Knowledgebase
humidity and vpd in a greenhouse #819756
Asked February 04, 2023, 6:12 PM EST
Kent County Michigan
Expert Response
Hi Casey,
My name is Jeremy and I'm a greenhouse agent with MSU Extension. Those are all great questions!
Is there a good way to keep my humidity up another way? If we're looking to increase the relative humidity (RH) of a growing environment, we'll need to add water vapor to the system and then try to keep it from escaping. Most greenhouses aren't airtight, so growers need to consistently add water vapor over time. Commercial growers will often construct tents to help keep RH higher in specific areas -- usually when they're rooting cuttings. I have attached photos from three different greenhouses that illustrate this concept. Note that the materials vary in permeability. Sheet plastic isn't very permeable, so it will trap more moisture than woven fabric.
Would misting in the morning or throughout the day help keep that humidity up? You're on the right track with misters. Yes, using them throughout the day should increase RH (depending on the air temp, of course).
On the other hand I am learning more about vpd. It seems with seedling/ cutting you want it lower to prevent fungal infections? No and yes. New vegetative cuttings do not have a root system, but they're still alive and photosynthesizing, so growers try to keep RH pretty high to reduce transpiration rates. Seedlings in plug trays with root systems, on the other hand, don't need nearly has high of an RH. In fact, rooted seedlings are usually better off with a lower RH and higher VPD.
Yes, a high RH environment is usually more favorable for pathogen development. New cuttings need a high RH, though, so it becomes a bit of a balancing act. Commercial growers take cuttings off the mist as soon as they develop some roots. Part of the reason is to reduce the chance of disease.
As you are learning, RH is dramatically affected by air temperature, so VPD can be a useful metric because it removes temperature from the equation.
It seems that the lower the humidity the higher the vpd. is that how it works? Yep, pretty much!
I'm not sure if you've read these yet, but these two articles do a nice job of explaining VPD.
https://www.canr.msu.edu/resources/water-vpd
https://www.canr.msu.edu/resources/vpd_vs_relative_humidity
I hope that helps. Let me know if I can be of further assistance!
Warmly,
Jeremy