If you’ve been researching cannabis cultivation, you’ve probably noticed a strange contradiction:
Some growers insist on 40–50% RH (Relative Humidity) in flowering, while others say keep it around 70%.
So… who’s right?
The short answer: both, depending on when in flowering they’re talking about and what conditions you’re working with.
Stage-Specific Needs
Cannabis plants have very different humidity needs during different parts of the flowering cycle.
Early Flower (First 1–2 Weeks After Switching to 12/12)
Buds are still small and airy.
Mold risk is low.
Plants still have lots of leaf surface and are actively stretching.
60–70% RH helps them transpire efficiently and grow strong.
Mid-to-Late Flower (When Buds Are Dense)
Buds pack on weight and trap moisture.
High humidity here can cause bud rot or powdery mildew.
40–50% RH helps protect against mold while still allowing healthy plant function.
VPD (Vapor Pressure Deficit) Targets
Many modern growers don’t follow just “raw humidity numbers” — they follow VPD charts, which balance temperature and humidity for optimal transpiration.
Early flower VPD targets allow for higher RH (60–70%).
Late flower VPD targets push RH lower (40–50%) to match the plant’s slower water use and prevent mold.
VPD isn’t just about preventing disease — it’s also about maximizing nutrient uptake and plant metabolism.
Environment & Risk Factors
Your grow environment plays a big role in how safe it is to keep higher humidity in flower.
High airflow / strong dehumidification → You can maintain higher RH in early flower without problems.
Small tents / still air → Lower RH earlier in flower to avoid mold risk.
The Bottom Line
Both numbers are “right” — they’re just for different parts of flowering.
Safe, all-purpose rule for most growers:
Weeks 1–2 of flower: 60–65% RH
Weeks 3–Harvest: 40–50% RH
If you have a top-tier setup with excellent airflow and environmental controls, you can push the early weeks higher for better growth — but never keep 70% RH into late flower unless you want to discover fuzzy gray surprises inside your colas.
Recommended Stage-Based Humidity Schedule
Here’s a safe breakdown you can follow:
Weeks 1–2 (Early Flower)
RH: 60–65%
Buds are forming; mold risk is low.
Keep strong airflow.
Weeks 3–Harvest
RH: 40–50%
Buds are dense — avoid moisture traps.
Use dehumidifiers if needed.
Final Thoughts
When in doubt, lower humidity in late flower — it’s far easier to lose a few grams from slightly drier air than to lose your entire crop to bud rot.
PLNTRK
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