What is damping down, and how do I do it properly?
Wetting the floor of a greenhouse cools it, raises humidity and suppresses red spider mite. When it helps, when it causes disease, and the plants that hate it.
- Difficulty
- beginner
- Time
- 5 min
- Read
- 3 min
Short answer
Throw water over the path, the floor and the staging legs on hot mornings and again at midday. Evaporation absorbs heat and raises humidity, which reduces plant stress and discourages red spider mite. Do it early in the day, never in the evening, and never in cool or dull weather.
Damping down is one of the oldest greenhouse techniques and it is still the cheapest cooling available. The physics is straightforward: evaporating water absorbs a large amount of energy, so wetting a warm concrete floor takes real heat out of the air. The humidity it produces is a second benefit for most summer crops and a serious risk in the wrong season.
Step by step
- Damp down in the morning, once the day is clearly going to be hot.Water thrown on the floor at eight or nine in the morning has all day to evaporate and does its cooling when you need it. A watering can with the rose off, or a hose on a gentle spray, over the path and under the staging.
- Repeat at midday on very hot days.A second application at the hottest part of the day is standard practice in a heatwave. On a solid floor in strong sun it will have dried within two or three hours.
- Wet the hard surfaces, not the plants.Path, floor, staging legs, the base of the walls. Wetting foliage does very little for air temperature and leaves droplets that scorch in sun and encourage fungal infection.
- Stop by mid-afternoon.Everything must be dry by nightfall. Damping down in the evening leaves the greenhouse humid and still overnight, which is exactly the condition botrytis needs.
- Combine it with ventilation, not instead of it.Damping down in a closed greenhouse creates a sauna. Vents open, air moving, then damp down — the moving air is what carries the evaporated water and the heat away.
- Know which plants want it and which do not.Cucumbers, melons, aubergines, ferns and most foliage plants love the humidity. Tomatoes tolerate it and benefit at midday for pollen release. Peppers and chillies are fine. Cacti, succulents, pelargoniums and anything grey-leaved should be kept out of a damped house.
- Do not damp down outside the hot season.From roughly September to May the greenhouse problem is too much humidity, not too little. Damping down in that period causes grey mould and downy mildew.
Tips
- Damping down is far more effective on concrete or paving than on gravel, and largely wasted on bare dry soil which just absorbs the water.
- It is one of the standard cultural controls for red spider mite, which needs hot dry air. If you have mite, damping down twice a day genuinely reduces the population and supports the predatory mite used against it.
- Tapping the tomato supports just after damping down at midday helps pollen release, because pollen sheds better at moderate humidity than in very dry air.
Common mistakes
- Damping down in the evening — Still, saturated air overnight is the perfect condition for grey mould on tomato stems, fruit and cucumbers. Everything should be dry by dusk.
- Damping down a closed greenhouse — Without ventilation the humidity simply builds. You get the disease risk without the cooling benefit, because the moisture-laden air has nowhere to go.
- Spraying the plants instead of the floor — Water on leaves in strong sun causes scorch, and it evaporates too fast to cool the air meaningfully. The floor is a much larger surface and it is not damaged by being wet.
Questions people ask
How often should I damp down?
Once in the morning on any hot sunny day, and a second time around midday in a genuine heatwave. Not at all on dull days, cool days, or between autumn and late spring.
Does damping down really cool a greenhouse?
Yes, measurably — evaporation absorbs a large amount of energy. It is not a substitute for ventilation and shading, but combined with them it takes a further few degrees off the peak for the cost of a watering can.