Is there a way to incorporate VPD for night; (lights off) so there will be two VPD settings for day and night
VPD at night doesn’t really matter much, so I would strongly suggest you focus on other aspects of the environment while your plants are resting - pest mitigation, etc.
Night time VPD doesn’t really matter anywhere near as much as daytime, so do what keeps your plants healthy & happy, and whatever is convenient for you.
Some research I did (specifically on nighttime transpiration):
According to the best of my research the difference between day & night transpiration is pretty big. See the figures and referenced studies below. Where did you arrive at your numbers?
Hemp:
From Tang, K., Fracasso, A., Struik, P. C., Yin, X., & Amaducci, S. (2018). Water- and Nitrogen-Use Efficiencies of Hemp (Cannabis sativa L.) Based on Whole-Canopy Measurements and Modeling. Frontiers in Plant Science, 9. doi:10.3389/fpls.2018.00951
Grapes:
From Medrano, H., Tomás, M., Martorell, S., Flexas, J., Hernández, E., Rosselló, J., … Bota, J. (2015). From leaf to whole-plant water use efficiency (WUE) in complex canopies: Limitations of leaf WUE as a selection target. The Crop Journal, 3(3), 220–228. doi:10.1016/j.cj.2015.04.002
From ESCALONA, J. M., TOMÀS, M., MARTORELL, S., MEDRANO, H., RIBAS-CARBO, M., & FLEXAS, J. (2012). Carbon balance in grapevines under different soil water supply: importance of whole plant respiration. Australian Journal of Grape and Wine Research, 18(3), 308–318. doi:10.1111/j.1755-0238.2012.00193.x
Hey GeMann,
I’ve been changing my leaf temp offset for night manually with the Pulse One to dial it in and then back in the morning. Working well so far. I agree it would be super nice if we could just have what you ask for because…
VPD drives respiration and respiration is extremely important at all times. Respiration is the reverse reaction of photosynthesis where the plant is converting the glucose made during photosynthesis into usable energy.
Plants will close their stomata if VPD is too high at anytime day or night, which will slow respiration. Photosynthesis requires light so only occurs during the day, but respiration happens day and night. ( Ever notice plants seem to grow more at night?)
When the lights go off the plants are off gassing more CO2 than O2 and the CO2 could get too high in a sealed environment, so that is a good time to exhaust excess CO2 and bring in fresh O2 through a HEPA filter on the intake while exhausting through carbon.
The plants need O2 at night and if VPD is dialed there will be considerable transpiration from respiration. Typically more dehumidification is required at night. The plants will be uptaking H2O and nutes, the drier the rootzone the more concentrated the nutes become. The plants still need water at night for respiration( if stomata were closed they might not) and it is important not to let the rootzone get too dry at night. Also see below regarding why night temp should lower than day to avoid stress.
https://www.pthorticulture.com/en/training-center/basics-of-plant-respiration/
“Plant respiration occurs 24 hours per day, but night respiration is more evident since the photosynthesis process ceases . During the night, it is very important that the temperature is cooler than during the day because plants can experience stress.”
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Hi @SteveH , seems like this is something that is relatively frequently asked for…
Can you make a wishlist post for this & we’ll add it to the roadmap?