# 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.

- Canonical: https://genetixbay.com/knowledge/environment/humidity-by-stage.html
- Category: Environment · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Transition, Flower, Ripening, Harvest, Cure · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What humidity should I have in flower?** Cannabis in flower does best at about 50–60 % relative humidity in weeks 1–4 at 23–27 °C, dropping to 40–50 % from week 5 to harvest at 20–25 °C. At lights-off, keep RH at or below 55 % and surfaces at least 3 °C above the dew point. These grower-practice bands target a leaf [VPD](https://genetixbay.com/knowledge/glossary/vpd.md) of roughly 0.9–1.5 kPa and limit [bud rot](https://genetixbay.com/knowledge/glossary/botrytis.md).

**Is 70 % humidity too high in late flower?** Yes, 70 % relative humidity is too high for cannabis in late flower, where the target is about 40–50 %. Dense buds plus humid nights is how bud rot starts, and a 2025 controlled cannabis study at roughly 78–98 % RH found delayed flowering, lower biomass and lower [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) content compared with 37–58 % RH.

**Why does humidity spike when the lights go off?** Humidity spikes when grow lights go off because the air temperature drops: the same amount of water vapour in cooler air means higher relative humidity, and surfaces near the dew point collect condensation. Ramp temperature down over 30–60 minutes instead of a sudden cliff, keep the dehumidifier running in the dark, or add heat during the dark period.

**How big a dehumidifier do I need?** As a first estimate, a grow-room dehumidifier should remove about as many litres per day as the plants are irrigated, because most irrigation water is transpired into the air. Four large plants in a 1.2 × 1.2 m tent can add 6–8 L of vapour daily. Size for real conditions: units rated at 30 °C and 80 % RH remove far less at 24 °C and 50 %.

**Can low humidity increase [trichomes](https://genetixbay.com/knowledge/glossary/trichome.md)?** No controlled evidence shows that low humidity increases cannabis trichomes. Dropping relative humidity to around 30 % to 'boost trichomes' is a myth as a reliable technique, contested at best, and mainly stresses the plant. Late-flower targets of about 40–50 % RH are set for disease control and a healthy leaf VPD, not for resin.

**Short answer.** RH targets only make sense with temperature, so set humidity to hit a [leaf VPD](https://genetixbay.com/knowledge/environment/vpd-explained.md) band. As a starting point: **clones/[seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) 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](https://genetixbay.com/knowledge/glossary/transpiration.md) 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).
- At lights-off temperature drops; the same [water content](https://genetixbay.com/knowledge/glossary/moisture-content.md) 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](https://genetixbay.com/knowledge/glossary/leaf-vpd.md) (≈, [LED](https://genetixbay.com/knowledge/glossary/led.md)) |
|---|---|---|---|
| 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](https://genetixbay.com/knowledge/harvest-cure/drying.md) |

**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

1. Calibrate the hygrometer (salt test).
2. Log RH over 48 h; look at the lights-off spike, not the daytime average.
3. 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](https://genetixbay.com/knowledge/glossary/runoff.md) trays.
- **[Too dry](https://genetixbay.com/knowledge/glossary/over-drying.md):** 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](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) 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.

## Sources

1. Corredor-Perilla IC, Kwon H, Park J, 2025. High relative humidity effects in cannabis. Frontiers in Plant Science 16:1678142. https://doi.org/10.3389/fpls.2025.1678142
2. Punja ZK, 2021. Emerging diseases of Cannabis sativa and sustainable management. Pest Management Science 77:3857–3870. https://pmc.ncbi.nlm.nih.gov/articles/PMC8451794/
3. Utah State University Extension. Hemp bud rot (IPM fact sheet). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-bud-rot.php
4. Allen RG et al., 1998. FAO-56 Chapter 3 (vapour pressure). https://www.fao.org/4/x0490e/x0490e07.htm