Stop Thinking “Temperature and Humidity Control” in Your Grow Rooms!

Why? Because it’s not the right way to think about what your plants need. The primary environmental concern of every strain of cannabis is the Vapor Pressure Deficit (VPD).

You need to think like a plant (yeah, I know, but work with me here…). What the plant thinks about is not an arbitrary grow room temperature setting, nor some “ideal” humidity level. The plant’s one and only happy place is the ideal “Vapor Pressure Deficit” for its particular stage of growth.

“My VPD is perfect and I feel good!”

So what is vapor pressure?

All gasses in the air exert a certain “pressure.” The more water vapor in the air, the greater the vapor pressure. From a plant’s perspective, high vapor pressure can be thought of as an unseen force in the air pushing on the plants from all directions. This pressure is exerted onto the leaves by the high concentration of water vapor in the air. At the same time the water within the cannabis leaves creates an internal vapor pressure.

If the air vapor pressure and the internal leaf vapor pressure are the same, no transpiration can take place via the stomatal pore . This is “no bueno” for plant.

If the air vapor pressure and the leaf vapor pressure are extremely different, the leaves will transpire too much water. Again we have an unhappy plant.

The difference in air vapor pressure and the vapor pressure within the plant is the Vapor Pressure Deficit. This is truly the only thing a plant cares about when it comes to transpiration.

The rate of transpiration is determined by the vapor pressure deficit and the vapor pressure deficit is controlled by the coincident temperature and relative humidity.  

The key word in the above statement is “coincident”. In order for your crop to have a happy ending, you must control both the temperature and humidity at precise “coincident” values through the various stages of growth. Controlling both the temperature and humidity levels in conjunction with each other will provide the plant with the correct transpiration throughout its life cycle.

So what does this mean in practical terms? It means you need a good control system for your cooling and dehumidification equipment – and this is where the problem occurs for most growers.

        Grow Room Controls – NOT!

High-end digital control systems and components are available – but at a price few growers are willing to pay. A typical Siemens digital control system for a single grow room can easily run between $20,000 and $50,000! While these systems are very precise they can also be quite complex for a grower to manage. Unless you are running a very large grow facility, plant tenders have better things to do than spend hours tinkering with complex software systems.

15000 Inc. took up the challenge to come up with a simple and cost effective VPD control system for small to medium size grow rooms. Working with local control vendors we have developed a straightforward system to control the cooling and dehumidification process using off-the-shelf components. The system employs sensors within the canopy for effective transpiration control. The control interface is simple and easy to adjust making it ideal for your grow room operation.

15000 Inc. is committed to supporting the cannabis industry by applying good engineering practices with common sense and affordable solutions. We can add tremendous value to your project and give you a competitive edge over your competition.

If you’re curious as to how we can help you simply call us today. We’d be more than happy to talk to you about your operation.

Call us today at 707-577-0363



About the Author
Gary Welch is a principal of 15000 Inc. with over 40 years experience in the field of HVAC, plumbing and fire protection design. He can be reached at