Environment
Temperature, humidity, VPD, airflow, CO₂ and light.
Airflow: air exchange, canopy air speed and fan placement
Aim for gentle, constant movement across and under the canopy (roughly 0.3–1.0 m/s at leaf level) plus enough exhaust to remove heat and humidity, typically one full tent air exchange every 1–3 minutes for a lit tent. Moving air thins the leaf boundary layer, which raised canopy photosynthesis in non-cannabis research, and dries out the microclimates where mould starts.
CO₂ enrichment: when it pays, how much, and how to stay safe
Raising CO₂ from ambient (~420 ppm) to 800–1,200 ppm during lights-on raises cannabis photosynthesis substantially, but only pays when light is already high (roughly 900+ µmol/m²/s) and the room is sealed enough to hold it. Keep occupied rooms below the 5,000 ppm 8-hour workplace limit and never enter a room above 30,000 ppm.
Heat and light stress: taco leaves, bleaching and foxtails
Heat and light stress show at the top of the plant, closest to the light: leaves curl up or taco, bracts bleach white or yellow, buds may foxtail. Measure leaf temperature and canopy PPFD before blaming nutrients. Typical fixes: dim or raise the light 10–20 %, bring leaf temperature under about 30 °C (86 °F), restore airflow and root-zone water uptake.
Humidity by stage: RH targets from clone to cure
Run about 65–80 % RH for clones and seedlings, 55–70 % in veg, 50–60 % early flower and 40–50 % in late flower, adjusted to hit a leaf VPD band at your temperature. High RH in flower raises bud-rot and mildew risk and, in a 2025 study, delayed flowering and lowered cannabinoids. Dry and cure near 60 % RH.
Light spectrum: blue, red, far-red and UV, what's real
Photon quantity (PPFD/DLI) and fixture efficacy matter far more than spectrum. Less blue slightly increased cannabis yield (about 12 % from 4 % to 20 % blue). Far-red stretches plants; its yield effect is contested and seems to trade off against cannabinoid concentration. UV has repeatedly failed to raise cannabinoids in controlled trials and can cut yield at high doses.
Photoperiod and light leaks: 18/6, 12/12 and the dark period
Photoperiod cannabis stays vegetative under long days (18/6 up to 24/0; no sleep period is needed in veg) and flowers when the uninterrupted night is long enough: 12/12 or 13/11, and some cultivars flower at 14–15 h, usually as ~12 h at high intensity plus sunrise and sunset ramps. Light during the dark period, even very dim, can delay or weaken flowering in sensitive cultivars; leaks can also expose herm-prone genetics, triggering male flowers on susceptible plants. Some cultivars yield more at 13–14 h days, at a possible cost in THC concentration.
PPFD and DLI by stage, with the evidence
In the best indoor trial, cannabis flower yield rose linearly with canopy PPFD from 120 to 1,800 µmol/m²/s at ambient CO₂ (116 to 519 g/m²) with no change in cannabinoid potency. Practical targets: 100–300 µmol/m²/s for seedlings and clones, 300–700 (up to ~900) in veg, and 800–1,200 in flower, or more with CO₂, as long as the plant shows no stress and the electricity is worth it.
Temperature by stage: day, night and root-zone targets
Run roughly 24–28 °C (75–82 °F) by day in veg, 22–26 °C (72–79 °F) in flower, cooling to 20–24 °C (68–75 °F) in late flower, with nights 3–6 °C (5–10 °F) cooler. Above about 30 °C (86 °F) photosynthesis in cannabis falls unless light and CO₂ are high. Keep the root zone and hydro reservoirs at 18–22 °C (64–72 °F).
VPD explained: vapour pressure deficit for cannabis growers
VPD is how hard the air pulls water out of the leaf, in kPa. Common targets are about 0.4–0.8 kPa for seedlings and clones, 0.8–1.2 kPa in veg and 1.0–1.5 kPa in flower, calculated from leaf temperature rather than air temperature. Most of these bands come from grower and greenhouse practice; cannabis-specific VPD trials are still scarce.
