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When we first begin to learn about plants, growing and life cycles, little emphasis is usually placed on environmental parameters, such as humidity. Too often, it’s all about nutrients, growing media and supplements—but we should not overlook the importance of the environment. Environmental metrics are on par with soil, sunlight and inputs when it comes to a plant’s health, and are a vital component of becoming a masterful grower in any indoor garden.

Indoor growers are constantly learning and finding new ways to promote plant health and avoid disease, and for a long time, relative humidity readings have been a major player in the field. But as our science and understanding have progressed, vapor pressure deficits (VPD) have become the more accurate and reliable measurement. There is still no set of “rules” to follow or exact figures to adhere to, so as always, growers should take any guidelines with a grain of “soil” and use their own judgment.

From Relative Humidity to Vapor Pressure Deficit

VPD begins with relative humidity (RH), or the percentage of water vapor in the air relative to how much vapor the air could hold at that particular temperature (until it reaches saturation). Warmer air and higher temperatures increase the air’s potential to hold vapor, while cooler air and lower temperatures decrease the air’s potential for holding vapor. This all means RH can and will change often.

Healthy plants transpire roughly 90 percent of the water they uptake, using just 10 percent to store in their tissue and to photosynthesize. The amount a plant transpires has a lot to do with humidity, so in order to maintain a healthy level of transpiration, a grow room’s RH should neither be too high nor too low. Evapotranspiration is an essential process through which plants cool themselves from the heat of the lights and uptake nutrients.

Should your RH be too high, the large amount of vapor in the air creates pressure within the room, “pushes” against the plants and makes it difficult for them to add more vapor to the air through transpiration. Conversely, a low RH means low pressure and drier air, which encourages the plant to transpire too much, and can stress the system as it tries to uptake more water through its roots, can increase water loss and may lead, in some extreme cases, to nutrient toxicity issues.

RH is a great start, and many growers manage to get by with careful manipulation of a grow room’s temperature and humidity. That said, advanced growers are turning to VPD readings for a more precise understanding of exactly how a plant behaves at any given moment in its environment.

In addition to RH, VPD takes into account the room temperature and leaf humidity, therefore including the plant into the calculation in a way that RH does not. Keeping in mind that healthy leaves operate at 100-percent humidity, growers can use VPD to calculate the deficit between the saturated vapor pressure of the plant and the air vapor pressure in the room.

Essentially, VPD—measured in kilopascals (kPa)—can tell a grower how humid or dry the environment is in the grow room from the plants’ perspective.

How VPD Affects Plant Health

To understand how VPD affects plant health, the basic rule of thumb is that a high VPD creates drying conditions for the leaves, whereas a low VPD encourages plants to transpire less.

There is also somewhat of a “Goldilocks zone” for VPD. When the VPD is 0.4 kPa or lower, plants may not transpire enough and retain water. (They may also take in less water.) This means they may lack the nutrients they would usually find in water taken up from the growing medium, and it can also encourage microorganism development on the plant’s surfaces. In situations where the VPD is 1.6 kPa or higher, plants may transpire too much, which can over-stress them and force them to uptake too much water and nutrients, leading to toxicity problems. In those conditions, plants can also wilt faster if watering is inadequate, as they can lose water faster than they can absorb it.

Of course, the plant’s stomata (the microscopic openings or pores in the epidermis of leaves and young stems, as defined by Encylopaedia Brittanica) can help mitigate the situation when the VPD is excessively high or low. In a high-VPD situation, stomata will close to minimize water loss. But closed stomata adversely affect photosynthetic rate, as the leaf requires open stomata to absorb CO2.

Some growers prefer a high-VPD/low-RH environment to avoid the risk of fungal disease that comes with minimal transpiration. However, the real problem is not so much the RH, but rather the spikes in RH during the day/night transition and the dew point (defined below).

The moment when the temperature dips, the air contracts and RH increases. As molecules get closer together, less space exists between them to store water vapor, thus increasing RH. This is how the humidity is ‘relative’ to the temperature: When the temperature drops to a point where there is more vapor in the air than it can hold, the humidity is forced out of the air. This is the dew point. Water droplets will form on any surface that is cooler than the air, and in the case of a grow room, this can mean the plants. As soon as this point is reached, the risk of mold spores germinating on the leaf increases.

Naturally, the temperature of a grow room will begin to drop as soon as the lights go out. An option to avoid peaks in RH or to avoid reaching the dew point is to progressively lower the RH just before the lights go off. This will give the atmosphere time to adjust to the temperature loss and help maintain the RH on a more even level.

When the VPD is in a healthy range of roughly 0.85 kPa to 1.15 kPa, the plants can thrive thanks to a balanced level of water uptake and transpiration driving nutrient uptake in the plants’ favor.

VPD is Not ‘One Size Fits All’

As higher and lower VPD can encourage certain behaviors in plants, lead growers may use these values and manipulate the environment based on the plant’s current growth phase.

For example, during the early growth stages, a plant doesn’t yet have the developed root system it needs for high water/nutrient uptake. This is the case during cloning or transplant shock. Growers therefore may aim for a low VPD of 0.8 to prevent excessive water loss from the leaves to alleviate plant stress.

As the plants move into the late vegetation phase and into early flowering, increasing the VPD to roughly 0.8 kPa to 1.2 kPa will help maintain a healthy rate of uptake and transpiration.

When plants move into the mid to late flowering stage, growers can look to increase the VPD further. By encouraging leaf evapotranspiration with a low RH, plants can stay cooler, avoid mold, and uptake more water and nutrients that can help create more yield and better flavor, just in time for harvest.

David Bernard-Perron is The Green Organic Dutchman’s vice president of growing operations and an agrologist with a Master of Science in Plant Sciences from McGill University. He began working in greenhouse production in 2001, and worked for McGill University’s greenhouses and Horticultural Research Center. Bernard-Perron worked as the lead agrologist at Whistler Medical Marijuana Corporation and designed its certified organic growing system.