Like in most things UV degrades most things, but any Styrofoam that is UV resistant is likely to be expensive. But I have never herd of any problems using regular home Styrofoam insulation, in fact the Styrofoam around one of the trash cans I used as a reservoir last summer has been sitting outside in direct sunlight all year and has no discoloration from UV yet. If you were worried I would just use a spray adhesive and line the top with a white plastic trash can bag. Personally I would not even bother. Even if I was worried, as the plants grow they will shade the top of the Styrofoam anyway. I get my Styrofoam from the local home improvement stores, they use it for home insulation. Here it comes in 1/2 inch, 1 inch, and 2 inch thick sheets.
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