Is 90 Percent Humidity Too High for a Greenhouse?

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.

Why 90% Humidity Is Problematic

1. Condensation Means Plant Disease Explosion

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

2. Poor Transpiration

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)

3. CO₂ Exchange Drops

Stomata remain more closed when humidity is high, which hinders photosynthesis and yield.

4. Bad for Budding & Fruit Production

Flowering plants, fruiting vegetables, and cannabis all perform poorly in RH above 85% for extended periods.

When Could 90% RH Be Acceptable?

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.

What's the Ideal Humidity Range?

How to Reduce Humidity in a Greenhouse

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

Summary

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.