Does Your Garden Have V.P.D.???

By Guy Ehydro

Put away the bro-science when it comes to humidity. Fear of mold and mildew continues to drive good growers to bad habits. Dropping the grow room humidity too low is stressing out plants, making them even more susceptible to infections. Consistently low humidity in the grow room will slow growth or even cause it to stop all together. Today, we introduce you to the real science behind Vapor Pressure Deficit (VPD) and what it means to your bottom line.

What is Vapor Pressure?

From a plant’s perspective, high vapor pressure is like an unseen force pushing on the plant leaf tissue from all directions.  The more water vapor (higher humidity) that exists floating in the air the greater the vapor pressure will be. A high concentration of water vapor in the air makes it harder for the plant to lose water into the air through transpiration. To put this all together, plants transpire less when the Relative Humidity (RH) is higher.

Conversely, in growing environments where the relative humidity is much lower, less pressure will be exerted on the leafs. The under abundance of water vapor in the air allows the plant to rapidly lose water, faster than perhaps it can sustain. This is what creates Vapor Pressure Deficit to exist as the plants begin to easily lose water into the dry air.

700860-01-small Commercial Grade Humidifier

Plant’s don’t have muscles and lungs to breathe with so they depend on changes in air pressure around these pores in the leaf cells called stomata to bring in fresh air. Hurricane Wall Mount Fans help plants breathe easier – but they can only do so much if the Vapor Pressure Deficit is too extreme. Sometimes a Commercial Grade Humidifier is the best option against a dry climate.

To cope with changes to the humidity in the growing environment plants adjust the size of their stomata. By controlling how open or closed these stomata are, the plant can regulate the rate that water is lost. I find that an indoor grow room temperature of 76 degrees Fahrenheit and 60% Relative Humidity encourages more vigorous growth and I’ve actually had FEWER issues with pests and diseases with my healthier plants.

We understand what Vapor Pressure Deficit does, it makes my plants lose water way too fast. But what is VPD really? What we are really looking at is the difference (deficit) between the amount of moisture currently in the air versus how much moisture the air could hold if it were totally saturated. Once the air becomes saturated water will condense out to form dew or films of water over leaves. It is this last instance that makes VPD such an important factor for greenhouse environment regulation.

QuestDual_150_Product-e1431649188927Most growers are well aware that with high humidity comes an increased risk of fungal infections, that’s why you’ll find a Quest Dehumidifier in almost every professional garden and greenhouse. Droplets of water will form on leaves when the water vapor condenses out of the air when temperature drops, allowing for diseases like botrytis and powdery mildew to take hold. If humidity remains too high for the temperature fungal diseases are enivitable.

If a film of water forms on a plant leaf it becomes far more susceptible to rot. On the other hand, as the VPD increases the plant needs to draw more water from its roots. In the case of cuttings, the plant may dry out and die. Where is Goldilocks when we need her?

Most cultivated plants prefer between 8 and 10 millibars at specific temperature and humidity levels, though many will tolerate between 5 and 13 millibars on the old barometer. Use the reference chart below to find a healthy humidity level for your grow room temperature if you don’t have a weather station in your grow space.


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