Pay Attention to Vapor-Pressure Deficit for Healthier Crops
by Michael Kovalycsik, National Sales & Marketing Director, Delta T Solutions
Growers know it’s important to pay attention to humidity in the greenhouse, but a more appropriate measurement — vapor-pressure deficit (VPD) — is often overlooked.
What Is VPD?
“VPD is the difference between the amount of moisture in the air and how much moisture the air could potentially hold when it is saturated,” explains Michigan State University Extension Educator Heidi Wollaeger.
Relative humidity, which is more commonly used when considering the greenhouse environment, is dependent on temperature. “For every increase of 20° F, the water holding capacity doubles,” says Wollaeger.
But, VPD can tell a grower a lot more about the greenhouse environment than relative humidity can.
“VPD is important to consider because it describes the driving force of water loss from the leaves, which is known as transpiration,” Wollaeger says.
Ideally, a greenhouse used for propagation should have a relatively low VPD. For example, growers should aim for a VPD of 0.3 kiloPascals (kPa) when rooting cuttings. This will help prevent young plants from drying out.
When finishing plants in the greenhouse, a VPD greater than 0.5 kPa is recommended to help plants cool themselves and improve uptake of water and nutrients. This kind of environment is also less likely to promote disease.
Adjusting Vapor-Pressure Deficit
Most environmental control systems use an algorithm to calculate vapor pressure from wet and dry bulb temperatures, performing the calculations so the grower doesn’t have to. Once a grower knows the VPD level in the greenhouse, adjustments can be made accordingly to create an ideal growing environment.
For example, if the greenhouse isn’t humid enough, and VPD is too high, water vapor can be increased in the air through misting, fogging or steam injection.
Low VPD, on the other hand, can be especially problematic in cold climates. During the winter, there is usually little greenhouse ventilation, so the air can become nearly saturated with water vapor (low VPD). The only way to increase the VPD is to bring in outside air, which usually has a low water vapor content. As that air is warmed, it can hold more water vapor and, as a result, the VPD increases.
“Growers are sometimes reluctant to ventilate when it’s cold outside, because additional heating is then required to raise the temperature of that cold air,” Wollaeger says. “However, ‘purging’ the air, or replacing warm, humid air with cold, dry air, can lead to fewer production problems.”
Contact Delta T Solutions for details on how one of our performance-engineered heating or irrigation systems will meet your needs. Call 800-552-5058 or email firstname.lastname@example.org.
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