This tool uses broad horticultural reference ranges. Species, light intensity, leaf temperature, irrigation and root-zone conditions can alter what is suitable.
Temperature For Seedlings And Cuttings
Young seedlings and newly rooted cuttings generally benefit from a stable environment because their root systems are still developing.
Large temperature swings can change atmospheric demand quickly and make moisture management more difficult.
At this stage, combine stable temperature with relatively high humidity, gentle air movement and appropriate irrigation.
For more detail, read our complete plant propagation guide.
Temperature During Vegetative Growth
Established plants generally tolerate a wider temperature range than newly rooted plants.
During active vegetative development, increasing leaf area means plants transpire more water and can add more moisture to the grow room. Temperature therefore needs to be managed alongside extraction and humidity control.
Read our vegetative growth guide for a complete look at this stage.
Temperature During Flowering And Fruiting
As plants mature, the canopy often becomes denser and environmental control becomes more demanding.
Stable temperature, controlled humidity and good air circulation help reduce the development of hot or humid microclimates within the canopy.
Why Day And Night Temperatures Matter
Indoor grow rooms rarely remain at one temperature for 24 hours.
When lighting switches on, electrical energy from lights, drivers and other equipment adds heat to the room. When lighting switches off, that heat input falls and temperature commonly drops.
A moderate day-to-night change is normal. Problems arise when the swing becomes large enough to cause abrupt changes in humidity, VPD or condensation risk.
For example, if room temperature falls rapidly after lights-off, relative humidity may rise sharply even if moisture content remains similar.
Our interactive grow room humidity guide includes a lights-off humidity simulator that demonstrates this relationship.
Why Does A Grow Room Get Too Hot?
Grow room overheating normally results from a mismatch between the amount of heat entering the space and the room's ability to remove or redistribute that heat.
Common causes include:
- Lighting heat load
- Insufficient extraction
- Warm intake air
- Restricted ducting
- Dirty or undersized filters
- Equipment producing additional heat
- Poor circulation
- High outside temperatures
- Inadequate room insulation or positioning
Grow Room Heat Troubleshooter
Choose the problem that best matches your room. The tool will highlight the most useful areas to investigate first.
How Lighting Affects Grow Room Temperature
All grow lights ultimately contribute heat to the growing environment.
LED grow lights are generally more efficient than older lighting technologies, but they still convert a significant amount of electrical energy into heat.
The location of drivers also matters. A driver mounted inside the grow space contributes its heat directly to the room, while a remotely mounted driver can move part of that heat load elsewhere.
When assessing a temperature problem, look at how quickly the room warms after lighting switches on. A consistent rise during the lighting period is a useful clue that lighting heat load and heat removal need to be considered together.
How Ventilation Controls Temperature
Extraction removes warm air from a grow room and replaces it with incoming air.
This can be extremely effective when the replacement air is cooler than the room.
However, extraction cannot cool below the temperature of the incoming air without additional cooling. If outside or intake air is already 29°C, simply installing a larger extractor cannot realistically create a 23°C room on its own.
For a complete explanation of extraction, intake and air exchange, read our Grow Room Ventilation Guide.
Why Intake Air Temperature Matters
The temperature of incoming air places a practical limit on what ventilation alone can achieve.
Cool intake air can absorb heat as it enters the room and then carry that heat away through extraction.
Warm intake air has much less cooling capacity.
This is why summer heat problems often persist even when extraction fans are already operating at high speed.
Air Circulation Vs Extraction
Circulation fans and extraction fans perform different jobs.
Extraction removes heat from the room by replacing indoor air.
Circulation redistributes air already inside the room.
A circulation fan can reduce local hot spots around lighting and within plant canopies, but it does not remove the total heat load from the space.
How To Lower Grow Room Temperature
If a grow room is consistently too warm, work through the possible causes methodically.
Improve Extraction
Check that the extraction fan is appropriately sized, operating correctly and not excessively restricted by filters, long duct runs or tight bends.
Improve The Intake Air Supply
Make sure enough replacement air can enter the room. Where possible, source intake air from a cooler location.
Reduce Ducting Resistance
Long duct runs, crushed ducting and numerous sharp bends reduce real airflow.
Move Heat-Producing Equipment
Where equipment allows it, moving drivers or other heat sources outside the grow room can reduce the internal load.
Manage The Lighting Period
During hot weather, running lights during the cooler part of the day can reduce the temperature of incoming air.
Use Active Cooling Where Necessary
If incoming air itself is too warm, air conditioning or another cooling system may be required to bring room temperature below ambient conditions.
Why Does A Grow Room Get Too Cold?
Low grow room temperature is most common during cold weather, lights-off periods or in rooms with excessive extraction relative to their heat input.
Potential causes include:
- Cold intake air
- Unheated buildings
- Large night-time temperature drops
- Excessive extraction
- Poor insulation
- Cold floors or external walls
- Insufficient heating
How To Increase Grow Room Temperature
When temperatures are too low, possible solutions include:
- Adding thermostatically controlled heating
- Reducing excessive extraction where safe and appropriate
- Improving insulation
- Drawing intake air from a warmer location
- Reducing large lights-off temperature drops
- Protecting the root zone from cold floors
Heating should be controlled rather than continuously adding heat without feedback from a temperature sensor.
Temperature, Humidity And VPD
Temperature and humidity combine to determine atmospheric demand.
At the same relative humidity, warmer air generally produces a higher VPD.
At the same temperature, reducing humidity generally increases VPD.
This is why changing grow room temperature can alter plant water use even when the RH percentage appears unchanged.
Use our interactive VPD guide and calculator to explore this relationship directly.
Air Temperature Vs Leaf Temperature
The temperature shown by a room sensor measures the surrounding air, but the leaves themselves may be warmer or cooler.
Leaf temperature is affected by:
- Lighting intensity
- Radiant heat
- Air movement
- Transpiration
- Room temperature
- Plant water status
Because VPD at the leaf surface depends on leaf temperature, measuring leaves with an infrared thermometer or suitable plant sensor can provide additional environmental information.
Why Root-Zone Temperature Matters
Air temperature and root-zone temperature are related but not identical.
Roots may be affected by cold floors, warm nutrient reservoirs, pump heat, room insulation and the temperature of irrigation water.
Extreme root-zone temperatures can influence root activity, dissolved oxygen and plant water uptake.
Do not assume that a suitable canopy temperature automatically means the root zone is also within a suitable range.
Where Should A Temperature Sensor Be Positioned?
A temperature sensor should normally be positioned around canopy height in a representative part of the growing area.
Avoid placing the main sensor:
- Directly beneath intense lighting
- Directly in front of a circulation fan
- Beside a heater
- Against the wall of a grow tent
- Directly beside an intake vent
- Far above the plant canopy
Large rooms may benefit from multiple sensors because one reading cannot always reveal temperature differences across the whole space.
Grow Room Temperature During A UK Heatwave
Hot UK weather creates a particular challenge because ventilation depends on the conditions of the incoming air.
If outdoor air is already hot, extraction can exchange air but may not provide enough cooling to reach the desired room temperature.
During warm periods, consider:
- Running lighting during cooler hours
- Maximising efficient air exchange
- Keeping ducting unrestricted
- Reducing unnecessary internal heat loads
- Monitoring plant water use
- Using active cooling where required
Read our UK Heatwave Growing Guide for a more detailed strategy.
Grow Room Temperature During Winter
Winter creates the opposite problem.
Cold intake air can lower room temperature rapidly, especially after lights switch off.
Once cold outside air is heated, its relative humidity can also fall significantly, potentially creating a high-VPD environment.
Winter environmental control therefore often requires balancing heating, extraction, humidity and fresh-air exchange.
Common Grow Room Temperature Mistakes
Chasing One Perfect Temperature
A single number cannot represent every plant species, stage and environmental condition.
Ignoring Humidity
Changing temperature changes the relationship between RH and VPD.
Ignoring Lights-Off Conditions
A room that looks perfect during the lighting period may become too cold and humid after lights-off.
Increasing Extraction When Intake Air Is Already Hot
More airflow does not automatically mean lower temperatures if the incoming air has little cooling capacity.
Placing The Sensor In A Hot Spot
A sensor directly under a light may exaggerate the average temperature experienced by the canopy.
Relying On A Circulation Fan To Remove Heat
Circulation moves heat around; extraction or cooling removes it.
Smart Temperature Control And Automation
Modern environmental controllers can use temperature and humidity sensors to coordinate equipment automatically.
Depending on the controller, this may include:
Instead of reacting to problems manually, variable-speed systems can gradually increase or reduce equipment output as conditions change.
This usually creates a more stable environment than simple full-power on/off control.
Frequently Asked Questions About Grow Room Temperature
What temperature should a grow room be?
There is no universal grow room temperature. Many indoor plants perform within broad day temperatures in the low-to-mid 20s Celsius, but the correct range depends on species, growth stage, humidity, VPD and lighting.
What temperature is too hot for a grow room?
There is no single threshold for every plant. As temperature rises, atmospheric demand and plant water use can increase, and prolonged excessive heat can reduce plant performance. Judge temperature alongside humidity, leaf temperature and plant response.
How do I cool a grow room?
Improve extraction and intake airflow, reduce ducting resistance, move unnecessary heat sources, run lights during cooler hours and use active cooling if intake air itself is too warm.
Does an extractor fan cool a grow room?
Yes, when the incoming replacement air is cooler than the room. Extraction cannot normally cool the room below intake-air temperature without active cooling.
Why is my grow room hotter than outside?
Lighting, drivers, pumps and other electrical equipment add heat. If heat is entering the room faster than ventilation removes it, indoor temperature rises above ambient.
Why does grow room humidity rise when temperature drops?
Cooler air has a lower saturation vapour pressure. If moisture content remains similar while temperature falls, relative humidity rises.
Where should I put my temperature sensor?
Position it around canopy height in a representative area, away from direct lighting heat, heaters, intake vents and strong fan airflow.
Is leaf temperature different from room temperature?
Yes. Lighting, transpiration and airflow can make leaves warmer or cooler than the surrounding air.
Should grow room temperature be the same day and night?
Not necessarily. A modest day-to-night change is normal, but large swings can cause rapid changes in RH, VPD and condensation risk.
Final Thoughts: Temperature Is Part Of A Complete Environmental System
Grow room temperature cannot be separated from the rest of the indoor environment.
Lighting adds heat. Ventilation removes heat. Temperature changes relative humidity. Temperature and humidity determine VPD. Plants add moisture through transpiration.
The goal is therefore not to chase one perfect temperature number. The aim is to create a stable environment where temperature, humidity, airflow, irrigation and lighting work together.
At Holland Horticulture, our aim is to help growers understand not just which environmental equipment to use, but why each part of the growing environment behaves the way it does and how to control it more effectively.