Greenhouse humidification and humidifiers
The relative humidity ( hereafter RH ) is defined as the amount of water vapour in the air compared with that in saturated air of the same temperature. Thus at 100% RH the air can no longer hold any more water vapour and the excess will condense out in the form of mist, fog or rain. No plant can live without water - indeed most plants are composed mostly out of water, and the health of a plant will depend directly on the RH of its surroundings.
Leaves lose water through evaporation through their stomata - small openings in special cells on the leaf surface where gaseous exchange takes place. If the RH is too low, a plant will start to give up some of its water into the surrounding air - the result is that the plant will start to dry out and wilt - exceptions to this are thick skinned cacti which have adapted over the years so as to thrive in dessert conditions.
It is clearly very important, therefore, to ensure that the RH in your greenhouse is kept at a level suitable to the plants that you are propagating and growing. It is equally important not to allow too high an RH as this can lead to fungi or bacteria growing on your plants.
RH can be monitored relatively easily - small electronic thermometers often include a humidistat which measures the humidity as well as the temperature. The RH can be increased by means of a humidifier, of which there are any number on the market.
The simplest humidifier is a hand held spray bottle used to manually spray water onto the leaves - somewhat impractical for the larger greenhouse. The best solution is a humidifier that is controlled automatically by the humidistat - whenever the RH drops below a predetermined level it sends a signal to switch on the humidifier, which cuts out when the desired RH is reached. By far the most common type of humidifier used in a greenhouse is the mister or fogger, and this is dealt with in a separate article.
Care should be taken regarding the time when humidification is allowed to take place. It is important not to let this happen during the hottest part of the day - i.e. between say 1100 and 1600 - water lying on the leaves between these times can be the cause serious burns to the plant by the strong sun, thus doing more harm than good. The best times for misters to be turned on are either early morning or late afternoon.