Humidity and VPD for Indoor Cannabis

Why humidity matters indoors

Relative humidity (RH) controls how fast plants transpire — water moving from roots through leaves into the air. Too low and stomata close, slowing growth. Too high in flower and bud rot becomes a real risk.

A tent in a bedroom can swing 30% RH between lights-on and lights-off. Managing that swing separates healthy canopies from powdery mildew and crispy tips.

RH targets by stage

Use these as indoor starting points — adjust for strain and airflow:

• Seedling: 65–75% RH — roots still developing
• Vegetative: 55–65% RH
• Early flower (weeks 1–4): 50–55% RH
• Late flower (weeks 5+): 40–45% RH — mold prevention zone
• Dry/cure room: 55–60% RH on the line, not in the tent

VPD in plain language

Vapor Pressure Deficit (VPD) combines temperature and humidity into one number describing how hard the air pulls moisture from leaves. The "sweet spot" for cannabis veg is often quoted around 0.8–1.2 kPa; flower slightly lower.

You don't need expensive gear day one — hit RH targets at your tent temperature and VPD mostly takes care of itself. Controllers that read VPD directly help in sealed rooms.

• High temp + low RH = high VPD → stress, leaf edge burn
• Low temp + high RH = low VPD → slow transpiration, mold risk
• Match dehumidifier/humidifier to tent volume, not room size

Humidifier and dehumidifier in tents

Seedlings in a big tent often need a small humidifier outside the tent ducted in, or a dome on each plant. Exhaust fans pull dry house air — counter that in veg or seedlings suffer.

Late flower in 4×4 with dense colas often needs a dehumidifier inside or in the room feeding the tent. Empty the reservoir daily — standing water grows bacteria.

Troubleshooting swings

Common tent humidity problems:

• RH crashes when lights on → humidifier timed with lights, or upsize exhaust speed on a controller
• RH spikes lights off → reduce passive intake, run dehu overnight
• Mold despite 45% RH → airflow problem, not humidity alone — add under-canopy fan
• Wet walls 24/7 → negative pressure too weak or duct leaking cold air