What is VPD and how does it affect your indoor growing?

By Stoney Tark

Relative humidity explained

Before getting into the specific details of how V.P.D is calculated, it is a good idea to understand the meaning of relative humidity. Indoor grow rooms will have a hydrometer which will display the level of humidity that is present in the air. Along with this reading the temperature will be provided simply meaning that at that point your environment is a set reading. For example during the cloning stage, seedling and entire vegetative stage the relative humidity required is around 70%-75%, whilst during the flowering stage 30% is the ideal level.

Now if you think that during different temperatures the level of humidity will fluctuate up or down. As this happen plants transpire accordingly and either through the roots or the foliage to adapt to their growing environment. In the exact same way that in the middle of summer, the heat combined with high humidity can be highly uncomfortable and it can be challenging to cool down.

Our bodies would perform osmosis differently and sweat more meaning we would urinate less in a way to preserve water levels and salt levels within the body. Plants and humans are exactly the same when it comes to adapting to their environment, plants feel the same uncomfortable force as we do, so once you can understand that much, learning how V.P.D is designed to allow peak performance can seriously improve the overall health and vigour of your plants.

Understanding vapour pressure deficit

Indoor grow book will always advise to keep your relative humidity and temperatures in accordance with the life cycle of the plants. At no point in any of the older grow bibles is V.P.D mentioned and the concept of applying it into indoor gardens is becoming more of a trend to the clinical, calculated grower. The way to think of V.P.D is to think In the same way that cold water holds more dissolved oxygen than hot water, the same principle applies with the air. As the air cools it has the ability to hold more water meaning the warmer that air becomes, the ability to hold water will become less and less until conditions are dry.

V.P.D is normally measured in kilopascals which is a derived unit of pressure. When relative humidity is high then V.P.D is low and the opposite when relative humidity is low then V.P.D is high. A good way to think about it is the higher the value of V.P.D then the greater the chance of the air drying out and as a result removing moisture out of the leaves.

How To Calculate V.P.D

S.V.P is an abbreviation for saturation vapour pressure which is stated as the water vapour pressure at 100% maximum relative humidity. A.V.P stands for actual water vapour simply meaning the percentage of relative humidity present in the air. To calculate the relative humidity, then the percent of actual water vapour is divided by the value of the saturation vapour pressure at whatever the temperature is at that time. Below is the formula to calculate relative humidity.

%RH= (AIRAVP / AIRSVP) x 100 (% value)

To calculate V.P.D, you will need to know the room humidity (R.H), the level of saturation vapour pressure (S.V.P) and the leaf temperature. The way this is done is using a thermal camera to highlight the areas of the plant canopy, so you can then interpreted those values based off the V.P.D chart. VPD is the leaf-temperature SVP, minus the air-temperature SVP times the percentage of relative humidity. Below is the formula to calculate vapour pressure deficit.

VPD= LEAFSVP – (AIRSVP x AIR%RH)

As with any test it is important to obtain consistent values, therefore pinpointing the exact parts of the leaf tissue you are testing for temperature may differ from test to test. Also running strict VPD values can sometimes lead to stress or the increased chance of pathogens forming in higher humidity environments. The V.P.D chart is created as a guideline and not all plants are the same.

How Does V.P.D Affect CO2 Uptake?

When grow room lights first come on, the plants will begin to fully function and begin to open stomata found on the surface of the leaf, 60 minutes after receiving light. The role of the stomata is open and absorb Co2 back to the plants, so they can photosynthesise and exchange oxygen as a bi-product. As a key component in the chain of photosynthesis, providing an environment that the stomata can adapt with the most integrity can be a case of correct V.P.D levels.

High humidity causes stomata to open and low humidity causes them to close. Once Co2 is absorbed through the stomata opening and photosynthesis can occur, then providing higher humidity can increase the rate of photosynthesis. Of course that being said outside threats from pathogens and plant disease can always enter a grow room that provides a consistent environment that the spores may survive. Ultimately the level of relative humidity will dictate the exact function of the stomata which can cause plants to wilt and dry or look vigorous and maximum health.

Top tips for indoor climate control

• If you are not able to follow a V.P.D chart, then you can use a hygrometer and be vigilant to the temperature and humidity. This device will give you an accurate reading, as well as inform you of the highest and lowest data from a 24 hour period.
• Use a humidifier or dehumidifier in order to control the humidity levels. During the growing phase of 18/6, Cannabis plants enjoy a range from 75% down to 50% the closer you get to flowering.
• Keep exhaust fans, oscillating fans and intake fans running 24/7. Creating a consistent environment will allow you to control the levels of stagnant air being dispersed from the tent and the air fresh.
• Temperature fan controllers can be installed inside a grow room, and will allow the temperature and humidity to remain consistent to how the plants require. They are quite expensive, however will take your indoor growing game to the next level and will take a lot of work off your hands.
• If you are planning to use CO2, then be prepared to take your temperature past 30 degrees Celsius, with high humidity. This can be a red flag zone for many growers, so be careful is supplementing plants with CO2.