Extensive green roofs:
- Mat-forming species of Sedum, Sempervivum, and moss.
- Sedum acre, S. rupestre, and S. album.
- Delosperma could be tried in a sunny frost-free area.
- Polypodium vulgare and Asplenium trichomanes could be used in dry shady conditions (Ferns)
Intensive Green Roofs:
- Drought resistant plants
Basic Understanding of Water Uptake in Plants
It is important to understand the process of water uptake in plants and how it contributes to evapotranspiration. All plants go through these two basic steps:
- Every plant has a root system that allows it to uptake water through permeable structures called fine roots
- As the roots migrate towards wetter areas through a process called hydrotropism, they uptake the water which goes through the xylem and out through the stomata
- During this process of stomata opening, carbon dioxide gas is exchanged with water leaving.
How does this relate to green roofs?
Plants on a green roof go through the same process of transpiration just like any other plant. However, when transpiring, water is released in an exothermic process that cools the plant. The plant becomes cooler and carbon dioxide is taken in. This means collectively, all of the plants lower the temperature of the roof and collect carbon dioxide, which are two contributions that this process gives to the benefit of a green roof. Furthermore, this refers back to the previous list of plants because generally, plants that are best suited for green roofs are usually those that can grow in a few inches of soil, be drought resistant, and have rapid growth. An example from the list above is succulents, such as sedum, which utilizes the crassulacean acid metabolism (CAM). This process allows them to open their stomata at night and allows for photosynthesis during the day which is the primary reason for the drought-resistant characteristic.