Temperature sets the speed of nearly every process in a plant: photosynthesis, respiration, water uptake and development. For cannabis, grow-room air temperature is the number most growers track, but leaf temperature is what the plant actually experiences, and root-zone temperature matters separately.
In practice
Common targets (air, under LED; HPS rooms run about 1–3 °C cooler for the same leaf temperature):
| Stage | Day | Night |
|---|---|---|
| Clones and seedlings | 24–27 °C (75–81 °F) | 21–24 °C (70–75 °F) |
| Veg | 24–28 °C (75–82 °F) | 20–24 °C (68–75 °F) |
| Flower | 23–28 °C (73–82 °F) | 18–22 °C (64–72 °F) |
| Late flower | 20–26 °C (68–79 °F) | 17–21 °C (63–70 °F) |
Chandra and colleagues (2008) found cannabis leaf photosynthesis peaked around 25–30 °C at ambient CO2. Measure at canopy height in the shade of a leaf, not near the floor or the fixture. Keep roots in hydro around 18–22 °C (64–72 °F) for good dissolved oxygen.
Don't confuse with
- Leaf temperature: often 1–3 °C below air under LED, and can be above air under HPS.
- DIF: the gap between day and night temperature.
- Root-zone temperature: medium or water temperature, a separate target.
The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.HPS (high-pressure sodium) is a gas-discharge lamp with orange-yellow light peaking near 589 nm; it was the standard flowering light before LEDs.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.CO2 enrichment raises carbon dioxide in a sealed grow space from about 420 ppm to roughly 800–1,500 ppm, letting plants use higher light intensities productively.DIF is day temperature minus night temperature; a larger positive DIF lengthens stems, while zero or negative DIF tends to keep plants shorter.Heat stress is damage or slowed growth when leaf or canopy temperature rises above what the plant can handle, typically above about 30–32 °C (86–90 °F) in cannabis.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Cold nights are deliberately low night temperatures in late flower; they can bring out purple colour in cultivars that carry it, but don't raise terpenes or resin.
