Vapor Pressure Deficit is Real, People!

Spectrum King LED reminds growers at all levels the importance of understanding vapor pressure deficit.
Vapor Pressure Deficit
Courtesy Spectrum King LED

When we first brought up VPD (Vapor Pressure Deficit) a short while back we found that it was basically a new concept for a lot of people. Not just for home growers but even for industrial grow facility workers who are running large commercial indoor growing systems. It was clear that this topic needs to be spread to as many eyes and ears that grow as we all need to see and hear more about it.

VPD is as important as the nutrients you feed and the lights you place above your plants whether high intensity discharge lights like HPS bulbs or full spectrum LED grow lights. It’s true that plants eat photons and like a balanced diet of them (yes, you read that right—plants do eat photons!) but your environmental conditions needs to be on point for you to maximize your crop yields and overall plant health.

One of the biggest details that is overlooked is the placement of the probe that tracks the temperature and humidity in your grow site. Probes should be at plant canopy height and preferably towards the middle of your space and in larger industrial grow sites you should have multiple probes setup across your rooms so that you can see how balanced your rooms are. Should you see much higher temperatures here yet much cooler temperatures on the other side then you know that your room is out of balance. The same goes for humidity. Take a look at the chart below and see the sweet spots where you find temperature and humidity are in balance.

Having good air circulation will result in your temperature and humidity levels being more even and stable in your grow space. Get yourself a handheld laser thermometer and just walk around and take readings and note them down. You can find laser thermometers in many places these days and now they are very cost effective. They will make your life better and help you dial in your grow and fine tune it to get the performance you desire and crave.

Courtesy Spectrum King

By taking readings on top of the canopy, underneath it and in various spots around the room and noting them down you will see patterns. Adjusting fans, moving the probes, adding a humidifier and dehumidifier or several of each depending upon the size of your operation will help you fine tune that environment. One of the keys to knowing you have improved the space for your plants is that they will be eating more and the leaves will be “high-fiving the room”.

Nothing better than seeing plants at peak performance craving more to eat as that tells you that your hard work getting the environment dialed in is paying off. Knowing how to read and set up VPD so you can use this information as one of your main garden tools is very important and then to track it 24/7 from seed to harvest to ensure you have not missed anything along the way.

Data logging may sound nerdy and unnecessary but I assure you it is not. Having been to an industrial grow that was not monitoring conditions properly I can assure you that losing a whole room to root rot, botrytis (bud rot), and powdery mildew got their attention. They had data logging, probes, thermostats, humidistats… all of them but were not monitoring the data 24/7. As it turned out they had an improperly configured dehumidifier and it was getting up to 98% humidity every night in their grow rooms which means it was basically raining in there from condensation.

The crops were ruined for that cycle but if they had just checked the data every morning from the previous night’s readings they would have known instantly there was a problem and could have saved that whole huge room of buds. Having the tools to monitor VPD is great but knowing how to use these tools and checking in as close as real time as possible can have you correct an issue before it becomes a serious problem and have you capable of dialing in a room for a specific cultivar or a family of them  to where you can group plants together and know from the data that they will do well together.

VPD gives you a safe range to operate and push your plants in. That’s whether the lights are on or off as the temperature and humidity have to be balanced to keep conditions optimal for your crops. One of the things that can have people scratching their heads is that when the temperature goes down the humidity level goes up generally.

Moisture levels are kept in balance by your temperatures lights on and lights off combined with probe monitoring. If too damp trigger the dehumidifier, if too dry trigger the humidifier or exchange the air. Too drastic a temperature swing or humidity swing and you can have serious problems in your grow. Your temperature and humidity probes need to be properly located. Placing them on the wall near the door where they don’t get direct light and can be exposed to air currents and drafts from opening and closing of the door won’t get you accurate readings or good data. Having the right tools and knowing where to place them for best use is the key to getting your VPD on point.

  1. I have a Pulse monitor. They recommend a cardboard shade on top of the unit to keep the light from warming it up and giving an inaccurate reading. Even so, they don’t want the sensor directly under the light.

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