This is an herb spiral, in which you plant drought hardy plants at the top and water-thirsty plants at the bottom. By adding wood, it mimics hugelkultur, in which buried wood adds nutrients and reserves water -From Grow Food Not Lawns
If you are not familiar with th ese, you are probably wondering, "What in the world is an herb spiral garden?" It is a type of her b garden that allows you to: Grow more plants in a smaller space. Grow herbs according to their needs and in varying soil.