Ask most people what kills plants in a greenhouse and they’ll say cold, or maybe too much sun. In our experience it’s neither. It’s stale, saturated air. A glasshouse is a sealed box that traps moisture from the soil, the leaves, and every watering can you tip out. When that moisture has nowhere to go, it settles on the coldest surface it can find, which is usually the glass, the frame, and your plants. That film of water is where mould, mildew, and rot get their start.

The frustrating part is that a humidity problem rarely looks like a humidity problem. It shows up as spotty leaves, soft stems, and seedlings that damp off overnight, and people chase all sorts of remedies before they land on the real cause. So let’s name it properly.

Why humidity is the quiet killer

Warm, still, wet air is exactly what fungal disease wants. Botrytis, powdery mildew, and downy mildew all thrive in it, and once they take hold in a closed greenhouse they spread fast because nothing is moving the spores along. High humidity also stops plants transpiring properly, so they can’t draw nutrients up the way they should. You end up with weak growth that looks like a feeding problem when it’s really a breathing problem.

Reading your greenhouse: the warning signs

You don’t need a hygrometer to catch this early. Walk in first thing and run through the list. Anything you can tick is a red flag worth acting on.

• Heavy condensation running down the inside of the glass most mornings.
• Water droplets sitting on leaves that never quite dry out by midday.
• A grey, dusty, or fuzzy film on stems, soil, or fallen leaves.
• That thick, still, earthy air you can feel the moment you step inside.
• Seedlings collapsing at the base overnight for no obvious reason.

The fix is airflow, not heat

Here’s the thing people get wrong: they crank up the heat to “dry it out”. Warm air holds more moisture, so unless you also move that air out and pull fresh air in, you’ve just made a warmer version of the same problem. The real answer is ventilation. Moving air strips the moisture film off leaves, evens out the temperature, and carries spores away before they can settle.

On a small house, cracking a roof vent and a low side vent on opposite ends creates a gentle chimney of airflow. On anything larger, or on a house you can’t babysit all day, powered ventilation does the job consistently. A steady extraction fan like the powervent motors we fit keeps a constant, quiet exchange of air going even when you’re not there to open a sash by hand.

A simple daily and weekly routine

Beating humidity is mostly habit. This is the rhythm we suggest to owners:

1. Open vents early. Get air moving before the sun loads the house with heat and moisture, not after.
2. Water in the morning, at the roots. Evening watering leaves standing moisture overnight, which is prime disease time.
3. Space your plants. Crowded foliage traps damp air between leaves. A little breathing room goes a long way.
4. Clear dead leaves and spent growth weekly. Rotting material is both a moisture source and a disease reservoir.

Questions we get asked a lot