Short answer. Keep the grow room at about 24–27 °C (75–81 °F) and 65–75 % RH for seedlings, 22–28 °C (72–82 °F) and 55–70 % in veg, 20–26 °C (68–79 °F) and 45–55 % in flower, and 40–50 % RH in the last 2–3 weeks, with nights 3–6 °C (5–10 °F) cooler. Those pairs give a leaf VPD of roughly 0.4–0.8, 0.8–1.2 and 1.0–1.5 kPa. Read your air temperature and RH, look up the VPD in the cheat sheet below, and nudge humidity first.
Climate is the boring backbone of every grow. Plants forgive a lot, but not a week at 32 °C or a flower room stuck at 70 % RH. This guide gives the targets, a one-table cheat sheet and the routine; the physics is in VPD explained.
What do you need to control the climate?
You need a way to measure, a way to move air and one device for whichever way your room drifts. Generic classes, no brands:
- [ ] A digital thermo-hygrometer with min/max memory, placed at canopy height in the shade of a leaf (not in the light beam)
- [ ] Ideally a second sensor or a logging one, so you see the night as well as the day
- [ ] An inline exhaust fan with a speed controller, venting through a carbon filter (smell control basics)
- [ ] One or two clip or oscillating fans moving air gently over and under the canopy (airflow)
- [ ] A dehumidifier if your room runs humid, or a humidifier if it runs dry (most rooms need one or the other, rarely both)
- [ ] Optional: an IR thermometer to read leaf temperature directly
- [ ] Optional: a small heater with a thermostat and tip-over cut-off for cold rooms (see electrical and water safety)
What temperature and humidity should you run at each stage?
Run warm and humid for young plants and progressively cooler and drier as flowers form. The table is Grower practice anchored to cannabis photosynthesis data [3] and disease research [4][5].
| Stage | Air temp, lights on | Lights off | RH | Leaf VPD target |
|---|---|---|---|---|
| Seedling and clone | 24–27 °C (75–81 °F) | 20–24 °C (68–75 °F) | 65–75 % | 0.4–0.8 kPa |
| Veg | 22–28 °C (72–82 °F) | 18–24 °C (64–75 °F) | 55–70 % | 0.8–1.2 kPa |
| Early and mid flower | 20–26 °C (68–79 °F) | 18–22 °C (64–72 °F) | 45–55 % | 1.0–1.5 kPa |
| Late flower (last 2–3 weeks) | 20–24 °C (68–75 °F) | 17–21 °C (63–70 °F) | 40–50 % | 1.2–1.5 kPa |
| Drying room | 15–21 °C (60–70 °F) | Same, dark | 55–62 % | Not used; see drying |
Net photosynthesis in cannabis peaked at about 25–30 °C and fell above that at ambient CO₂ [3], so 30 °C (86 °F) is the practical ceiling for a normal tent. Keep the root zone and any hydro reservoir at 18–22 °C (64–72 °F) (temperature by stage).
What is VPD, in one paragraph?
VPD (vapour pressure deficit) is how hard the air pulls water out of the leaf, measured in kilopascals (kPa). It combines temperature and relative humidity into one number that tracks transpiration. Low VPD (below about 0.4 kPa) means the leaf barely transpires, so the plant moves little water and calcium and mould thrives; high VPD (above about 1.6 kPa) makes plants close their stomata to save water. Calculate it from the leaf, not the air: leaf VPD uses leaf temperature, which under most indoor lights runs about 1–2 °C below air temperature [2].
What does the VPD cheat sheet say?
The cheat sheet gives leaf VPD in kPa for your air temperature and RH, assuming the leaf is 2 °C cooler than the air. It uses the standard Tetens equation for saturation vapour pressure, es(T) = 0.6108 × exp(17.27 T / (T + 237.3)) kPa [1], with leaf VPD = es(leaf) − es(air) × RH/100.
| Air temp | 40 % | 45 % | 50 % | 55 % | 60 % | 65 % | 70 % | 75 % |
|---|---|---|---|---|---|---|---|---|
| 20 °C (68 °F) | 1.13 | 1.01 | 0.89 | 0.78 | 0.66 | 0.54 | 0.43 | 0.31 |
| 22 °C (72 °F) | 1.28 | 1.15 | 1.02 | 0.88 | 0.75 | 0.62 | 0.49 | 0.36 |
| 24 °C (75 °F) | 1.45 | 1.30 | 1.15 | 1.00 | 0.85 | 0.70 | 0.56 | 0.41 |
| 26 °C (79 °F) | 1.64 | 1.47 | 1.30 | 1.14 | 0.97 | 0.80 | 0.63 | 0.46 |
| 28 °C (82 °F) | 1.85 | 1.66 | 1.47 | 1.28 | 1.09 | 0.90 | 0.72 | 0.53 |
| 30 °C (86 °F) | 2.08 | 1.87 | 1.66 | 1.45 | 1.23 | 1.02 | 0.81 | 0.60 |
How to read it: seedlings want the 0.4–0.8 band (bottom-right cells), veg 0.8–1.2 (the middle), flower 1.0–1.5 (left-centre). Example: 26 °C at 60 % RH is 0.97 kPa, fine for veg; drop RH to 50 % and it becomes 1.30 kPa, right for flower.
Two cautions. Under strong LEDs with good airflow the leaf can sit close to air temperature, which makes real VPD about 0.2–0.3 kPa higher than the table [2]; measure leaf temperature with an IR thermometer if you want precision. And many VPD bands are greenhouse and grower practice: cannabis-specific VPD trials are still scarce (VPD explained).
How do you set up and run your climate, step by step?
- Run the empty room for 48 hours with the light and fans on their normal schedule. Log lights-on and lights-off temperature and RH every few hours, or use the min/max memory.
- Place the sensor correctly: at canopy height, shaded, not touching the tent wall, not in the exhaust stream. Move it up as the plants grow.
- Fix temperature first. Too hot: raise exhaust speed, cool the room the tent pulls from, or dim the light 10–20 %. Too cold: slow the exhaust and warm the surrounding room; avoid a heater blowing on plants.
- Then set RH for the stage. Too humid: more exhaust, then a dehumidifier in the room (or tent). Too dry: slow exhaust, then a humidifier.
- Check VPD on the cheat sheet once both numbers are in range. Nudge RH by about 5 points to move VPD by roughly 0.1–0.2 kPa.
- Keep the lights-off drop to 3–6 °C (5–10 °F). The dark period is when RH spikes; if night RH climbs above about 60 % in flower, run the dehumidifier or exhaust at night too.
- Move air under and over the canopy so leaves flutter gently, about 0.3–1.0 m/s at leaf level. No dead corners, no leaves whipped flat (airflow).
- Step the targets down at the flip and again for the last 2–3 weeks of flower, when dense buds are most at risk of bud rot.
- Change one setting at a time and wait 24 hours before judging. Log every change in your grow journal.
- Recheck after every big change: a new light height, a pot-up, a dense canopy and a new season all move the numbers.
Do cold nights improve colour or flavour?
Cold nights can bring out colour; they don't add terpenes. Anthocyanins, the pigments behind purple, build up with low temperature and other stresses in plants that carry the genes for them [7], so a cultivar with purple genetics may colour up with cooler nights. That part is real.
The GenetixBay house position: claims that cold nights add terpenes, flavour or resin are a Myth unless peer-reviewed data shows otherwise. Purple is bag appeal, not proof of quality. Large night drops (more than about 8–10 °C) also push RH up in the dark and set up condensation inside buds, which is how botrytis starts [5][6]. A 3–6 °C drop is plenty. See purple stems and leaves for when purple means something else.
How will you know the climate is working?
You'll know the climate is right when the logs are boring and the leaves look relaxed:
- Temperature stays within about 2–3 °C (4–5 °F) of target, lights-on, day after day.
- RH stays within about 5 points of target, including the first hour after lights-off.
- Leaves sit flat or slightly raised and flutter gently in the fan breeze; no upward taco or canoe, no droop.
- The pot dries from heavy to light in 2–4 days in veg; a pot that stays wet for a week often means VPD is too low or the plant is too small for the pot.
- No condensation on tent walls in the morning, and no grey fuzz or white powder in flower.
- Top leaves are not bleached or crispy; if they are, see heat and light stress.
What mistakes do beginners make with climate?
- Measuring in the wrong place: a sensor in the light beam reads 3–5 °C hot; one on the floor reads cold.
- Only checking during the day. Lights-off RH spikes are where mould starts.
- Chasing VPD decimals while temperature swings 8 °C between day and night.
- Running flower at veg humidity, 60–70 % RH in weeks 6–9, the classic route to bud rot [4][6].
- Sealing the tent too tight with no exhaust, so heat and humidity build up and CO₂ runs low.
- Big cold-night drops "for purple": colour maybe, terpenes no, condensation yes.
- Blowing a fan straight at one plant hard enough to cause windburn and clawed leaves.
- Changing exhaust, humidifier and light height on the same day, then not knowing which one helped.
Evidence
- Saturation vapour pressure and VPD maths: Established [1].
- Leaf temperature below air under indoor lights, and closer to air under LEDs: Established [2].
- Cannabis photosynthesis peak around 25–30 °C at ambient CO₂: Supported [3].
- High RH delaying flowering and lowering cannabinoids: Supported (one 2025 cannabis study) [4].
- High RH raising bud rot and mildew: Established [5][6].
- Stage VPD bands: Grower practice.
- Cold nights and anthocyanin colour: Supported in plants generally [7]; cold nights adding terpenes or resin: Myth (house position, no supporting cannabis data).
