Thu, Jun 4, 2020


Incorporating the Vapor Pressure Deficit growing chart as a guide for managing your relative humidity.


What priority does managing humidity in your growing environment play? Is it a factor you monitor in order to control and limit fungal growth and disease prevention exclusively? Does high humidity make you nervous? Do all these questions sound like a hack opening?


As a gardener who chooses water as their medium, I’ve always adhered to different practices to achieve results rather than what I would read about on forums. One of those practices included running my rooms at humidity levels 15 to 25 percent higher than what I understood the “agreed to” set points are. Like most things in my growing career, I stumbled across the benefits of a high-humidity environment quite by accident. Leave it up to an equipment “failure” to create a radical shift from my intention and toward that of the plants. I couldn’t help but notice a healthier, perkier, improved color display from the plants subsequent to running them unintentionally in higher humidity. And so I shifted my ranges for the next run and saw still more improvements running higher humidity. It was as if I had stumbled across some rare vortex where the plants I was cultivating thrived under a different set of conditions than what I believed “the rest” of the scene was using and chalked it up to the unique features of a deep water culture system.

Looking back on my experiences now with an expanded knowledge base provides a much more clear understanding of how my environment was being managed, or mismanaged as it were. Constantly battling the high heat loads of my high intensity discharge lights with as much air circulation and air conditioning as I could squeeze into a hobby grower’s budget, I was overlooking a critical factor: maintaining the humidity. Humidity plays a critical role in plant growth, vigor and health.

Humidity is the measurement of water vapor in the air at any given temperature. Water vapor is water in its gaseous form being held between the air molecules. Temperature plays a critical role in a humidity measurement. Higher temperature causes gas to expand while lower temps cause it to contract. I think of warmer air as having more space for water vapor to fit inside and as a result, warmer air often holds higher humidity than cooler air. This is why when lights turn off in an indoor environment, humidity will spike to dangerous levels if not regulated. The warm water vapor-rich air suddenly rapidly contracts as the temperature decreases from the lights going off. As a result, the air pushes out water droplets (water in liquid form) as the space to hold water vapor molecules in the air decreases — a circumstance most gardeners are familiar with.


In order to create a uniform and relatable measurement, “relative humidity” is used to describe the percentage of humidity in the air compared with the total amount of water vapor that could potentially be held at any given temperature. So, for example, when we say that the relative humidity is 60 percent, we are saying that at the current measured temperature, the air is holding 60 percent of the total amount of water vapor possible. So while less total water vapor may exist in lower temperature air, it may be supporting a higher relative humidity measurement than the same amount of water vapor being held at a higher temperature air simply because the higher temperature air can hold more total water vapor as it’s comparatively expanded.


Plants rely on pressure differentiation to pull up nutrients through the roots, transport minerals, maintain turgor pressure and a variety of other functions. Plants also have stomata: tiny pores, predominantly on the leaf surface that the plant can open or close, allowing for transpiration (the release of internal water vapor through the stomata into the surrounding environment). The stomata is also the mechanism through which the plant intakes Carbon Dioxide.

The relative humidity in your room is additionally a measure of the amount of pressure being exerted on your plants. If you think of your plants as maintaining 100 percent relative humidity inside the leaf surface, the plant will monitor and adjust the amount of water vapor it releases through transpiration based largely on the relative humidity outside that leaf surface. If the relative humidity or RH, outside the leaf surface is very high, there is literally more pressure pushing back onto the stomata, which limits the amount of transpiration the plant is required to perform and thus allows the stomata to open fully and stay open. Conversely, if the RH outside the leaf surface is very low, the air will “suck” more water vapor from the plant through the stomata and the plant will lose more water to transpiration. Plants will close or shrink their stomatal opening in response to higher transpiration rates as a defensive mechanism to conserve water.

The differential pressure created between the RH inside the leaf surface (fully saturated at 100 percent) and the actual RH outside the leaf surface (the room’s RH) is known as the Vapor Pressure Deficit (VPD). Since gases will move from areas of higher pressure to areas of lower pressure, the greater the VPD, the faster the plants will lose water due to the higher “evaporation power” of the air (think of high pressure air pushing on the walls of an inflated balloon, suddenly escaping to the lower pressure air of the room it occupies). VPD is measured in millibars or kilopascals and is a unit of pressure constantly acting upon your plants.

Maintaining a VPD between 7.5 to 10 is considered by many the ideal range for maximizing growth potential while minimizing risk factors of exceedingly low VPDs (high humidity). From the chart, one can see the higher the room’s temperature, the faster water vapor is removed from the air and consequently, a higher RH is required to maintain the ideal VPD range. Many indoor farmers’ skin will crawl at the idea of running rooms higher than 70 percent RH and with good reason as mold and disease is considerably harder to prevent under these conditions. That is one of the many reasons it is imperative to properly size your cooling needs to exceed the heat delta your gardens create so that an ideal VPD range can be supported without bringing the RH to such dangerous levels. It also shows that if your rooms run warmer and dry from all the cooling equipment, you can create an immediate improvement to your plant’s health by adding in a humidifier or other device capable of raising the RH in the room while lights are running.

The healthier, more vigorous plants that are produced through managing to this table have higher Systemic Acquired Resistance (SAR) response, providing a more naturally resistant plant to mold a fungal infestation. Furthermore when combining a sustained VPD during both periods of the plants light cycle any microbial defenses you are foliar feeding your plants with becomes increasingly effective. Freshly brewed microbial tea makes an outstanding inoculant, preventing all types of mold, mildew and disease infestation. The beneficial microbes easily outcompete the pathogenic spores when applied properly and allowed to survive. These microbes need to be applied in an environment supporting RH humidity levels ideally between 65 and 75 percent, and allowed to exist on the leaf surface. Most foliar sprays are most effective indoors when lights are off however, it’s during this dark period when most dehumidification equipment is running to prevent and control the humidity spike mentioned earlier. It’s not uncommon as a result, for microbial tea to be applied in an environment that is actively scrubbing water vapor from the air targeting RH humidity levels under 50 percent. In these circumstances, the tea is applied only to quickly be evaporated off the leaves, all but completely mitigating its efficacy. When following the VPD chart and minimizing the variance between daytime and evening temperatures, microbial tea is applied in an environment considerably closer to — if not inside — the ideal range for the microbes in the tea to live for extended periods of time on the leaf surface and work their magic.

As the Cannabis plant reaches more and more cultivators hands imparting the knowledge and lessons she has to share, we as farmers will continue to evolve our techniques and understanding of creating an ideal environment to maximize genetic potential and minimize risk. Incorporating the Vapor Pressure Deficit growing chart as a guide for managing your relative humidity can be just one of the tools in the arsenal of today’s indoor gardener.

For more information on how to achieve or maintain humidity in a specific environment or for general questions on Vapor Pressure Deficit, please don’t hesitate to contact me at and as always, happy gardening!!!