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19 May 2021

How Vapor Pressure Deficit (VPD) Impacts Cannabis Plant Growth

Scientific research into cannabis cultivation is finally beginning to receive the recognition it deserves after years of being suppressed due to legalization restrictions. As science has delved deeper into genetics, nutrition, lighting, and environmental controls, studies and data now provide answers for many unknowns surrounding the shrouded plant. 

Old-school growers from the past had to learn growing techniques from trial and error experiments. There were not many published articles on growing cannabis, much less information on environmental influences. Fortunately, times have changed as the science behind ideal conditions for optimal cannabis plant growth now opens the door for advanced cultivation practices.

One of the recent discoveries determined to have an influence on cannabis plant development is VPD. Researchers found a direct correlation between proper temperature and humidity levels when looking at achieving maximum yield and potency. Their findings reveal a better understanding of how Vapor Pressure Deficit impacts cannabis plant growth.

To better understand VPD, we must first learn how a cannabis plant distributes water throughout its system.

What is Transpiration?

Water provided to a cannabis plant’s root system displays white tendrils readily absorbing the hydration. Once fulfilled, water begins moving up through the plant. Through a process called transpiration, water and nutrients are transported up from the root system to supply foliage and flowers with their essential needs. When the plant is satisfied, it releases the excess in the form of water vapour from leaf surfaces.

Interestingly, most of the water applied to the root system isn’t used for growth, like as much as 99%. Instead, its function is to help move cellular activity within the plant through transpiration. As water transports through the plant, it is converted to water vapour and released back into the air.

Water vapour disperses from tiny pores on the underside of leaves called stomata. These microscopic openings are also responsible for the transport of CO2 into the plant through photosynthesis. Stomata open and close naturally in response to light but are also triggered to shut down in stressful situations.

Transpiration is affected by air temperature, and relative humidity in a grow room. If temperatures get too high, stomata close to hold moisture in the plant. If the relative humidity is too high, the marijuana plant cannot transpire due to excess water vapour in the air – the rate of evaporation reduces.

Think of transpiration as the evaporative mechanism in water transfer through plants. Vapour pressure deficit is a measurement of the drying power of the air within a growing chamber.

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How VPD Affects Transpiration

Vapour pressure deficit determines the difference between the vapour pressure emitted from the cannabis plant’s leaf surfaces and the atmospheric vapour pressure in the surrounding air. The determining factors to VPD are also temperature and relative humidity. Indoor marijuana growers can harness these environmental controls to create the ideal atmosphere to drive transpiration.

Water vapour escaping the cannabis plant’s stomata collides with water vapour in the air upon release. As nature provides its own balancing act, the atmosphere regulates the amount of moisture it can hold. The environmental conditions present at any given point determine the plant’s response to the situation.

For instance, if the indoor grow room is muggy and the air is thick with moisture, there’s not much room for water vapour released from the plant to intermingle with the saturated air. High humidity in the surroundings will slow transpiration to a crawl. On the other hand, extremely hot and dry conditions will prompt stomata to close in response to the plant stress.

The trick is to find that sweet spot, where the environment welcomes and encourages the cyclic motions of transpiration as water enters and exits the plant. Optimal humidity and temperature levels in the growing chamber energize the cannabis plant to move the nutrient and water uptake through the system.

Steady transpiration leads to increased nutrient uptake and carbon dioxide intake from photosynthesis resulting in healthy, vigorous growth.

Vapour Pressure Deficit vs Surplus

Vapour pressure differentials measure the difference between water vapour readings in the air and those found on leaf surfaces from the marijuana plant’s canopy. Depending on the temperature and relative humidity levels, the differentials may show a surplus or a deficit of available vapour pressure.

For example, airspace reaching the maximum amount of vapour pressure holding capacity at a particular temperature is called the Saturation Vapor Pressure (SVP). When the temperature drops, the SVP decreases since the air cannot take in any more water.

After that point, the surplus water vapour begins condensing back into a liquid form, visible as dew when morning temperatures are lower. The rate of plant transpiration slows significantly as a surplus of water vapour inhibits the air.

As temperatures rise, on the other hand, the amount of water the air can also hold increases. This deficit gives a cannabis plant the green light for transpiration as abundant airspace is available to release its water vapour. Again, if the temperature is too hot and humidity levels drop, stomata shut down as a defence mechanism to reserve water within the plant.

Maintaining vapour pressure deficit with environmental controls is the driving factor for optimum plant growth.

Tools for Measuring VPD

Before delving into the calculation, let’s first look at some measurements needed from the grow room. As stated above, temperature and humidity are two variables determining VPD. A hygro-thermometer, which should be in every grow room, quickly displays current readings of these two measurements.

An infrared temperature gun reads a cannabis plant’s leaf temperature. These handy gadgets display specific temps wherever the gun is aimed. Infrared guns are inexpensive and fun to shoot around the room, revealing more about temperature fluctuations within a growing environment.

Readings on a particular cannabis cultivar will vary depending on where you point the gun at the canopy. Shaded, leafy areas towards the bottom of the plant will be a few degrees cooler than those taken from top branches receiving more light intensity. An average of several readings taken throughout the plant provides a good point for VPD calculations.

Calculating VPD in a Cannabis Grow Set-up

How much airspace the marijuana plant has to capitalize on for good transpiration flow is determined by vapour pressure differentials. This process first requires finding out the SAV, which is the temperature reading of the marijuana plant’s leaf surfaces.

Finding out the actual vapour pressure (AVP) of the air and subtracting it from the SVP reveals the vapour pressure differentials at that particular spot in the grow room.

VPD = SVP – AVP

The following formula calculates SVP, where T is the temperature in Celsius measured from leaf surfaces using a measurement called pascals (Pa). The calculation for Fahrenheit to Celcius is  (°F – 32) / 1.8 = °C

SVP = 610.7*107.5T/(237.3+T)

To convert VPD into kilopascals (kPa), divide the above formula by 1000 to find the standardized number.

This same equation determines the actual amount of water vapor in the room with the added component of relative humidity (RH) readings incorporated into the formula.

AVP = SVP (Pascals) = 610.7*107.5T/(237.3+T) x RH/100.

(Again, dividing the first half of the equation by 1000 will give kPa measurements for VPD)

OK, so there are numbers on the paper, but what do they tell a cannabis grower about VPD? Thankfully science has come before us, and now VPD charts are easily obtained from internet sources. On a side note, now that you know how to do the math, it would only be fair to admit VPD Calculators are also easily attainable with a quick search.

Vapor pressure deficit impacting cannabis plant

What do the Numbers Mean?

As cannabis plants evolve through their growth cycles, their transpiration rates change. Looking at new seedlings or young clones trying to build their root base, VPD levels between 0.8 and 1.0 kPa are the ideal range to encourage transpiration. Warm temperatures and ample humidity in the air helps young plants develop strong root systems.

Similarly, plant growth in the vegetative stage thrives in sultry conditions, performing well with low VPD levels between 1.0 – 1.2 kPa. Dropping relative humidity by even 5 – 10% from earlier growth stages encourages ample photosynthesis and transpiration activity, pushing plant growth.

As the marijuana plant transforms into the flowering stage, high VPD levels between 1.2 – 1.6 kPa at moderate temps with lower humidity levels ensure no mould will be growing inside compact buds. As you see, there is quite a lot to learn about how VPD effects cannabis plant growth throughout different stages.

Profiting from VPD

While science has determined, and charts reveal, a broad spectrum of ideal parameters for ideal vapour pressure readings, every growing environment is different. Cannabis growers implementing the ideal VPD into their grow room can dial in optimum temperature and humidity levels by keeping accurate data recordings and making the necessary adjustments.

Learning how to control vapour pressure deficit in a grow set-up will lead to healthier plants and bountiful harvests.

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Post author
Charle Thibodeau
Charle’ Thibodeau is a freelance writer with almost a decade´s experience, specializing in cannabis content for the past two years. A strong motivation to educate, inform, and promote the culture surrounding this miraculous plant is her earnest mission.
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