How do I stop condensation and damp in a greenhouse?

In summer a greenhouse ventilates itself through open vents and doors. In winter it is shut, unheated, full of plants that are barely transpiring, and the air inside sits at close to saturation — which is the ideal condition for botrytis.

Step by step

1. Ventilate on every mild day.Even in winter, open a roof vent or the door for an hour or two whenever it is above freezing and not blowing a gale. Changing the air once a day prevents most of the problem.
2. Use low and high openings together.A low side vent lets cool air in and a roof vent lets warm air out, which drives a continuous flow without a fan. An open door alone is one opening and gives very little through-flow.
3. Water in the morning.Water applied in the morning is largely taken up or evaporated by evening. Water at dusk sits on the surfaces all night at the coldest, dampest point of the cycle.
4. Water the compost, not the plant.Wet leaves and wet crowns are where botrytis starts. Water into the pot at soil level and keep the foliage dry.
5. Water far less in winter.Dormant and semi-dormant plants use very little. Most winter losses in a greenhouse are rot from overwatering rather than cold.
6. Space the plants out.Air has to move between them. A bench packed solid with pots holds still, damp air at leaf level regardless of how much you ventilate.
7. Remove dead material weekly.Fallen leaves, spent flowers and dead stems are where grey mould establishes and then spreads to living tissue. A five-minute tidy each week does more than any spray.
8. Get pots off the floor.Cold, wet floors keep the compost cold and wet. Benching or even a layer of slats keeps air moving under the pots.
9. Add gentle air movement if you have power.A small circulating fan on low, running continuously, prevents the stagnant pockets that mould needs. It works better than heating for damp control.
10. Clean the glass in autumn.Clean glass lets more light in, which raises daytime temperature and dries the air. It also removes overwintering pests and disease from the frame joints.

Tips

• Water in the morning, at the roots, and much less than you think in winter.
• One vent open is not ventilation — you need air in at the bottom and out at the top.
• A weekly five-minute tidy of dead leaves prevents more mould than any fungicide.

Common mistakes

• Keeping the greenhouse shut all winter to keep it warm — An unheated greenhouse is barely warmer than outside overnight anyway, and the still saturated air rots the plants you were protecting. Ventilate on mild days regardless.
• Damping down the floor in winter — Damping down is a summer cooling technique. In winter it adds moisture to air that is already saturated.

If it doesn't work

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