For cannabis in the flowering stage, humidity control gets much stricter because dense buds trap moisture and create ideal conditions for mold.

Target humidity for flowering stage

Early flower (weeks 1–3): 50–60% RH

Mid flower (weeks 4–6): 45–55% RH

Late flower (final 2–3 weeks): 40–50% RH (ideal)

What counts as “too high”

>60% RH: Elevated risk—acceptable only briefly

>65% RH: Too high—conditions favor mold inside buds

>70% RH: High probability of infection, especially in dense canopies

Why humidity matters

At higher humidity levels:

Bud interiors stay damp → perfect for Botrytis cinerea (bud rot)

Surface moisture supports powdery mildew

Transpiration slows, reducing nutrient transport and resin development

VPD target (more precise control)

During flowering, aim for:

VPD: 1.2–1.5 kPa

This corresponds roughly to:

75°F (24°C) → ~45–50% RH

80°F (27°C) → ~40–45% RH

Practical humidity control strategy

Keep nighttime RH in check (it often spikes when lights go off).

Use dehumidification and air circulation in the greenhouse. Not just fans—fans alone don’t remove moisture. Seedmax industrial dehumidifier for greenhouse is a good choice for commercial growers. The mobile dehumidifier with water tank is easy to move and handle. The ceiling mount dehumidifier is better for a professional grow room that needs highly automated management. Welcome to your consultation.

Avoid leaf crowding; maintain airflow through the canopy.

Water early in the light cycle so excess moisture dissipates.

Bottom line

For flowering cannabis, anything consistently above 60% RH is too high, and 65%+ becomes a real risk zone. Keeping it in the 40–50% range late in flower is one of the most effective ways to protect yield and quality.