Do cannabis seedlings need a humidity dome?

Q: 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.

Short answer

Cannabis seedlings don't strictly need a dome, but one helps for the first 3–7 days in a dry room by holding 65–75% RH. Vent it daily and remove it gradually once true leaves 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. 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 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 target	0.4–0.8 kPa

What to do

Measure room RH first. If it's already 60–70%, skip the dome.
If it's dry, dome the seedlings and open the lid for a few minutes each day.
Wipe away condensation; it means the air is saturated.
Open vents as true leaves form, then prop the lid for a day or two.
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 (it makes it worse).
Setting a domed tray right under a hot lamp.

Full article: Seedling care: the first three weeks
Damping-off · Humidity by stage
Humidity and light for clones
Terms: cloning dome · hardening off

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.