Views: 1 Author: Site Editor Publish Time: 2025-12-10 Origin: Site
A humidity level of 90% is too high for most greenhouses. It may be acceptable for a very short period, but sustained 90% RH creates serious disease and plant-health risks. Most greenhouse crops thrive when the humidity stays between 60%-80% RH, depending on the species. Once humidity reaches 90% or higher, the air becomes nearly saturated with moisture - meaning any surface (leaves, buds, fruits, frames, soil) can develop condensation, which is the perfect environment for diseases.
At 90% RH, tiny temperature drops cause water droplets to form on leaves. This leads to:
Powdery mildew
Botrytis (gray mold)
Root rot and stem rot
Foliar diseases
Faster spread of pathogens
Plants release moisture through their stomata (transpiration). At 90% humidity, this slows way down, causing:
Nutrient uptake problems
Slower growth
Calcium deficiencies (tip burn, blossom-end rot)
Stomata remain more closed when humidity is high, which hinders photosynthesis and yield.
Flowering plants, fruiting vegetables, and cannabis all perform poorly in RH above 85% for extended periods.
Seed germination (short-term, controlled)
Certain tropical plants
Propagating softwood cuttings
Nighttime humidity spikes (if it drops in the morning)
Most greenhouses expect night spikes, but the humidity should drop again quickly once the lights turn on or the sun rises.
Plant Type | Ideal RH |
Leafy greens (lettuce, herbs) | 60–75% |
Tomatoes, peppers, cucumbers | 60–70% |
Flowers (ornamentals) | 50–70% |
Cannabis | 50–70% (depending on stage) |
Seedlings | 70–85% |
If you see readings near 90%, take action quickly:
Use a dehumidifier for greenhouse (most reliable)
Improve ventilation/exhaust fans
Add horizontal airflow fans to prevent wet spots
Heat + vent (classic moisture-purge method)
Avoid overwatering; improve drainage
Space plants properly to avoid microclimates
Water early in the day
Use condensation-control films or thermal screens
90% RH is far too high for normal greenhouse operation. If that number shows up regularly, a dehumidifier or ventilation upgrade is strongly recommended to prevent disease, protect yields, and maintain plant health.
