Humidity is the quiet greenhouse variable that causes more losses than growers realise. Too high, and stagnant moist air breeds fungal disease; too low, and heat-stressed plants struggle. The goal is a healthy middle, managed mostly through airflow and watering habits.
Why humidity matters
- Too high + still air — the classic setup for botrytis“>botrytis (grey mould), downy mildew“>downy mildew and other diseases.
- Too low + heat — wilting, scorch and stressed growth, especially in summer.
Measure it
A hygrometer“>hygrometer shows relative humidity so you can act before problems appear. Place it at plant height out of direct sun.
Lowering humidity (the usual winter problem)
- Ventilate — even a little on milder days exchanges moist air; keep a circulation fan“>circulation fan running. See ventilation“>ventilation.
- Water in the morning, at the base — so foliage and surfaces dry before night.
- Space plants — avoid crowding so air moves and leaves dry.
Raising humidity (summer / propagation)
Damping down the floor, grouping plants and misting propagation areas raise humidity when heat dries the air. Balance it with airflow so you don’t tip into disease territory.
Quick questions
How do I reduce humidity in a greenhouse? Ventilate and keep air moving, water at the base in the morning, and space plants — trapped moist air is the main cause of greenhouse disease.
Related guides
- Ventilation“>Ventilation · greenhouse diseases · watering“>watering · temperature“>temperature