Short answer. RH targets only make sense with temperature, so set humidity to hit a leaf VPD band. As a starting point: clones/seedlings 65–80 %, veg 55–70 %, early flower 50–60 %, late flower 40–50 %, drying and curing ~55–62 %. The flower numbers are driven by disease risk: dense buds plus humid nights is how bud rot (Botrytis cinerea) starts [2][3].
What's happening
- Young plants with few roots lose water through leaves faster than they can replace it; high RH lowers that demand.
- Bigger plants transpire a lot of water into the room. A plant drinking 2 L a day puts most of that into the air; a 1.2 × 1.2 m (4 × 4 ft) tent of four such plants is adding ~6–8 L of water vapour a day (~90–95 % of irrigation water is transpired in a healthy crop)being reviewed.
- At lights-off temperature drops; the same water content now means higher RH, and surfaces near the dew point collect condensation. That is the hour bud rot loves.
- A 2025 controlled study of cannabis at roughly 78–98 % RH vs 37–58 % RH found delayed flowering, lower biomass and lower cannabinoid content at high RH [1].
Numbers that matter
| Stage | RH | At this air temperature | Leaf VPD (≈, LED) |
|---|---|---|---|
| Unrooted clones (dome) | 75–90 % | 23–26 °C (73–79 °F) | 0.2–0.5 kPa |
| Seedlings | 65–75 % | 23–26 °C (73–79 °F) | 0.4–0.7 kPa |
| Veg | 55–70 % | 24–28 °C (75–82 °F) | 0.7–1.1 kPa |
| Flower wk 1–4 | 50–60 % | 23–27 °C (73–81 °F) | 0.9–1.3 kPa |
| Flower wk 5–harvest | 40–50 % | 20–25 °C (68–77 °F) | 1.1–1.5 kPa |
| Lights-off in flower | ≤ 55 % and ≥ 3 °C above dew point | ||
| Drying | 55–62 % | 15–21 °C (59–70 °F) | see drying |
Dew-point rule of thumb: keep every surface (walls, buds, pots) at least 3 °C (5 °F) above the room's dew point. At 22 °C and 60 % RH the dew point is about 14 °C (57 °F) [4].
What to check first
- Calibrate the hygrometer (salt test).
- Log RH over 48 h; look at the lights-off spike, not the daytime average.
- Inside the canopy: a hygrometer inside a dense cola zone often reads 5–10 % higher than room air.
How to fix it
- Too humid: dehumidifier sized to the transpiration load (litres per day ≈ total irrigation litres per day as a first estimate), more air exchange if outside air is drier, keep the dehumidifier running at lights-off, thin interior leaves, don't leave standing water or runoff trays.
- Too dry: humidifier on a controller; fewer air exchanges; group plants; raise reservoir lids only briefly. Avoid misting buds.
- Spikes at lights-off: ramp temperature down over 30–60 minutes instead of a cliff, or run heat in the dark period.
How to confirm it worked
RH trace stays within ±5 % of target through the lights-off transition, no condensation on tent walls, no grey fuzz inside colas at harvest (check by breaking open the largest buds).
Common mistakes
- Undersized dehumidifier rated at test conditions (often 30 °C/80 % RH) that removes far less at 24 °C/50 %.
- "Lowering RH to 30 % boosts trichomes." No controlled evidence; it mainly stresses the plant (Myth as a reliable technique, Contested at best).
- Ignoring humidity inside the canopy.
Evidence
Humidity as a driver of bud rot and powdery mildew is Established [2][3]. High-RH harm to flowering and cannabinoids is Supported by one 2025 cannabis study [1]. Per-stage RH bands are Grower practice derived from VPD logic.
