It's all according to what kind of product they are putting out in the store. If it's shake then yea I can see people growing it at the house. If they actually put good strains out at a competitive price then that's different story. Growing is complicated and takes time if your wanting quality. Inside the house takes chemicals and light schedules while outside you would have to worry about your neighbor topping your crop. To much hassle when you could just go down to the 7-11 and getting a carton of green stick icky.
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