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This is the data from them hung at full power at 18 inches. So much lower than this at half power obviously. Yes the tent is 3x3. Generally speaking I hang my lights high the first few days and gradually lower them to avoid light stress and I did that with these as well. I can put them within a a foot of HO T5s and they don't stress. They are well established plants too, at least 6 months old on most and about a year on one. Next time I have to throw something into the tent I will hang them at 24 inches and turned down to half power to rule that out. My gut is telling me that's not it though, still will rule it out.
you should really give it a try with lower intensity.
it is of course hard to guess which ppfd you have in real, i would guess the manufactorer could give you a ballpark number beeing good for veg.
basically you should encounter good healthy growth at 250ppfd in veg, if they can take a 400, good, give it to them, but i never had vegging plants taking more so far.
i did used (older Cree CXB) 3500K Cobs in a 3x3 with a 240W driver btw, the classic growmau recommendation at that time.
there are always tons of factors in to the game, temp, salt stress, genetics, etc.
but if you say they do well under T5, i would guess its simply the intensity mostly.
PPFD Test of Household Bulbs, Ikea 10W Grow Bulb, and T5 (28W) CFL Fixture We’re back again with another, more thorough, household bulb test. We’re testing 2.2W, 5.7W, 10W (Ikea PAR30 Grow Light) LED bulbs as well as a 2x14W T5 (CFL) fixture. Here's our first LED bulb PPFD test. The goal with...
www.ledtonic.com
ledtonic published some tests, as you can see these 2 14W T5 put out just a 100 ppfd directly underneath them at 1".
you may get a idea how little ppfd you provide to your plants by using these cfl.