This is purely anecdotal as I don’t have the PAR measurements to back it up, but I started to have issues anytime plants got less than 18-20 inches from my COB setups. I ran mostly CREE CXB3590s and some Vero 29s at 1750 milliamps, for reference. I grew in a tent with that mylar reflective material. I had ok air movement, not great, so there is the potential that some of my issues were just temps at the flower surface. Ambient temps were usually mid-high 70s but could have been hotter at the canopy level. I never got to actually bleaching tips but when I turned the lights up full bore, some strains seemed to get finicky.
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