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Maximum umol measurement for sustaining dark cycle?

I had a question from one of our commercial customers that I didn’t know the answer to off the top of my head, but figured someone on here is familiar with: what the maximum umol a plant can withstand during the night photoperiod without it waking up or experiencing issues of hermaphroditism?

Some types of equipment my customers are usually worried about are CO2 generators, controller digital readout screens, etc… I know they’d like a way to measure if these are going to negatively effect their grow rooms prior to dealing with the consequences of the potential issues.

I’m sure there are some scholarly articles pointing to this info, just figured I’d ask here before jumping into Google Scholar @Theo @shibbyhockey04 :smiley:


Good question. I don’t know. A bit darker than full moon I suppose :wink:

The greenhouses I worked with had Ludvig Svenson screens and less than 1 umol m-2 s-1


@Nate: TL;DR: 2 umolm-2s-1 is enough the trigger “day” for a long day plant!

I recently “attended” a virtual lighting conference for supplemental lighting in green houses held by Green House Magazine. There were several talks from leading horticulture researchers, one of which was Chris Currey from Iowa State University. In his talk he commented that 2 umolm-2s-1 is enough to trigger “day” for long day plants such as cannabis. This is an extremely small irradiance to wake the plant up!

Therefore light pollution during dark hours may be a concern for some growers. Security lights, car headlights, and light from adjacent bays were some sources he pointed out. Light from controllers and things may also be a source of light pollution and cause rest issues for plants exposed.

I’d suggest looking at the placement/directionality of these lighting controllers and then either by manufacture data or through measurement find out about the output of light from the display. If the light level at the plant surface is around that minimum value above, then some blackout curtains or coverings might be a good idea.


hey Nate,
I am working on a piece of hardware right now that will track plant photosynthetic and soil health properties alongside finished parameters like THC/CBD potency and yield. This would be a perfect use case where you could track your plants, and see how much umol your plants are exposed to and how it effects those final outcomes. Maybe you would discover ways to better optimize your grow from there. Get in touch!