Short answer. Cannabis photosynthesises best somewhere around 25–30 °C (77–86 °F) leaf temperature when light is strong; above ~30 °C (86 °F) net photosynthesis and water-use efficiency fall in the one cannabis gas-exchange study we have [1]. In practice: veg 24–28 °C (75–82 °F) day, flower 22–26 °C (72–79 °F) day, late flower 20–24 °C (68–75 °F), nights 3–6 °C (5–10 °F) cooler, root zone 18–22 °C (64–72 °F).
What's happening
Temperature sets the speed of every enzyme in the plant. Photosynthesis climbs with temperature until respiration and photorespiration catch up; at high light and high CO₂ the optimum shifts warmer, which is why CO₂-enriched rooms run 2–4 °C hotter [1][2]. Night temperature mostly controls respiration (sugar burnt) and stretch: a warm night relative to day (negative DIF) shortens internodes, a big positive DIF lengthens them (well documented in ornamentals; Grower practice in cannabis).
Leaf temperature is what matters, and it is not air temperature. Under LEDs well-watered leaves usually run slightly cooler than air; under HID they run warmer [3]. See VPD explained.
Numbers that matter
| Stage | Day air | Night air | Root zone / reservoir | Notes |
|---|---|---|---|---|
| Germination | 22–28 °C (72–82 °F) | same | 22–26 °C (72–79 °F) | Hemp seed optimum ~19–30 °C [5] |
| Seedlings / clones | 23–27 °C (73–81 °F) | 20–24 °C (68–75 °F) | 21–24 °C (70–75 °F) for rooting | Warm root zone speeds rooting |
| Veg | 24–28 °C (75–82 °F) | 19–23 °C (66–73 °F) | 18–22 °C (64–72 °F) | Add 2–3 °C with CO₂ 800–1,200 ppm |
| Transition (flower wk 1–3) | 23–27 °C (73–81 °F) | 19–22 °C (66–72 °F) | 18–22 °C (64–72 °F) | Smaller DIF limits stretch |
| Mid flower (wk 4–6) | 22–26 °C (72–79 °F) | 18–22 °C (64–72 °F) | 18–22 °C (64–72 °F) | |
| Late flower / ripening | 20–24 °C (68–75 °F) | 16–20 °C (61–68 °F) | 18–21 °C (64–70 °F) | Cooler days avoid heat losses of volatile terpenes (Grower practice); cold nights don't add terpenes |
| Hard limits | Above 30–32 °C (86–90 °F) sustained, or below 15 °C (59 °F) | Above 24 °C (75 °F) in DWC/RDWC raises root-rot risk [4] |
What to check first
- Canopy-height reading, shaded probe, moving air. Ceiling and floor readings lie.
- Leaf temperature with an IR thermometer; compare with air.
- Day-night swing: plot 48 h. Sharp swings at lights-on/off cause condensation.
- Reservoir or pot temperature, especially in hydro and fabric pots on warm floors.
How to fix it
- Too hot: raise or dim lights 10–20 %, improve exhaust or add cooling, move ballasts/drivers outside the room, run lights during the cooler part of the day.
- Too cold: insulate, add a thermostatically controlled heater (never an open-element heater near water or plastic), shorten the dark-period swing.
- Warm reservoir: chiller, insulated reservoir, reservoir off the floor in shade, larger volume.
How to confirm it worked
New growth is evenly spaced (no sudden stretch), leaves don't taco or droop at mid-day, water use is steady, and roots in hydro stay white.
Common mistakes
- Copying a CO₂-enriched commercial setpoint (28–30 °C) into an un-enriched tent.
- Ignoring leaf temperature under HID.
- Big cold nights to "force purple" or "boost terpenes". Anthocyanin colour is mostly genetic; cold can bring it out in some cultivars (real), but it also slows ripening and invites condensation. More terpenes, flavour or resin from cold: Myth.
- Warm reservoirs in summer and wondering why roots go brown.
Evidence
The cannabis gas-exchange response to temperature and light is Supported by Chandra et al. [1][2] (one cultivar, leaf level, short-term measurements). Stage setpoints and DIF effects are Grower practice informed by greenhouse horticulture. Root-zone 18–22 °C for hydroponics is Established extension guidance [4]. Germination cardinal temperatures for hemp are Supported [5]. Cold nights bring out anthocyanin colour in cultivars that carry it; that's real. Claims that cold nights add terpenes, flavour or resin are a Myth (house position) unless peer-reviewed data shows otherwise.
