Relative to a green roof, what are the differences between using hearty succulents and grasses or perennials?
Many succulents operate with Crassulacean Acid Metabolism or CAM. In plants which use CAM, carbon dioxide is taken in through stomata with the simultaneous loss of water vapor. By only requiring the stomata to be open during the (cooler) night for carbon dioxide uptake and keeping them closed during the (hotter) day, the plants lose much less water. In extremely arid conditions, some CAM plants can permanently close their stomata so preventing any water loss. During these times, the plant growth might be inhibited, but the plants can can maintain the healthy state of the cells. What this means is that for an non-irrigated roof, non CAM plants such as perennials and grasses will be much more susceptible to heat and drought. Grasses & perennials generally have more mass and generally require more soil depth than succulents. These plants are designed by nature to build biomass and this additional mass creates additional requirements for the roof as compared to succulents. Such requirement
