Advanced Climate Control – VPD and Transpiration

VPD (Vapor Pressure Deficit – the difference in water vapour pressure) is an important factor for indoor growers. It is closely linked to temperature and relative humidity, affects plant transpiration processes and their ability to take up water and nutrients. Proper climate and VPD control are essential for healthy growth and development at every stage of a plant’s life. In today’s article, you will learn why it is important to monitor VPD indoors.

Even small changes in climate can have far-reaching consequences for plant development in indoor growing, and VPD is a good example of this. Maintaining the correct VPD values requires proper regulation of temperature and humidity, as well as adapting the climate in the growing space to the age and type of plants. In the following lines, we will look at how VPD works, why it is important to monitor it, and how to achieve ideal conditions at every stage of plant development.

What is VPD

VPD is closely related to the data we commonly monitor when growing plants indoors, specifically temperature and relative humidity (RH) in the growing space. Air temperature affects its ability to retain water in the form of vapour. Warmer air can hold more water than colder air. Relative humidity then indicates how much of the maximum possible water vapour capacity the air is currently holding. VPD expresses the difference between the amount of water vapour currently in the air and the amount it could hold at full saturation (i.e. at 100% relative humidity).

VPD is an important concept for understanding how the surrounding air affects plant transpiration processes. The unit of VPD is pressure expressed in kilopascals (kPa). The lower the VPD value, the more saturated the air is with water, which limits the plant’s ability to evaporate water and slows the rate of transpiration. This leads to lower water and nutrient uptake and creates a humid environment where mould, bacteria and other pathogens can easily spread. On the other hand, a high VPD value means that the air in the growing space is very dry and can hold more water. In such an environment, transpiration speeds up and the roots take up more water and nutrients. However, excessively rapid transpiration forces plants to close their stomata, which are pores on the leaves that serve for gas exchange (primarily CO₂ and O). with the surrounding environment. Excessively high VPD can lead to overfeeding and problems with water uptake.

VPD control

The exact VPD range may vary slightly depending on the species being grown; however, the decisive factor is primarily the age of the plants. In general, the younger the plants, the lower the VPD that is suitable for their optimal development.

• Optimal VPD range for seedlings and clones – 0.45 - 0.8 kPa - Clones, seedlings and young plants do not have enough leaves for rapid transpiration and require a humid environment. During the first weeks of a plant’s life, keep VPD at the lower end of the range.

• Optimal VPD for vegetative growth 0.75 – 1.25 kPa - During the vegetative growth period, plants have enough leaves to transpire large amounts of water and thus also take up larger amounts of nutrients. The aim is to increase VPD to the maximum without causing the stomata to close and reducing the supply of CO₂, which would slow growth.

• Optimal VPD for the flowering stage 1.2 – 1.5 kPa - During flowering and ripening, plants are sufficiently robust and able to transpire large amounts of water; however, flowers and fruits are sensitive to a humid environment. During the flowering stage, increase VPD to prevent water condensation.
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If you want to know the exact VPD values for a specific temperature and RH, look for online VPD calculators (VPD Calculator).

Steps for controlling VPD

Given how the VPD value relates to the current temperature and humidity in the growing space, the key to correct values is controlling these two variables. For example, when the air temperature drops after the lights are switched off, relative humidity usually increases and VPD decreases, which limits the rate of transpiration. In practice, this also means that more water condenses in the growing space. If you want to maintain the current VPD in such a situation, it is necessary to increase the air temperature in the grow room. Essential tools for controlling VPD are greenhouse heaters, dehumidifiers or humidifiers, and additional extract fans.

1. Measuring temperature and humidity: Place a sufficient number of thermometers with hygrometers in the growing space. Make sure you measure the readings not only at the tops, but also in the lower parts of the plants.
2. Temperature control: If the temperature in the growing space rises too high, increase the output of the extract fans, or consider installing air conditioning. In cooler conditions, you can use greenhouse heaters. Use a sufficient number of circulation fans to prevent cold or hot air from building up.
3. Humidity regulation: If the relative humidity in the growing space is too low, use humidifiers. They are especially suitable in the summer months and at the beginning of the growing cycle. Excessively high humidity can be solved with a dehumidifier, which is particularly important after the lights are switched off or during flowering, when there is a greater risk of mould.
4. Automating climate control: Climate control units (grow room controllers) automatically adjust temperature, humidity, ventilation and lighting based on the set parameters. Some modern devices can also calculate and monitor VPD.

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