I am looking for shallow root (4" or less) plants of about 3-4' for our ecoroof. Planting material is pumice. Now have sedums planted but also want larger plants in background.
This is pretty late to the party for replying to your message, but I will offer advice anyway. I am not an expert and I don't know anything about ecoroofs. However I would think that some ornamental grasses or sedges could do the trick nicely. Depending on your garden zone and winter temps, maybe some larger succulents like agave, aloe, or cacti? Also, perhaps a non-invasive variety of thistle?
Just some crazy ideas