Relative
Humidity
Energy conservation features, including the use of double polyethylene film, have increased greenhouse relative humidities [24]. Although computer control systems are designed to monitor and can control humidity, it is not always clear what levels are desirable in terms of plant growth, fruitset, and disease prevention. Also, humidity control is indirect and usually involves tradeoffs with air and leaf temperatures, CO2, and ventilation.
The issue of which vapor pressure deficits (vpds) should be maintained in the greenhouse is complicated. At low vapor pressure deficits (sometimes also called moisture deficits), plants are light-colored with large, soft leaves, weak growing points and low transpiration rates. In a recent study in the Netherlands [25], high humidity (low vpd): reduced leaf area because of lack of calcium; increased stomatal conductance; reduced final yield and reduced mean fruit weight. This study was conducted over a fairly limited range of vpds, however: 0.35-1.0 by day and 0.21-0.71 kPa by night. It is generally accepted that vpds of 0.2 kPa or less also encourage diseases. However, low vpds increase root pressure, which decreases the incidence of blossom-end rot, a calcium-related fruit disorder.
It
is unclear to what extent high vpd is deleterious to the plant, assuming
that adequate water is available, but vpd’s over 1.0 are considered to
be too high. In Northern Europe and the cooler areas of North America,
vpds over 1 kPa are rarely seen but in most US greenhouses and some summer
production situations in northern latitudes with continental climates,
vpds will sometimes exceed this range. A greenhouse temperature of 26°C
and relative humidity of 60% would result in a vpd of 1.35, for example. High moisture deficit results in small, thick,
‘hard’ leaves. If plants transpire more water than can be supplied through
the roots, blossom-end rot may develop in the fruit and stomates may close,
resulting in poor growth.
Diseases also spread more rapidly under conditions of high relative humidity, especially if leaf surfaces are also wet from dripping on condensate. Options for decreasing relative humidity include venting the humid air and replacing it with cooler, drier outside air. As cool, dry air heats up in the greenhouse, it absorbs moisture and lowers the humidity. In glass houses with vents, the heat should be turned on and the vents opened. In houses with fans, the fans should be turned on for a few minutes and then the heater turned on to bring the air temperature up. Humidity reduction by bringing in outside can be effective even if the outside air is very humid, as long as it is cooler than the inside air.
Preventing condensate from dripping on the plants is also important. Sunny days increase leaf transpiration and media evaporation and the warm air holds the moisture as vapor. At night, as air cools to the dewpoint, condensation occurs and water droplets are formed on cooler surfaces such as the leaves and the inside skin of the greenhouse. This moisture promotes the germination of fungal pathogen spores such as Botrytis or powdery mildew. Some greenhouse plastics are designed to prevent condensation from forming by incorporating a wetting agent. Moisture condensing on the plastic remains as a film, draining off rather than dripping onto the plants. These wetting agents last 1-2 years, which is longer than wetting agents applied directly to the inside surface of the plastic. Glass greenhouses usually do not require the use of wetting agents as moisture runs off the steeply pitched roof rather than accumulating.
Increasing air movement to 1 m/s (leaves move slightly) in the greenhouse also reduces dripping because it reduces the differences between the leaf surface and the air. This reduces condensation on the leaves because leaf surfaces do not cool below the dewpoint. Air movement can be increased by either running the fans on hot air furnaces or by fan-jet systems. In fan-jet systems, the fan is connected to a perforated plastic tube (polytube) set to run continuously, either drawing in outside air through a louver or recirculating air within the greenhouse. The air in the polytube is forced out through the small holes and mixes with the air in the structure. Correct sizing and spacing of the holes is important and must be carefully calculated from formulas provided by the manufacturer. Horizontal air flow fans are also used to increase air circulation and mixing. Small fans (1/15 horsepower, 0.4-0.5 m) placed along the sides of the house push air in one direction on one side of the greenhouse and in the opposite direction on the other side. These fans operate continuously except when the exhaust fans are turned on.