If you have good light and decent nutrients but plants still look unhappy, the problem is usually the air. Temperature and humidity decide how fast a plant drinks, how well it takes up nutrients and whether mould gets a foothold. This guide gives you the target numbers for each stage, explains VPD in plain English, and lists the practical fixes that work in a UK home.
The target numbers
Measure at canopy height, with the lights on, using a hygrometer with a probe. The reading by the tent door is not what your plants feel.
| Stage | Temperature (lights on) | Temperature (lights off) | Relative humidity |
|---|---|---|---|
| Seedlings and cuttings | 22-26°C | 18-22°C | 70-80% |
| Vegetative growth | 22-28°C | 18-22°C | 55-70% |
| Early flowering / fruit set | 22-26°C | 18-21°C | 45-55% |
| Late flowering / ripening | 20-25°C | 16-20°C | 40-50% |
Two rules sit behind the table:
- Keep the day-night swing under 8°C. A tent at 26°C with lights on and 15°C with lights off stresses plants and causes condensation that leads to mould.
- Humidity comes down as plants mature. Young plants have small roots and take in water through their leaves, so they like it humid. Mature plants with dense foliage need drier air so moisture does not sit in the canopy.
What VPD means and why growers talk about it
VPD stands for vapour pressure deficit. It is a single number that combines temperature and humidity to describe how hard the air is pulling water out of the leaves.
- Low VPD (cool and humid): plants barely transpire, nutrient uptake slows and mould risk rises.
- High VPD (warm and dry): plants transpire too fast, close their pores to protect themselves, and growth stalls. Leaf edges curl and tips burn.
- The sweet spot is roughly 0.8-1.2 kPa for most crops in veg and 1.2-1.5 kPa in flower.
You do not need to calculate it. What VPD teaches you is that temperature and humidity move together. If your tent is 28°C you need higher humidity (around 65%) than if it is 23°C (around 55%) to get the same result. A warm, dry tent is the most common UK summer problem; a cool, damp tent is the most common winter one.
Measuring properly
A £10 hygrometer with an external probe is the best value piece of kit in the shop. Hang the probe at canopy height, out of direct light. Check it at lights-on and an hour before lights-off, because that is when conditions are at their extremes. If you want to go further, a digital controller with a sensor will log the readings and switch equipment for you.
How to lower humidity (the summer and late-flower problem)
Increase extraction. Humidity is mostly a ventilation problem. If your extractor fan is on a speed controller, turn it up. If it is already at full speed and humidity is still high, the fan is too small for the tent.
Add air movement. A clip fan or two moving air through the canopy stops pockets of still, wet air forming around the leaves. This is the cheapest fix and the most often skipped.
Water in the morning. Watering just before lights-off leaves wet media sitting in a cooling tent all night. Water at the start of the light period so plants use it and the surface dries.
Remove excess leaf. Dense lower foliage traps moisture. Thinning it improves airflow.
Use a dehumidifier. For a persistently damp room (common in cellars and older houses) a small compressor dehumidifier in the room the tent draws air from is the proper fix. Aim the tent's passive intake at it.
Check your intake air. The tent can only be as dry as the air coming in. If the room is at 75% humidity because of drying washing, the tent will be too.
How to raise humidity (the winter and seedling problem)
Slow the extraction. Running a fan flat out in a cold, dry room strips moisture. A fan speed controller lets you find a balance.
Use a humidifier. An ultrasonic humidifier placed in the tent or at the intake works well for propagation and veg. Use a hygrostat so it switches off at your target.
Propagators and domes. For cuttings and seedlings, a propagator lid keeps humidity at 80%+ around the plants without raising the whole tent.
Wet towels and trays. Low-tech but effective: a tray of water or a damp towel in front of the circulation fan will lift humidity by 5-10%.
Group plants together. Plants humidify their own air. A full tent is easier to keep humid than a tent with two plants in one corner.
How to manage temperature
Too hot:
- Switch to LED if you are on HPS. This is the single biggest change you can make.
- Run lights at night. Electricity is often cheaper on an off-peak tariff and the room is cooler.
- Upgrade the extractor fan or add an intake fan.
- Dim the light slightly during a heatwave. Losing 10% output is better than cooked plants.
- Use an EC fan with a temperature controller so it speeds up automatically.
Too cold:
- A grow tent heater on a thermostat is the reliable answer. A 1kW or 2kW fan heater in the room, or a tube heater inside the tent, keeps lights-off temperatures up.
- Insulate the tent with bubble foil or put it on a rug if it stands on a cold concrete floor.
- Reduce the extractor speed at night with a controller that has a minimum speed setting.
- Keep the root zone warm. Cold roots cannot take up nutrients even if the air is fine. A heat mat under seedling trays helps a lot.
Common mistakes
- Trusting the hygrometer on the tent wall. Measure at the canopy.
- Running the extractor at one speed all day. Conditions change between lights-on and lights-off. A controller pays for itself.
- Ignoring the room. The tent breathes the room's air. Fix the room first.
- Chasing numbers to the decimal. Plants cope with a range. Aim for the band in the table, keep swings small and watch the leaves.
Kit that helps
Most of the kit in this guide is in stock in the shop: hygrometers and meters, extractor fans, circulation and clip fans, grow tent heaters and a grow room humidifier. If your readings are off and you are not sure which fix to try first, send us a photo of your hygrometer and your tent size on WhatsApp or call 0121 553 7872. We will tell you straight.