Using the Vapour Pressure Deficit (VPD) in Cannabis Cultivation
In this fascinating interview taken from Hanf-Magazin, you will learn about the potential hidden in your plants‘ genetics and how to unlock it when you master the interaction of temperature and relative humidity by means of the Vapour Pressure Deficit, VPD. By making use of Purolyt’s priming effect, your next harvest will reach numbers that have not been considered possible before.
With targeted use of Purolyt, plants can mature in a high humidity, even in the later stages of the bloom. At the same time, plants are protected against diseases and infestations. This leads to healthy plants and a constant increase in yields.
Introduction
Cannabis cultivation methods and conditions are discussed with great enthusiasm and with large portions of idealism. Since there are so many varieties of cannabis and the plant is cultivated in all kinds of climates around the globe, we must assume that there are also equally diverse cultivation methods.
Nevertheless, there is also the fundamental knowledge that you must have if you want to successfully grow cannabis plants. Light, water and various basic nutrients, for example, are essential for almost all plants to thrive. Based on this, there are ways to create an environment for cannabis in indoor cultivation that have a positive effect on the health and condition of the plant and, last but not least, can increase yields in a targeted manner. Factors such as temperature and humidity play a major role here.
Working with the VPD (Vapour Pressure Deficit) table
To ensure a successful cultivation with cannabis plants and to achieve the highest possible yield, you can make use of the so-called VPD (Vapour Pressure Deficit) tables. A VPD table combines the room temperature and humidity into a single unit of pressure. A VPD chart shows the range where both parameters correlate best with each other. The VPD value is given in kilopascals (kPa). As plants have different needs in each stage of growth, specific VPD tables were developed for each stage.
The Vapour Pressure Deficit, or VPD for short, describes the difference between the moisture contained in the air and the moisture the air could possibly contain. If the temperature in the grow room is higher, the air can hold more water particles. Growing at optimal VPD values influences several key processes in the plant's metabolism such as stomatal opening, CO2 uptake, transpiration (evaporation of water through the leaves) and nutrient uptake. The speed at which metabolic processes take place can thus be increased in a controlled manner.
The company Corpus Verde has not only dealt with the correlation between temperature and humidity, but also with ways of combining the protection of plants against diseases and pests with methods of optimising yields that take both factors into account. With CEO Guido Friedrich, we were able to discuss these exciting topics in a little more detail and clarify questions about cultivation taking into account the Vapour Pressure Deficit (VPD) in combination with their product Purolyt for a safe and abundant harvest.
Interview with Guido Friedrich, CEO of Corpus Verde
Hanf Magazin: The abbreviation VPD refers to the "Vapour Pressure Deficit". How would you explain this to someone for whom growing is still new territory?
Guido Friedrich: It's about the perfect interaction between leaf temperature, relative humidity and air temperature. If all values are in harmony, the plant can fully develop its genetic potential. Both too high and too low VPD values lead to unnecessary stress for the plant. With the help of a VPD table, I can set the ideal humidity for each temperature and vice versa.
Hanf Magazin: What humidity and temperature values are ideal for the different phases in the life cycle of the plants and how is this expressed in the VPD value?
Guido Friedrich: Considering the whole growth cycle of the plant, we find ourselves in a general VPD range of roughly 0.3–1.6 kPa. The traditional VPD values for cannabis in each phase are as follows:
Propagation phase: The young plants cannot handle stress yet as they are busy growing roots. Thus, you want to aim for a higher humidity and low VPD values at the bottom of the general range.
While the cuttings take root, a VPD between 0.1–0.3 kPa is recommended. At 22–23 °C, this corresponds to a relative humidity of 75–80%.
Some growers even keep their seedlings at a VPD of 0.1 kPa during the first 21 days, at up to 90–100% humidity, to train them for higher humidity conditions. This is the so-called high humidity plant training!
Growth phase: The plants are now bigger and more robust. Thus, you can decrease the humidity in their environment to increase the VPD. This boosts the water and nutrient intake. However, if the VPD is increased too much, the plants‘ stomata will close again and will absorb less CO2.
A good VPD range for the early vegetative stage thus lies around the middle of the general range, between 0.4–0.8 kPa. At 24–34 °C, this corresponds to a relative humidity of 75–80%.
A good VPD range for the late vegetative stage lies even closer to the middle of the general range, between 0.7–1.0 kPa. At 24–34 °C, this corresponds to a relative humidity of 55–75%.
Pro tip: It seems like you will get the most significant mass build-up at 0.7–0.8 kPa. However, this can vary and is heavily dependent on the plant variety.
Bloom phase: The plants are robust now, but the blossoms are quite sensitive to various influences. Excessive humidity should be avoided.
A good VPD range for the early bloom stage lies at the upper end of the general range, between 0.8–1.2 kPa. At 24–34 °C, this corresponds to a relative humidity of 50–75%.
A good VPD range for the late bloom stage lies even closer to the upper end of the general range, between 1.2–1.6 kPa. At 24–34 °C, this corresponds to a relative humidity of 40–65%.
The table below provides a great guideline as a rule of thumb, but you should look up the detailed values in the corresponding VPD table.
| Plant Cycle Phase |
VPD
[kPa]
|
Temperature
[°C]
|
Rel. Humidity
[%]
|
| Min |
Max |
Min |
Max |
Min |
Max |
| Propagation |
0.1 |
0.3 |
22 |
23 |
75 |
85 |
| Early Growth Phase |
0.4 |
0.8 |
24 |
34 |
75 |
80 |
| Late Growth Phase |
0.7 |
1.0 |
24 |
34 |
55 |
75 |
| Early Bloom Phase |
0.8 |
1.2 |
24 |
34 |
50 |
75 |
| Late Bloom Phase |
1.2 |
1.6 |
24 |
34 |
40 |
65 |
Hanf Magazin: For the inexperienced grower, it's easy to say, "A plant will basically grow by itself if I put the seeds in the ground and then water it from time to time." Why should you bother with things like relative humidity or VPD tables?
Guido Friedrich: This is a great question, as on the first glance, this whole process seems a bit complicated and expensive. But it does not have to be that way. I will further explain later how even hobby growers can achieve great results at a low price.
But you’re still waiting for my actual answer: It’s all about the climate! Let’s have a look at the distribution of the influencing factors you will find in horticulture. It will soon become obvious that these offer the most possibilities for the gardener to set for success.
The influencing factors are divided into three areas: Genetics, climate, and water.
I would give the choice of the most advantageous genetics a weighting of impressive 50%. The second most important factor will come as a surprise for most horticulturists, as many would think that the size of the yield is mainly decided by the choice of fertiliser or the use of an additional booster. However, they forget that these agents can only take their full effect when the climatic conditions are ideally set.
Thus, I would weight the influence of the most advantageous climate, i.e. light, temperature, relative humidity and CO2, with 45%, and the influence of irrigation and fertilisation with only 5%.
|
Influencing Factors for the Success in
Indoor Cultivation / Horticulture
|
Share in % |
| Genetics |
50% |
| Climate (light, temperature, rel. humidity, CO2) |
45% |
| Water (irrigation, substrate, fertiliser, stimulants) |
5% |
| Total |
100% |
If you want to work with your yields in the 1-2 g/watt range, then you should make that effort. It always depends on what you want to achieve. I have met many growers over the last 30 years. Some are quite satisfied with 0.5 g/watt light output. Others, however, set 2g/W as the lower efficiency limit. With the right climate and well-selected genetics, this goal can be achieved consistently.
Hanf Magazin: Are there devices that help me determine the VPD or do I have to calculate it myself based on the relative humidity and other parameters?
Guido Friedrich: There are indeed various manufacturers who provide devices with infrared cameras that will help to constantly and automatically determine the optimal VPD value. These will also control the climate automatically. At larger production scales, this investment would absolutely make sense. However, I would strongly advise against the use of inexpensive devices without an infrared camera, as those would have the same accuracy as if you’d toss a coin.
In the hobby and experimental area, on the other hand, you can just use the available VPD tables or online VPD calculators where you can look up the values. A commercially available infrared thermometer is then needed to determine the leaf temperature, as the room temperature does not correspond to the leaf temperature. In addition, a ventilation system is required to regulate temperature and relative humidity and to set day and night values. Depending on requirements, either a humidifier or dehumidifier should also be used.
Hanf Magazin: Using the Purolyt disinfectant concentrate to clean surfaces is probably the most obvious thing to do for most people. However, it is actually much more versatile. How can I use it for the cultivation?
Guido Friedrich: You are absolutely right. The possible applications are very diverse. Unfortunately, for legal reasons, we are only approved as disinfectant for air, water and surfaces in the EU. This means that the gardener can disinfect all tools, rooms, irrigation systems as well as the water and air in the greenhouse. In other countries such as the USA, Canada or Japan, this non-toxic and environmentally friendly technology has been allowed to be used in many other areas for decades, e.g., for fresh-cut lettuce or for treating fruit and vegetables after the harvest.
The most interesting application in indoor cultivation has been the subject of numerous international studies for two decades: regular direct spraying on the plants from the first day in the propagator until harvest (and possibly also when drying herbs to prevent mould).
Direct spraying leads to a strengthening of the plant, based on two very effective mechanisms:
a) the external antimicrobial protective effect
b) the plant is stimulated to produce defence proteins which significantly strengthen its resistance from within. This effect is called "priming" in science.
To achieve these positive effects, all you must do is to apply a 1:25 dilution of Purolyt and water 2–3 times a week, depending on the humidity.
This is the Game Changer:
If the plant is strengthened this way, it is possible to have a high humidity, at a VPD of only 0.7–0.9 kPa, during the whole bloom period. As mentioned above, we see the most relevant mass build-up of the plant in this range. Thus, the yield will be significantly increased by the optimised climate. If Purolyt is used regularly, the risk of developing the dreaded Bud Rot (Botrytis) is eliminated. Hence, with Purolyt, the traditional compromise to increase the VPD value during the bloom to 1.6 kPa and thus to decrease the humidity is no longer necessary.
At the same time, the use of toxic pesticides is decreased. For example, a cannabis farmer from California was able to increase his average annual yield by 26%, while the use of pesticides could be minimised by over 80%.
Hanf Magazin: The preventive use of Purolyt serves to increase the possibilities of the plant's immune system. Therefore, it becomes more resistant to various diseases and pests. For which diseases and pests does this work best? Which experiences do you get the most positive feedback about?
Guido Friedrich: Diseases such as mildew, grey mould and root rot can be kept at bay by the preventive use of Purolyt. If there is already an existing infestation, Purolyt can only prevent the further spread of the disease. Scientific studies have also found that the plants become much more tolerant to any kind of stress (heat, drought, etc.).
Since Purolyt is not toxic to insects, one can take advantage of an indirect effect with pests:
Let's take the extremely unloved common spider mite (Tetranychus urticae). Spider mites multiply at high temperatures and low humidity. Unfortunately, these are also the same climatic conditions that our plant needs when in flower. That is why spider mites always become a problem when the plant is in flower. We change the climatic conditions from the growth to the flowering phase from 80% relative humidity to 60–40%.
Why do we do this? If we have a closer look at the VPD tables, we will see that optimal VPD values can be achieved even with significantly higher temperature and humidity values.
For example, an optimum value is found at 32 °C and 72% relative humidity. At these values, spider mites cannot reproduce, while their predators (predatory mites) start to thrive in those conditions. Now, the experienced grower will immediately think, "But my flowers will start to rot." And this is exactly what you can prevent with the right use of Purolyt!
Through the regular use of Purolyt, a VPD of 0.7–0.9 kPa with relative humidity > 60% can be set throughout the entire flowering phase. Thus, by using Purolyt preventively and regularly, we can create climatic conditions in which pests fare poorly and beneficial insects fare optimally. This guarantees protection against many pests such as spider mites and brings greatly increased yields.
The priming effect plus the optimal climate according to the VPD table combine to create the perfect conditions for the plants and beneficial organisms, while the pests have no chance.
Hanf Magazin: But what about the taste? It seems very unusual to spray the blossoms. Won’t many growers be opposed to this?
Guido Friedrich: At first, people are always very sceptical towards innovation. When the steam locomotive first exceeded a speed of 30 km/h, researchers warned that the human body could dissolve at such high speeds. Today I’m racing through the woods on my e-mountain bike at a speed of 50 km/h and my body wants even more. If you tried it once, you can’t go back. Our reality has changed.
To say it upfront: Purolyt does not impact the taste negatively, as Purolyt does not leave any reactive residues.
Quite to the contrary: if we set the VPD ranges to the abovementioned values, you will experience a whole new taste. The more humid the climate is (low VPD value), the less terpenes will be lost through outgassing and will remain in the plant material. Conversely, this means that the drier the climate is (high VPD value), the more terpenes will be lost. The more intense the smell in my tent, the less terpenes I will get in the glass. These valuable substances will be lost in the activated carbon filter.
Hanf Magazin: How is Purolyt actually produced and how does it work?
Guido Friedrich: Purolyt’s principle of operation is like the way the human body fights off bacteria and viruses:
When the immune system detects a threat, white blood cells are activated which create an antimicrobial substance called hypochlorous acid (HOCl). HOCl is also the active ingredient in Purolyt. It is scientifically proven that HOCl has extremely efficient disinfecting properties. Plants also use HOCl in their immune response as a defence against bacteria, fungi and viruses! Therefore, the abovementioned priming effect can occur in the first place.
Purolyt is produced by electrolysis (i.e. with electric current) from common salt (NaCl = sodium chloride) and water. In the process, the above-mentioned HOCl is formed at the positively charged electrode (= anode, hence the term anolyte).
However, the molecules are virtually subjected to an electrical voltage (scientists call it "electrochemically activated") and strive to react back to their "relaxed" original state, i.e. water + salt. If Purolyt comes into contact with, for example, a bacterium, it releases its electrical voltage and thereby dissolves its cell membrane. In the process, Purolyt decomposes back into water and salt. There are no unsafe residues! Even the salt is only present in negligible traces.
HOCl is not dangerous for all higher organisms (humans, animals, plants, insects), as they have long since learned during evolution to produce HOCl themselves and to use it effectively. In comparison to alcohol which, as a disinfectant, is known to dry out the skin, HOCl is used very successfully in the field of wound healing - it even stimulates human skin to heal instead of damaging it. Isn't that wonderful?
Hanf Magazin: I find the reconditioning of used substrates an interesting application for Purolyt disinfectant. Does it work with all substrates that I can use in indoor growing?
Guido Friedrich: Since Purolyt decomposes very quickly into water and traces of sodium chloride (which results in its non-toxicity), it is only possible with inert substrates e.g., rockwool, Hydroton, seramis, perlite, etc. It will not work with soil.
Hanf Magazin: To summarise this veritable flood of information, this means for cultivation:
If you consider the correlation between temperature and humidity right from the very first day of culturing and set the parameters according to the VPD tables, you can – with help of Purolyt – enable the plants to mature at increased humidity levels even in the late stages of bloom.
Among other benefits, this will result in:
- a) an excellent resistance against diseases
- b) the creation of advantageous climatic conditions that support the reproduction of beneficial insects and virtually prevent the reproduction of pests
- c) a significant increase of the blossom yields
- d) valuable terpenes being maintained in the plant
This sounds like a real win-win situation for the plant and the grower.
Author: DIETER KLAUS GLASMANN
Release date: 20. January 2021
Category: Interviews
This interview was published on the website of the Austrian Hanf-Magazin (a German-language magazine about hemp) and later edited by us. You will find the original article here: https://www.hanf-magazin.com/kolumnen/interviews/cannabis-anbau-unter-beruecksichtigung-des-saettigungsdefizits/