Dan is right. At a minimum of 400*F curing temp for powder coating, the plastic flower pot probably looks like the witch in the Wizard of Oz when she was melting. I powder coat most everything I can, but not that part. Might give the innards a really good inspection and compare to the pictures of the flower pot on this site.
FYI, I bought some of that asphalt/tar material from the Classic Center. I believe it was around 3/8" thick. There are many others who carry it.