Sweating High Humidity
Humidity levels in the greenhouse can be as important to growers as temperature. The problems associated with high humidity are well known: water dripping off the roof and walls and a high incidence of Botrytis, powdery mildew and other fungal problems. Unfortunately, how to control humidity levels is less well known. Solutions for high humidity range from simple, cultural practices to integrated heating and ventilation strategies. But a key to managing humidity is understanding more about it.
Relative Humidity Basics
Humidity is the amount of water vapor in the air that accumulates as a result of evaporation and plant transpiration. Sling cyclometers, electronic humidistats or the comparison of wet bulb temperature to dry bulb temperature in a constant air volume can measure humidity. Humidity levels are expressed as percent relative humidity (percent RH), which means the percentage of the total water vapor the air can hold at a particular temperature. For instance, at 60¡ F 1 lb. of air will hold 77 grains of water vapor and will be at 100 percent RH. If that same air is ç warmed up to 80¡ F, it will hold 156 grains of water vapor, and the 77 grains present would be roughly 50 percent of its capacity. Warmer air will have a lower percent RH reading than cooler air containing the same amount of water vapor.
When air reaches 100-percent RH, the water vapor will condense on all surfaces and form droplets in the air (rain). Water also condenses on surfaces cooler than the surrounding air at RH levels below 100 percent. Condensation forming on leaf surfaces fosters disease and creates a "rainforest" effect in early morning or late afternoon. When the relative humidity rises, the air becomes closer to saturation, and the surface temperature that will cause condensation becomes warmer. This surface temperature is known as the dewpoint temperature.
The relationship among air temperature, relative humidity and dewpoint temperature is important. At an indoor air temperature of 68¡ F and a RH of 75 percent, the dewpoint temperature is 60¡ F. This means condensation will form on the structure or glazing when the surface temperature drops to 60¡ F. The RH drops to 65 percent by raising the air temperature to 72¡ F without removing any water vapor. Similarly, when it is raining outside the RH is 100 percent. But if it is 45¡ F outside, and that cool, saturated air is brought into the greenhouse and warmed up to 68¡ F, the RH of that air will be 45 percent.
Prevention is the easiest and cheapest way to combat high relative humidity. If water vapor doesn't accumulate in the air, there will be no need to remove it. Careful irrigation practices can have an impact on how much moisture is in the air because freestanding water is a main source of water vapor. Puddles under benches become water vapor, but they are not the only problem with indiscriminate irrigation; spraying water on pots, benches and leaves unnecessarily increases humidity (and potentially spreads disease).
Irrigation systems also impact the evaporation process. While sub-irrigation has many benefits, it has the downside of delivering water via a large flat surface. This surface remains wet, and the water eventually moves into the air. Overhead sprinklers are fairly cost-effective systems but, like sloppy hand watering, leave large volumes of unused water in the greenhouse. Dripper systems may be a more costly and less convenient choice of irrigation, but the only remaining unused water is the leachate, and this can be minimized if the applied volume is controlled properly.
Timing is also important. Because water vapor accumulates ç relentlessly if there is no ventilation, it is not wise to water prior to closing the greenhouse for the night. Leave plenty of time to vent out the warm, wet air before evening. Also, reconsider large-scale irrigations on rainy or overcast days. Try to water the day prior to expected bad weather, or hold off until the next day if clearing is anticipated. If there is not enough sun to support ventilation, the water stress on the crop will be considerably less than normal, and damage from drought is much less likely.
Unfortunately, once water vapor is in the air, there are fewer choices for preventing problems. One option is to direct the resulting structural condensation away from the plant material to prevent dripping on the crop. Some glazing materials are chemically coated to prevent condensation from forming droplets; the water runs down the glazing to drip channels or aisles before falling from the roof. Spacers between the glazing and the purlins allow condensation to move down to the gutters unimpeded. The angle and inflation pressure of double poly can also affect where droplets form. But those locations that drip should be consistent, so aligning aisles or workspaces under drip areas will lessen the amount of crop that gets wet.
Lowering the water content of the air, commonly referred to as dehumidification, can only be accomplished by removing water vapor. This is done by venting or exhausting wet air out of the greenhouse and replacing it with drier, cooler outside air. Humidity problems generally occur more when the house is closed up for extended periods with no release of wet air, such as during inclement weather or late afternoon until morning. Even minimal ventilation during these times will help lower the humidity; it can be as simple as manually cracking a vent or door to allow some air migration out of the zone. Unfortunately, this increases heating costs. Environmental control systems offer dehumidification strategies that can minimize costs, but ultimately, dehumidification will always create additional costs.
Environmental control systems offer many automated alternatives for lowering relative humidity; they vary in detail and effectiveness with the complexity of the environmental control system. Most environmental control systems supply at least a simple RH set point and settings to allow a cooling stage to be invoked to remove the humid air. More sophisticated, computer-based systems offer integrated strategies to maximize the process effectiveness.
One common tool is venting only a portion of each hour: For instance, venting five minutes out of every 15. Another strategy is limiting the dehumidification period to just before sunrise and sunset when the zone air, structure and dewpoint temperatures are changing rapidly, ç causing condensation. Venting at this time will expel the nearly saturated air before the vapor can condense, and it will prevent vapor from becoming free water that will cause disease problems and accumulate in the air later as it evaporates.
By applying some simple humidity concepts, growers can increase the effectiveness of additional heating. For example, because warmer air holds more water, it is useful to raise air temperature slightly before venting. This will have two beneficial effects: The RH will drop slightly, delaying the onset of venting, and when the RH rises back to the set point and venting occurs, the expelled air will contain more water than the previously cooler air.
For growers with an environmental control system, a dehumidification regime may utilize many integrated actions to combat the problem. During the day and much of the night, a grower may accept an unchecked humidity level for cost-saving reasons. But about 30-60 minutes before sunrise and sunset, a high humidity set point of 75-percent RH may be selected.
The dehumidification program is initiated when the zone enters the desired time period, and the humidity level climbs to 75 percent. The environmental control raises the zone temperature by a couple degrees, lowering the RH by about 5 percent. Slowly, with continued transpiration, the RH climbs back to 75 percent. The environmental control now vents for five minutes, removing the warm, wet air. This brings cooler air in, which mixes with the warm, wet air and results in a slightly lower temperature and decreased RH level. After five minutes of venting, the environmental control compensates for the lower air temperature by heating slightly, thereby lowering the resulting RH level further. Once the desired air temperature is restored, the environmental control system returns to its natural control state and waits for the RH level to rise again to 75 percent, at which time the cycle is repeated. This regime continues until 30-60 minutes after sunrise or sunset, and the resulting air mass contains substantially less water vapor for an extended period, lowering the amount of condensation and disease problems.
The Bottom Line
The adage "an ounce of prevention is worth a pound of cure" applies to humidity control. The good news for most growers is that preventative measures are generally easier and less expensive than the actions required to alleviate high humidity levels. However, even superb cultural practices cannot fully prevent humidity levels from rising when the greenhouse is closed up, as transpiration is a basic plant process. How growers handle inevitable high humidity becomes a decision based on economics and availability of the tools necessary for dehumidification. Hopefully, with a little knowledge of the problem and its causes, that decision will be more informed and fit with production practices.