Why have we chosen EPS (expanded polysytrene) over hard plastic?  "Styrofoam" is a version of polystyrene and has gotten a bad rap over the years.  Most people don't realize that the environmental affects of making hard plastic are worse  than EPS.  There are more steps in the manufacturing process, therefore more toxins exposed to the environment.  Expanded polystyrene requires less processing and holds up better in the sun.  It also insulates the roots much better from heat and cold.  Hard plastic pots have no insulating properties and will break within a year or two of being outside.  EPS was recently approved by the EPA for organic growing.  There is no leaching into the roots,  therefore, EPS is a safe, long-lasting material for successfully growing plants.