I came across the same problem just a few days ago. I'm essentially in the 3rd week of veg and I was wondering why my plant wouldnt grow any more in the ~5 in. pot I had it in. Took the plant and soil out of the pot and found out it was COMPLETELY root-bound. Transplanted my baby into a 5 gal pot and shes finally growing again!

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Sometimes life gives you lemons. Me? I say fuck the lemons and give me some goddamn weed NOW motherfucker!