I'm constructing an outdoor vivarium for my rescued Grey Tree Frogs. They were tadpoles in a garbage bin and now they are ready for release, however, nearby construction has eliminated many tree and lack of habitat has stirred up frog eating animals. I've grown attached to my little friends. They will overwinter in a new habitat which will stand 6'x2 on pilings 2' about ground. There will be suitable screening and protection against raccoons, snakes and birds. I am going to put native plants in this "cage" but dirt will only be 4" deep with loose leaf on top of ABG substrate. What shrub, grass or plant will withstand Zone 7 temperatures yet have root structure that can survive in only 4" of substrate? I'm wondering what else I can use in this structure that will provide protection against heat and winter cold.