Grow science · 10
Ideal temperature
Temperature doesn’t just make plants grow faster or slower. It changes every other number in the room.
Targets
| Stage | Lights on | Lights off |
|---|---|---|
| Seedlings & clones | 22–26 °C | 20–24 °C |
| Veg | 24–28 °C | 20–25 °C |
| Flower | 22–27 °C | 18–23 °C |
Air temperature, typical guideline values. Below about 18 °C growth slows down a lot, above about 30 °C most plants are stressed, unless you run extra CO₂.
Why LED rooms run a bit warmer
Under LEDs leaves are usually 1–3 °C cooler than the air (leaf vs. air). The plant cares about its leaf, so LED growers often set the room 1–2 °C warmer than they would under HPS lamps.
Temperature changes your VPD
At the same 60 % humidity, the VPD climbs fast with temperature:
| Air temp at 60 % RH | VPD |
|---|---|
| 20 °C | 0.94 kPa |
| 24 °C | 1.19 kPa |
| 28 °C | 1.51 kPa |
| 32 °C | 1.90 kPa |
So when you change the temperature, check your humidity too: ideal humidity by temperature.
The night drop
A small drop of 2–4 °C at lights-off is enough. A bigger drop pushes humidity up fast and brings cold surfaces close to the dew point. More on that in day vs. night VPD and dew point & mold.
Don’t forget the roots
The root zone likes it cooler than the leaves: around 18–22 °C in the substrate or nutrient solution. Warmer water holds less oxygen, which is why hydro systems in hot rooms often run into root problems first.
Questions growers ask
What is the ideal grow room temperature?
Around 24–28 °C with lights on in veg and 22–27 °C in flower, with a drop of 2–4 °C at night. Seedlings and clones like 22–26 °C.
How much cooler should it be at night?
About 2–4 °C cooler than during the day. A bigger drop makes humidity spike and brings surfaces close to the dew point.
Is 30 °C too hot for plants?
For most plants 30 °C is the upper limit without extra CO₂. Above that, stomata start to close, growth slows and the VPD climbs quickly.
What water temperature is best for roots?
Around 18–22 °C. Warmer water holds less dissolved oxygen, which makes root problems more likely.