The principle behind plant growth is: light, food, and water. As you may know, plants consume CO₂ (carbon dioxide) and excrete O₂ (oxygen). They take in water and nutrients mainly through the roots, and transport it to the leaves where the light source then hits to energise the metabolic processes in the developing plant. With VPD, you can control (and hopefully increase) the rate at which these metabolic processes occur—though technically indirectly.
Underneath the leaves, there are tiny structures called stomata. These are openings that mediate the entry of water molecules and carbon dioxide, as well as serving as an exit port for oxygen and water molecules.
VPD, or Vapour Pressure Deficit, is a technique used to discover and implement optimal temperature and relative humidity conditions to achieve greater cannabis plant performance.
You may think you have dialed in your grow room perfectly; but in all practicality, there is likely a great margin for improvement. It only makes sense to use VPD to dial in perfect conditions if you are able to fine tune to precise values of both temperature and humidity. In practical terms, most of the time you will simply be adjusting temperature.
Sure, everyone knows that during flowering, you will want to keep the temperatures at around 25°C and below 55–65% RH to avoid bud-rot. Under 35% RH may be too stressful for the plant. If you run CO₂ supplementation, you can easily run both higher temperatures and humidity levels safely.
We will try to keep it as simple as possible. The mathematics behind this concept are quite complex indeed, but we do not need to get into them. Thankfully, there are many charts out there that will make your calculations as simple as crossing two numbers together—temperature x relative humidity.
VPD has been known for quite a while, but fell out of the agro-horticulture circuits because large-scale operations rely on natural climate, as opposed to indoor growing. Only more recently, with the advent of the greenhouse and an influx of hobby cannabis growers, has it become possible to actually control environmental conditions with evermore precision to make practical use of VPD.
But what if you could significantly increase your final yield, simply by knowing with absolute certainty the exact combination of temperature and RH to turbo-charge the stomata’s efficiency? This is the power of VPD.
The higher the ambient temperature, the higher quantity of water molecules it can absorb. This increases the surrounding air pressure. It is this air pressure that constricts the stomata from being able to open. They will do their best, but will not be as efficient, and far from working at peak performance.
VPD can be a challenge to understand at first, but it is extremely easy to put into use. We highly recommend you try it out, your plant will love you for it.
That way, VPD is essential when it comes to offer the best conditions for the development of plants, also to understand its water requirements according to the nutrients used and thus adjust them correctly to meet the grower’s expectations. We have seen that the best way to keep a suitable VPD value is to control the temperature and/or relative humidity, which is something we have multiple options for, as we will see later.
Saturated steam pressure values chart
In the chart above you can see the right temperature and moisture values to maximize the development of cannabis plants. The green zone shows you optimal values of VPD, in which it would be ideal to maintain the environment. The orange zone shows correct values, in which plants should not have problems to develop normally. However, red areas show values in which your plants will be very far from hit their full potential, since the relation between temperature and moisture is not optimal.
Thus, we can connect an air humidifier in case you need to raise the moisture or fans if what we want is to decrease it. With the climate controllers we can also modify the extraction equipment speed and adjust it in order to expel the correct amount of heat and moisture. If the temperature is too high, we can use less powerful bulbs or LED systems, increase extraction and ventilation or use air conditioning.
On the other hand, if it is necessary to increase the temperature, we can incorporate to the assembly some heating device like tubular heaters for greenhouses or heat mats. Reducing ventilation and extraction will also help, although you will need to control the increase in relative moisture. In short, climate control units allow us to set the temperature and moisture values that will determine the operation of all devices connected, which certainly represents a great help for the grower.
VPD= ((100-RH) / 100) * SVP
Dehumidifiers decrease environmental moisture and release heat, which can be helpful in certain cases like very moist and cool areas, such as basements. We can also add CO2 to the environment, although this is usually reserved for experienced growers who have their grow room fully optimized. As we see, there are numerous options in the market to meet all the needs and achieve the ideal environment for our plants, which has a direct impact on their performance.
You can check the VPD chart for the bloom stage in this link.
Dew is formed when the environment is saturated with water vapor
Temperature and relative humidity play an important role in indoor marijuana cultivation since much of the plant’s activity depends on these factors.