HortResearch Publication - Humidity in Greenhouses
Low air humidity increases crop transpiration, but too low air humidity in combination with high radiation amplifies the transpiration more than the plants can handle. If the water loss by the leaves exceeds the water uptake by the roots, the plants start wilting. Such water stress (or actually drought stress) means that most processes, including growth, come to a temporary stand-still. Water stress in combination with burning sun shine may cause permanent damage in the form of leaf burn. If the dry conditions last for a longer period of time the crop will adjust by forming smaller, thicker, sometimes brittle leaves, which gives the crop a very unhealthy appearance. Its performance is probably still high thanks too the high radiation, but lower than it would be when the leaves were vigorous. Hot dry conditions may cause special problems in some crops, e.g. blossom-end rot in tomato and capsicum. Low air humidity also favours some pests, e.g. spider mites lay more eggs and spider mite infection develops faster under low humidity Some predator mites are not effective under low humidity For all these reasons we would like to mitigate the conditions in summer.
The first method to increase air humidity in the greenhouse is by supporting crop transpiration, as the transpiration brings masses of water vapour into the air. Two things are important: sufficient water supply and optimal water uptake. Ideally, the water supply capacity in a soil-less system must be so high that even on the hottest days still some drain water comes out of the system. The water uptake can peak to as high as 8 liter per m2 per day, which amount must be available in the substrate or supplied when needed.
Secondly, the plants should be trained to be able to maintain a high rate of transpiration when needed. Therefore, on occasional dull days the transpiration must be increased by extra ventilation. On hot days some extra ventilation (and pipe heating) early in the morning can push the transpiration, in order to have the plant active when the radiation rises fast. This may conflict with CO2 enrichment. However, on hot dry days, the transpiration is of critical importance and should get the highest priority.
White-wash is often used for maintaining a better climate in the greenhouse. By keeping excessive radiation out, it avoids boiling hot temperatures and thus prevents the VPD to become too high [high VPD means low air humidity, and VPD depends on temperature]. There is a counterbalancing effect: white-wash reduces the incoming radiation which reduces the rate of transpiration. Generally this effect does not completely compensate the temperature effect, so the VPD in a white-washed greenhouse is generally not as high as under clear glass. However, white-wash 'permanently' reduces light also in hours when light is not in excess, which reduces growth and production. White-wash should therefore be used sparsely and mainly on hot spots, like north wall, main path, work places).
Sprinklers outside on the roof are meant for cooling, and not directly for increasing the humidity. The principle is that water sprinkled on the glass roof evaporates, which uses lots of energy (heat) from the surrounding. This reduces the temperature of glass and air. In a glasshouse with roof windows and roof sprinklers, the incoming air is cool and humid. The extent of the effect of roof sprinklers depends very much on the air humidity of the outside air. On days with low air humidity, the cooling effect can be many degrees, e.g. 6 oC or more.
Fogging is spraying very small droplets of water (5-10 micron), which float in the air and evaporate into water vapour. This increases the relative air humidity (and reduces the VPD) and also reduces the air temperature. The crop may stay dry, unless the humidity is already high, or the fogging is on too long, or the nozzles are too close to the crop. 'Misting' has much bigger droplets than fogging, and misting always makes the crop wet, which reduces photosynthesis and stimulates disease development. It is obvious that fogging is preferred and also that fogging is more effective when the humidity is lower.
A fogging installation typically consist of a pressure pump, (copper) pipelines for high pressure water (e.g. 1000 PSI), and special nozzles. There are also systems that use high pressure air and low pressure water. One would install one nozzle on each 6-10 m2 (or more dense in small greenhouses). The water usage is around 0.1 to 0.2 litre per minute per nozzle, but the nozzle should only be on in intervals (e.g. 30 seconds on, several minutes off). The capacity of the fogging installation should be around ... litre per hour for a 1000 m2 greenhouse.
A general problem of fogging is blocking of the nozzles. It is very important to have excellent water quality and to filter the water. The pH of the water must be neutral. The nozzles need some maintenance (cleaning) now and then.
The major drawback of fogging is its costs. This makes fogging merely economic for regions with regular extremely dry periods, where other methods (see above) are insufficient. Fogging helps to avoid blossom-end rot, but also for this problem are other (cheaper) remedies. Some growers use the fog installation for spraying chemicals, thus saving on labour costs, which makes the fogging installation more economic. Of course only chemicals that dissolve extremely well in water, or liquid chemicals, can be sprayed through the fogger.
Fogging seems to be the ideal method to avoid low humidity, but it has some other disadvantages too, apart from the price. A main concern is that fogging reduces the transpiration, so that the effect of fogging is counteracted by the crop itself. Therefore, fogging can make the crop 'lazy' - if a crop is grown under almost continuous fogging the root system is not capable of high water uptake anymore. So fogging must be used wisely and selectively, actually it should be considered as a back-stop. If other measures (like stimulating transpiration) are not sufficient, fogging can be used during a few hot hours per day. In crops with a low leaf area, the transpiration is often insufficient to maintain a reasonable humidity level and fogging may help. Crops struggling with Pythium or other root problems can be fostered by fogging during the hot hours of the day. Ideally the fogging is controlled via the computer, and activated on the basis of time and measured conditions (e.g. air humidity, radiation, temperature, preferable also air humidity outside).