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I use the 4ft 8 bulb T5 for a grow. With 5 Hashberry under them, I hardly have enough light. I had to add 2 single 4' T5's for the sides and that didn't quite cut it either. Like StinkyAttic recommends, I have a very thick canopy and have to turn the plants daily to ensure adequate light coverage. They are 4 weeks into flower but boy is it a struggle with the T5's. Now for veg, they were awesome. Make sure you use the 6500's for veg and 3000 mixed with 6500 for flower. I am using 6 3000 and 4 6500 for flower. This is my first T5 grow. I will keep everyone posted.
Of course flowering is going to be hard if you don't have enough lumens. You have max 50,000 lumens for 5 plants not to mention and oddly shaped light that has a rectangular array. I would grow at max 2 plants under one t5 8 lamp fixture. I am ordering 2-3 8 bulb fixtures for a 4x4 4 strain scrog soon. I am going to use Nlite bulbs most likely to maximize the amount of usable light, also you can get a better spectrum, closer to the absorption spectrum of cannabis, than with HID. That's not an opinion, thats going off spectral distribution.