Do you know what VPD is? Discover the key to ideal conditions for plant growth

VPD (vapor pressure deficit = the difference in water vapor pressure) is still a big unknown for many growers. On the other hand, more and more home growers practicing indoor gardening in their grow tent are getting acquainted with VPD lately. Join them with the information you’ll absorb in this article. For professionals, understanding VPD and using it in cultivation practice is a matter of course. Join them and reach a new level!

All growers certainly want to provide their plants with perfect conditions for development. To give them an ideal climate inside the cultivation space so they can later enjoy a wonderful, flawless, abundant harvest. However, when it comes to the climate inside a grow tent or an indoor grow room, most growers deal only with factors such as temperature or humidity. These factors can be directly influenced using common equipment, whether it's a quality air handling system or appropriately chosen artificial lighting for plants. Even the actual measurement of temperature and humidity is not rocket science.

VPD and the importance of plant transpiration

Transpiration is the process by which plants “breathe” and release water through their stomata, the pores on leaves. Intensive transpiration through leaf stomata is extremely important for plants because the amount of water exhaled by a plant is always closely related to how much water with nutrients (irrigation) it can absorb through its root system.

Now an important fact comes into play. The rate of plant transpiration largely depends on the difference between the water vapor pressure inside their stomata and the water vapor pressure in the surrounding air. With that we’ve, without unnecessary technical formulations, clarified the term VPD itself. The difference in water vapor pressure simply tells us how strong the air’s “drying” power around the plants is, which directly affects the intensity and overall quality of their transpiration.

What the water vapor pressure around the plants will be always depends on the temperature and humidity of the air in the cultivation space. The humidity inside the pores on the leaves can generally be considered 100%. The pressure is mainly influenced by the leaf temperature. If, in addition to air temperature and humidity, we also measure the leaf temperature, we can calculate the difference in water vapor pressure between the stomata and the leaf surroundings from the measured values. Depending on the plant’s current life stage, we can maintain and control VPD. And that’s great news, because if you can keep an eye on VPD during the cultivation cycle, your plants will do noticeably better!

How to achieve correct VPD?

You might wonder where the catch is. The best thing is: practically none. You just need to responsibly measure the main factors that influence the difference in water vapor pressure. Repetition is the mother of wisdom and in this case of high yields, so for completeness we list them below:

• air temperature
• leaf temperature
• relative air humidity
• light intensity
• substrate moisture and EC of the substrate

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Accurate measurement and control of the individual factors that influence VPD is essential to using this knowledge for a better harvest. You can also find various tables on the internet with generally recommended values for vapor pressure difference. For a start, remember a simple but most important rule: the surface of the leaves must always be cooler than the surrounding air! If it were the other way around, the plants’ transpiration rate would not be sufficient, plants would not cool properly and ultimately could suffer irreversible damage or even die.

The description of the opposite and undesirable state clearly shows again how important the difference in water vapor pressure is in indoor cultivation. At some point, most growers (especially beginners) encounter apparently inexplicable problems. Plants do not look vital at first glance, show signs of nutrient deficiencies and their leaves wither. Growers ask themselves and more experienced colleagues where the problem is and search in vain for the reason they cannot achieve satisfactory results. After all, they apply fertilizers exactly according to the manufacturer’s dosing tables and often devote more care to their plants than to themselves!

Practical examples

• If the vapor pressure difference is too high, plants cannot cool sufficiently because the surrounding air dries them out too quickly. As a result, plants do not develop as they should. They usually also show symptoms of nutrient deficiencies, which we addressed in a separate article: Symptoms of important nutrient deficiencies. In these situations, plant leaves often dry out, curl, or gradually die.
• How does this happen? In indoor gardening it is most often caused by a combination of low air humidity and a powerful discharge lamp.

Do you want to ask us something? We are here for you, either by phone or at the well-known e-mail address info@higarden.cz. We look forward to your questions!

We also recommend these products:

• Ultrasonic humidifier HUMIPRO 4L
• Garden Highpro Clip Fan 15CM / 15W 2 speeds
• Ventilator PRIMA KLIMA EC WHISPERBLOWER

Further reading:

• Symptoms of important nutrient deficiencies
• Discover modern growboxes Garden Highpro
• Substrates and fertilizers CANNA: 30 years of research for successful cultivation of plants indoor and outdoor