What is VPD?

Vapor Pressure Deficit (VPD) is the difference, or deficit, between the vapor pressure of water in the air versus vapor pressure of water in the leaf of a plant. Thus, VPD is a means of measuring potential water stress within a plant. It is a more accurate measurement than relative humidity (RH) because it accounts for the relationship of temperature and humidity. RH and VPD are typically inversely related, such that a low RH will mean a high VPD. That high VPD value means that a plant will feel a strong pressure to transpire significantly more water.

Why should I care about VPD?

While VPD is not a direct measurement of water flow or water loss, it’s an indicator of stress in the plant. The VPD value tells you how strongly a plant will feel the need to transpire.

You can use this to your advantage, because an increase in the transpiration rate will also increase how much water your plant is pulling through its roots. An increase in the water absorption also means nutrient uptake increases. Thus a high VPD (or low RH) will increase nutrient uptake.

Conversely, high VPD values will result in plants closing their stomata to prevent water loss. If you want your plants’ stomata wide open to increase CO2 absorption, you will want a low VPD value (or high RH).

How can I use VPD?

You can use VPD measurements for different stages of your plants life cycles. In the cutting or seedling stage, a high RH or low VPD is ideal to reduce stress and ease the new plant into growing. You also don’t want temperature variations. With cuttings, this is typically achieved by placing them into a plastic dome.

In the vegetative stage, you can have a lower RH/higher VPD, somewhere between 60-80% RH. Keeping the humidity relatively high, but increasing the temperature with some temperature swings increases the plant’s metabolism and encourages growth.

In the bloom stage, you will want an even lower RH and higher VPD. This is in order to prevent pathogenesis and add just enough stress to give the plants some extra flavor. This will, of course, depend on the particular strain you’re growing. At this stage, there will be larger temperature swings to trick the plant into thinking winter is coming.

During the entirety of the plants life cycles, make sure to take notes about the VPD and photographs and any other documentation you can think of. When you go back over these notes, you can track how well your plants respond to specific conditions.

What do I need for VPD control?

You’ll need a few key things:

• A controller that can integrate information from multiple sensors and give meaningful values. Ideally, this controller can also actuate equipment to keep the environment close to ideal set points.
• Sensors to accompany the controller, placed in a variety of locations around the canopy.
• Good airflow equipment to reduce microenvironments. This means fans and other means of air circulation. Good thinning techniques will also help.
• Humidifiers and dehumidifiers. These are necessary for maintaining proper humidity levels. 

But there’s more than just that. It’s also recommended that you have:

• An HVAC unit capable of handling inputs from your controller.
• CO2 injection to maximize the benefits of VPD control.
• An automatic irrigation system that can take input from your controller.
• Appropriate lighting for your grow op.

What steps should I take to get started?

Make a small testing area that you can control. Ideally, use a grow tent as a testing room. In that room, test everything that you can think of. Push your plants as far as they can go, then dial it a step back. Find out what it takes to get the best product with the smallest amount of input. Then take what you learned from the test area and apply it on the larger scale.

What should I be wary about?

Pathogenesis, primarily. VPD control methods generally increase the humidity, so if you’re not keeping your grow operation clean and sanitary, you can run into issues with fungus and other pests. You also want to avoid condensation and the damage that comes with it. Dripping water means your humidity is too high.

Also be wary that different strains will respond differently to the same conditions. Use your testing room to figure out those differences.

I want to learn more. Where should I go?

This article has been paraphrased with permission from ​Growers Network​.
Want to read more? Head on over to Growers Network to read the ​full article​.

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