Greenhouse Temperature and Humidity

The relationship between temperature and humidity in a greenhouse is important.

Most people know that the purpose of a greenhouse is to help plants grow. Those thinking about building or acquiring a greenhouse, though, might not know that just putting the plants in a greenhouse isn't enough. Optimal conditions need to be maintained in a greenhouse to maximize the benefits. Among the elements that must be controlled is the balance between temperature and humidity.

Relative Humidity

Moisture content in the air is measured as "relative humidity" or "RH," a ratio of the weight of water in the air to the capacity of the air to hold water. Since warmer air can hold more water than colder air, humidity is temperature dependent. In other words, a given amount of water in the air will produce a higher relative humidity at a lower temperature than it will at a higher temperature.

The Significance of Changing Temperature

Understanding the difference between warm and cold air in terms of humidity reveals part of the importance of managing humidity levels. During the day, the greenhouse heats up, allowing the air to hold more moisture. At night, the temperature in the greenhouse drops. When that happens, the air loses its ability to hold the same amount of water. Excess water in the air will condense on surfaces and plants.

Condensation Concerns

You might think that water condensing on plants is good. The truth is, it can be problematic. For one thing, water on the leaves may promote the growth of fungi and mildew. Greenhouse surfaces can also build up layers of spores and other plant pathogens. As water condenses on those surfaces and drips, the pathogens drip with it. Splashing can spread disease from plant to plant.

Good Humidity Levels

Since temperature is part of the relative humidity equation, it is important to know the temperature range experienced by a particular greenhouse. A change of 20 degrees can double the water capacity of air. Dew point charts list the condensation at specific temperatures. As an example, if the relative humidity is 95 percent at 60 degrees Fahrenheit, then condensation will occur at 55 F.

Controlling Humidity

There are several ways to control humidity in a greenhouse. One approach is to control moisture at the source--watering. The key points to remember in that approach are that you should water only as much as plants need, and avoid wetting surfaces like floors in the process. Proper drainage helps, too. Bottom heating can also help stop condensation by keeping the plants warm at night. Proper ventilation can help equalize the humidity levels inside the greenhouse.

References

• UMass Amherst: Greenhouse Crops and Floriculture: Greenhouse Management/Engineering