In this Growers Spotlight, we interview Chris Vaughn, a lead grower with Higher Minds Horticulture about his experience with VPD control.

Why all the fuss?

What is VPD and why should growers care?

VPD stands for Vapor Pressure Deficit, and it’s the difference between a leaf’s vapor pressure and the air’s vapor pressure.

VPD control is related to the following:

• Modifying metabolic rate

• Improving yield quality

• Determining plant stresses

• Altering pathogenesis

• Injecting carbon dioxide

Managing VPD lets me get away with using fewer nutrients while also seeing increased trichome production.

Can you use VPD to steer growth?

Relative humidity (RH) and VPD are inversely related. This means that when relative humidity (RH) is high, VPD is low, and vice versa. Let’s examine how plants react to changes in VPD:

• The bulk flow of water changes within a plant’s xylem as VPD changes.

• High VPD/Low RH: The plant increases water usage, increasing stress.

• Low VPD/High RH: The plant reduces water usage, reducing stress.

• Nutrients follow the flow of water through the plant tissues.

• High VPD/Low RH: Nutrient uptake increases.

• Low VPD/High RH: Nutrient uptake falls.

• Plants open and close their stomata to regulate moisture loss. If you’re using carbon dioxide, you want the plants’ stomata to stay dilated to maximize gas exchange.

• High VPD/Low RH: Plants close their stomata, reducing CO2 uptake.

• Low VPD/High RH: Plants open their stomata, increasing CO2 uptake.

If your VPD is too low, then your plants aren’t going to acquire enough nutrients, slowing growth; if your VPD is too high, you’re going close the stomata, rendering your extra carbon dioxide ineffective. There’s a sweet spot in the middle.

Is there a difference between controlling VPD in a greenhouse grow op compared to a warehouse grow op?

Yes, but this is true of any environment. How you control your VPD is based on a lot of factors, including:

• Heat sources

• Number of plants in the room

• Environmental controls

• Air conditioners

• Humidifiers

• Ventilation

• Climate and time of year

Most indoor growers typically use big, powerful AC units to control heat. Air conditioners act as dehumidifiers, so indoor growers need to worry about how to reintroduce moisture into the room.

The humidity in greenhouses is generally elevated at most times of the year. The trick is that the humidity can vary wildly based on the climate. You need extra hardware to compensate for seasonal changes.

Good Technique

What techniques would complement VPD control?

Your basic techniques need to be dialed in before you try VPD control:

• Keep the environment as clean as possible. Higher humidities increases the likelihood of pathogenesis.

• Be diligent about airflow. Precise VPD control requires optimized air flow.

• Monitor your water usage. Be careful to not cut off oxygen to the roots.

When everything else is accounted for, the primary technique that complements VPD control is carbon dioxide injection. Fine-tuned VPD controls the behavior of the stomata, allowing increased gas exchange.

Can you use VPD Control in different life stages?

When you put clones under a dome, you’re keeping the RH high and VPD low. This reduces stress and gives the cuttings time to form roots. Additionally, most growers keep their vegetative humidity higher to reduce stress.

You want to keep your VPD relatively high (low RH) during the flowering cycle. Start with a moderate VPD during the first weeks of flowering, then increase your VPD (lower your RH) towards the end of flowering to prevent pathogenesis.

What are the limitations of VPD control?

The biggest drawback to running a low VPD (high RH) is that you can experience pathogenesis if your rooms aren’t clean. Along the same lines, room homogenization is a struggle. There are always microenvironments forming due to the nature of living organisms.

How do you scale VPD control effectively?

A big mistake people make when they scale up is failing to segment their new space into smaller rooms. Good growing is about having a series of smaller rooms that are easier to control.

How does VPD control affect different cultivars?

There’s not a lot of research out there. All strains can be affected differently in the same environment. Lineages from humid environments will likely prefer humid conditions, and the converse is also probably true.

What would an ideal setup look like to you? What kind of equipment should be integrated?

• Humidifiers for starters. Personally, I think that ultrasonic humidifiers work best.

• Double or triple redundancy on your air filtration if you’re making an environment hospitable for fungal pathogens.

• A controller that integrates your different systems and has multiple setpoints based on factors like time of day.

• Some sort of CO2 injection system.

• Grow tents are perfect for experimenting because they have a more manageable environment.

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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