Coat the foam with Cement and then seal it with cement sealer, or grout sealer, or clear coat for painting. . after it has dried for a while use a small old drill bit and hand drill the holes just bigger than the plant stems and then fill the holes with GE Silicone I Caulk. Don't use the GE Silicone II it has a anti-fungal, anti-bacterial chemical that is really bad for reptiles.