I'm having pretty much the same problem; actually there are a couple of things you can try if you think there root bound. You could try leaf feeding and use only water for direct feed and make sure your waters not clorinated by leaving it out for 24 hrs before watering and see what happens. Myself I think youv'e got a watering problem (to much maybe to often) are you letting her dry out between waterings?

I am currently having a few issue's also. My plat i s VERY healthy but seems to be growing slow. lights are not the issue and the dirt around the sides of the pot has space between it and the bucket, no roots out the bottom. I have been super sropping it though.. Anybody...

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Current Grow-Northern Lights