# How do I raise the humidity in my grow tent?

> Raise grow-tent humidity by keeping the water your plants transpire: slow the exhaust first, then add a humidifier on a humidistat. Targets are 65–75 % RH for seedlings, 55–70 % in veg and 50–60 % in early flower. Low RH matters most for seedlings and clones; in late flower, 40–50 % is the goal, so dry air is rarely a problem then.

- Canonical: https://genetixbay.com/knowledge/answers/how-to-raise-humidity-in-a-grow-tent.html
- Topic: Environment · Type: How-to
- Evidence: Grower practice · Status: Draft: awaiting expert review
- Full article: [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- Also asked as: Humidity too low in my grow tent | How do I increase RH in a grow room? | Low humidity in my grow tent: what can I do? | How do I raise humidity for seedlings?
- 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)"

## Short answer

Raise grow-tent humidity by holding on to the water your plants [transpire](https://genetixbay.com/knowledge/glossary/transpiration.md): turn the [exhaust](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) down first, then add a [humidifier](https://genetixbay.com/knowledge/glossary/humidifier.md) on a humidistat. Aim for **65–75 % [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) for [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md)**, **55–70 % in veg** and **50–60 % in early flower**. *Grower practice*, built on [VPD](https://genetixbay.com/knowledge/glossary/vpd.md) physics.

## Why

Relative humidity sets how hard the air pulls water from the leaf, the vapour pressure deficit [1]. Dry air at normal temperatures means a high VPD: [stomata](https://genetixbay.com/knowledge/glossary/stomata.md) partly close, CO₂ uptake drops and leaf edges crisp.

Seedlings and fresh clones have few roots. In dry air they lose water faster than they can replace it, which is why young plants get the highest RH.

Under [LEDs](https://genetixbay.com/knowledge/glossary/led.md), leaves often sit 1–3 °C (2–5 °F) below air temperature [2], so the leaf feels less dry than an air reading suggests. Read [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) before chasing a number.

Don't overshoot. Damp air and wet surfaces in flower invite *[Botrytis](https://genetixbay.com/knowledge/glossary/botrytis.md)* and [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) [3].

## The numbers

| Stage | RH target | Air temperature |
|---|---|---|
| Unrooted clones (dome) | 75–90 % | 23–26 °C (73–79 °F) |
| Seedlings | 65–75 % | 23–26 °C (73–79 °F) |
| Veg | 55–70 % | 24–28 °C (75–82 °F) |
| Flower weeks 1–4 | 50–60 % | 23–27 °C (73–81 °F) |
| Flower week 5 to harvest | 40–50 % | 20–25 °C (68–77 °F) |

## What to do

1. Check the hygrometer is accurate and sits at canopy height.
2. Slow the exhaust fan, keeping temperature in range.
3. Cool the room a degree or two if it runs hot; warm air reads drier.
4. For seedlings, use a vented dome or group the pots.
5. Add a humidifier with a humidistat, low in the tent, away from lights and plugs.
6. Use [RO](https://genetixbay.com/knowledge/glossary/ro-water.md) or distilled water in it if your [tap water](https://genetixbay.com/knowledge/glossary/tap-water.md) is hard.

## Common mistakes

- Misting plants instead of humidifying the air.
- Running the exhaust flat out in a dry winter room.
- A humidifier with no controller, soaking the tent overnight.
- Raising RH in late flower, when 40–50 % is the goal.

## Related

- Full article: [Humidity by stage](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- [How do I lower the humidity in my grow tent?](https://genetixbay.com/knowledge/answers/how-to-lower-humidity-in-a-grow-tent.md)
- [Should I mist my plants?](https://genetixbay.com/knowledge/answers/should-i-mist-my-plants.md)
- [VPD explained](https://genetixbay.com/knowledge/environment/vpd-explained.md)
- Terms: [humidifier](https://genetixbay.com/knowledge/glossary/humidifier.md) · [leaf VPD](https://genetixbay.com/knowledge/glossary/leaf-vpd.md)

## Follow-up questions

**How can I raise humidity without a humidifier?** Without a humidifier, raise grow-tent humidity by slowing the exhaust fan so the air the plants transpire stays in, as long as temperature stays in range. Group plants together, add more plants or a larger canopy, and lay a damp towel in a tray below the canopy for seedlings. Avoid spraying buds, and don't leave standing water near electrics.

**Is 30 % humidity too low for cannabis?** 30 % relative humidity is too low for most of a cannabis grow. Seedlings and clones want about 65–75 %, veg 55–70 % and early flower 50–60 %. At 30 % and normal room temperatures, leaf VPD runs well above 1.6 kPa, stomata partly close and leaf edges crisp. Late flower tolerates 40–50 %; going lower to 'boost trichomes' has no controlled evidence.

**Where should I put a humidifier in a grow tent?** Put a grow-tent humidifier low in the tent or at the intake side, away from the light, plugs and buds, with a humidistat set to the stage target. Mist from it should evaporate before it reaches leaves or walls. Fill it with RO or distilled water if your tap water is hard, or it will leave white mineral dust on leaves and the tent.

## Sources

1. Allen RG, Pereira LS, Raes D, Smith M, 1998. Crop evapotranspiration: guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56), Chapter 3. FAO. https://www.fao.org/4/x0490e/x0490e07.htm
2. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/
3. 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/