Indoor cannabis plants do best when the environment stays stable. Light, temperature, humidity and airflow all affect each other, so it helps to think of the grow space as one system instead of separate settings.
Quick answer
For most indoor grows, aim for:
| Stage | Temperature | Humidity | Notes |
|---|---|---|---|
| Seedling | 20–25°C | 60–70% | Gentle light, higher humidity |
| Vegetative | 22–26°C | 50–60% | Stronger light, steady airflow |
| Flowering | 20–24°C | 40–50% | Lower humidity to reduce mold risk |
These numbers are starting points, not hard rules. A plant in a stable 25°C room will usually do better than one bouncing between 18°C and 31°C every day.
Start with the symptoms
If your plant looks unhappy, the environment is often one of the first things to check.
- Stretching usually points to weak light or light too far away
- Curling or tacoing leaves often points to heat or light stress
- Drooping can be watering-related, but temperature and humidity can make it worse
- Slow growth often happens when the room is too cold, too hot, or too dry
If the issue is mostly showing on the leaves, this guide may help narrow it down:
https://ilgmforum.com/t/cannabis-leaf-problems-yellow-brown-curling-leaves-explained/
Light: strong enough, but not stressful
Light drives growth, but seedlings, vegging plants and flowering plants do not need the same intensity.
Seedlings need gentle light close enough to prevent stretching. Vegging plants can handle stronger light as they build structure. Flowering plants usually need the most light, but only if temperature, humidity and watering are also under control.
If your seedlings are tall and thin, the light is probably too weak or too far away:
https://ilgmforum.com/t/cannabis-seedlings-stretching-leggy-seedlings-fix-causes/
For a deeper light guide:
https://ilgmforum.com/t/how-much-light-do-cannabis-plants-need-ppfd-guide/
Temperature: avoid big swings
Cannabis can tolerate a range of temperatures, but big swings create stress.
As a general rule:
- Seedlings prefer 20–25°C
- Vegging plants often do well around 22–26°C
- Flowering plants usually do better slightly cooler, around 20–24°C
Too hot, and plants may curl, dry out quickly or slow down. Too cold, and growth slows while nutrient uptake becomes less reliable.
Humidity: higher early, lower in flower
Humidity should usually come down as the plant matures.
Young seedlings like more humidity because their roots are still developing. During veg, moderate humidity supports steady growth. In flower, lower humidity helps reduce mold risk as buds get denser.
A simple target:
- Seedlings: 60–70%
- Veg: 50–60%
- Flower: 40–50%
If humidity is too high, improve exhaust, airflow or use a dehumidifier. If it is too low, a humidifier is usually the cleanest fix.
Airflow: gentle movement, not windburn
Good airflow helps prevent stagnant pockets of humidity and strengthens stems. You want the leaves to move gently, not get blasted from one direction all day.
A basic indoor setup usually needs:
- an exhaust fan to exchange air
- at least one oscillating fan inside the tent
- no dead zones where air sits still
Airflow becomes especially important in flower when buds are dense and humidity can build up.
VPD: useful, but don’t overcomplicate it
VPD combines temperature and humidity into one number. It can be helpful once you have the basics under control, but beginners do not need to chase perfect VPD from day one.
Think of it this way:
- Too humid for the temperature = slow transpiration and mold risk
- Too dry for the temperature = plants dry out and stress faster
- Balanced temperature and humidity = easier growth
Start by getting temperature, humidity, light distance and airflow stable. Then use VPD to fine-tune.
How to dial in your grow room
Start simple:
- Get temperature into a reasonable range
- Set humidity for the plant’s stage
- Make sure the light is the right distance
- Add steady airflow and exhaust
- Watch the plant for 24–48 hours before making more changes
Changing everything at once makes problems harder to diagnose.
Need help dialing in your setup?
Post your:
- temperature and humidity
- light type and distance
- tent size
- plant stage
- a photo if something looks off
Small environment changes can make a big difference, and other growers can usually spot the issue quickly.