They are actually tall, square plastic pots that had small rose bushes in them. I did use milk cartons with my first bud producing grow back in '78. I used a carton full of water to act as a mold in the 5gal pails I transplanted them to by filling the pail to the height I wanted then placing the carton in and filling around it. Then watered it well and carefully pulling the carton out and replacing it with the plant in it’s exact sized rootball. Rootsquare? 
This is what the roots looked like before I sawed off the stringy ones.
