# Do cannabis seedlings need a humidity dome?

> Cannabis seedlings don't strictly need a humidity dome, but one helps in dry rooms for the first 3–7 days by holding 65–75% RH around them. Vent it daily, and remove it gradually once the first true leaves are growing well. A dome left closed for days breeds mould and damping-off. If your room already sits at 60–70% RH, skip the dome.

- Canonical: https://genetixbay.com/knowledge/answers/do-seedlings-need-a-dome.html
- Topic: Germination · Type: How-to
- Evidence: Grower practice · Status: Draft: awaiting expert review
- Full article: [Seedling care: the first three weeks](https://genetixbay.com/knowledge/germination/seedling-care.md)
- Also asked as: Should I use a humidity dome for seedlings? | Humidity dome for seedlings, yes or no? | When do I remove the dome from seedlings? | Do seedlings need high humidity?
- 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

Cannabis [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) don't strictly need a dome, but one helps for the first 3–7 days in a [dry room](https://genetixbay.com/knowledge/glossary/drying-room.md) by holding 65–75% [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md). Vent it daily and remove it gradually once [true leaves](https://genetixbay.com/knowledge/glossary/true-leaves.md) are growing. *Grower practice*.

## Why

A seedling has a tiny root and thin leaves. In dry air it loses water faster than the root can replace it, so it droops. Humid air lowers that pull.

The pull is the [vapour pressure deficit](https://genetixbay.com/knowledge/glossary/vpd.md). At 25 °C, air at 70% RH has a much smaller deficit than air at 40% RH, which follows directly from how vapour pressure rises with temperature [3].

The same still, saturated air suits [damping-off](https://genetixbay.com/knowledge/glossary/damping-off.md) pathogens. Extension guidance warns against keeping seedlings wet under covers with poor air movement [1][2].

So the dome is a short bridge, not a home. Once the root system grows, the seedling handles normal room humidity.

## The numbers

| Stage | RH | Air temperature | Dome |
|---|---|---|---|
| Days 0–5 (emerging) | 65–75% | 23–27 °C (73–81 °F) | On, vented daily |
| Days 5–10 (true leaves) | 65–75% | 23–27 °C | Vents open, lid propped |
| Days 10–21 | 60–70% | 23–27 °C | Off |
| [Leaf VPD](https://genetixbay.com/knowledge/glossary/leaf-vpd.md) target | 0.4–0.8 kPa | | |

## What to do

1. Measure room RH first. If it's already 60–70%, skip the dome.
2. If it's dry, dome the seedlings and open the lid for a few minutes each day.
3. Wipe away condensation; it means the air is saturated.
4. Open vents as true leaves form, then prop the lid for a day or two.
5. Remove it fully by about day 10, and keep gentle air moving.

## Common mistakes

- Sealing the dome for a week and finding fuzz on the mix.
- Removing it in one go in a dry, hot room.
- Using a dome to fix [overwatering](https://genetixbay.com/knowledge/glossary/overwatering.md) (it makes it worse).
- Setting a domed tray right under a hot lamp.

## Related

- Full article: [Seedling care: the first three weeks](https://genetixbay.com/knowledge/germination/seedling-care.md)
- [Damping-off](https://genetixbay.com/knowledge/germination/damping-off.md) · [Humidity by stage](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- [Humidity and light for clones](https://genetixbay.com/knowledge/answers/humidity-and-light-for-clones.md)
- Terms: [cloning dome](https://genetixbay.com/knowledge/glossary/cloning-dome.md) · [hardening off](https://genetixbay.com/knowledge/glossary/hardening-off.md)

## Follow-up questions

**When should I take the dome off seedlings?** Take the dome off cannabis seedlings gradually once the first set of true leaves is growing, usually around days 5–10. Open the vents first, then prop the lid for a day or two, then remove it. Taking it off in one go in a dry room can make them droop for a day. Condensation running down the lid means it is overdue for venting.

**What humidity do seedlings need?** Cannabis seedlings do best at about 65–75% relative humidity with air at 23–27 °C (73–81 °F), a leaf VPD of roughly 0.4–0.8 kPa. Their small roots can't replace water quickly, so very dry air makes them wilt. Much wetter than that, with no airflow, favours damping-off and soft stems. These targets are grower practice rather than cannabis-specific research.

**Can a humidity dome cause damping-off?** Yes. A humidity dome left sealed over cannabis seedlings, combined with a wet surface and still air, creates the conditions that damping-off organisms such as Pythium favour. The dome itself isn't the problem; neglect is. Vent it at least once a day, wipe heavy condensation, keep the mix just moist rather than wet, and remove the lid once true leaves are growing.

## Sources

1. University of Minnesota Extension. How to prevent seedling damping off. https://extension.umn.edu/solve-problem/how-prevent-seedling-damping
2. NC State Extension. Damping-off in flower and vegetable seedlings. https://content.ces.ncsu.edu/damping-off-in-flower-and-vegetable-seedlings
3. Allen RG et al., 1998. FAO-56 Chapter 3 (vapour pressure). https://www.fao.org/4/x0490e/x0490e07.htm