Last semester Evelyn and I were in a group working on a project that dealt with expandable foam. It is a two-part solution, and when mixed, the liquid expands into foam about 15 times its original volume as rigid or soft foam.
The rigid expandable foam is very strong and light, it works well in compression, and usually are used for flotation devices in the ocean. We also experimented with adding some materials into the foam. The rigid expandable foam is very strong, and mixing some kind of soil or nutritional substance would eliminate the structure+soil system, and instead has a system that is both structure and soil. There are some down sides to the foam, it is kind of ugly. But they could be covered with paint, or once the plant starts growing. These structures are very fast to cast.