Damping Down Greenhouses and Polytunnels

As the weather gets warmer in spring, rising greenhouse temperatures are controlled by opening ventilators and doors.  The air can get very dry, and this is a problem for plants which prefer it moist, such as cucumbers, foliage plants and many orchids.  Damping-down helps.  You sprinkle the ground (and staging if any) with water using a hose or watering-can.  The evaporation of the water humidifies and cools the air, reducing water stress on the plant leaves.  The humidity also prevents tomato pollen from drying out and encourages a better set of fruit.  In warm weather the water will quickly dry off and you may need to damp down more than once a day.  Capillary matting or old carpet holds more water than a concrete path and they keep the humidity up for longer.  In really hot weather you may need to spray the plant leaves as well.  Damping down discourages some pests like red spider and thrips which don’t like the humidity.  On the other hand, the same humidity can encourage the botrytis fungus that causes grey mould, so carefully remove all rubbish and dead leaves, and don’t damp down late in the day to let the greenhouse dry out before dark.