When creating a green roof, it is naturally a good idea to chose plants wisely and pick species and variants that are suitable for the prevailing conditions, such as climate and how much room that’s available.
For vegetated roofs in subtropical climates, plants from the Crassulaceae family are very popular since they don’t have any big requirements when it comes to watering. Also, they used the eponymous crassulacean acid metabolism (CAM), where carbon-dioxide that enters the stomata (plant pores) during the night is converted into organic acids, creating carbon-dioxide that will be used for the Calvin Cycle during the day when the stomata is closed. Having the stomata close during the day makes these plants more capable at surviving warm days in arid conditions.
As can be expected, Crassulaceae species tend to display xerophytic adaptations, such as thick and reduced leaves with a low surface-area-to-volume ratio. The cuticle is often thick and the stomata can be sunken into pits.
Crassulaceae species are generally herbaceous, but some species will grow into subshrubs, and there are also a few that will become treelike and thus be unsuitable for some types of green roofs.
The family Crassulaceae contains about 1400 named species, divided into roughly three dozen genera.
One example of a well-known member of the Crassulaceae family is Kalanchoe blossfeldiana, a herbaceous plant with bright colored flowers that can be anywhere from dark red to pink, orange, golden or white. This plant is known under many different names in English, including Christmas kalanchoe, Florist kalanchoe, Flaming Katy, and Madagascar’s Widow’s-thrill.
The Christmas kalanchoe is native to Madagascar, but is today grown world-wide as a house plant. It’s a popular choice as a potted plant since it requires very little care, and this trait has helped making it a popular plant for low-maintenance green roofs as well. As a succulent plant, it is drought resistant and can handle sunny days without wilting. In its native Madagascar, it grows in humus soil in the comparatively cool plateaus of the Tsarantanana Mountains. The Christmas kalanchoe is not a good choice for a green roof if there is a risk of the temperature dropping below 10 degrees C.
The Christmas kalanchoe is a perennial and bushy plant that remains green year round, and it rarely exceeds 45 cm in height. The ultimate spread can be up towards half a meter, so it is a good idea to give it plenty of room. The growth rate is slow, however, so there is no need to rush – it can start out in pretty crowded growing conditions if you are willing to give it more space eventually.
The foliage is textured and shiny, with scallop-edged ovate leaves that are 5-10 cm long.
Even though the plant is green year round, it only flowers from late fall to early winter.
The Christmas kalanchoe is a sturdy plant that rarely succumb to disease, but some pests can pose a problem for it, especially mealybugs and vine weevils.
Another well-recognized member of the Crassulaceae family is Crassula ovata, a plant native to South Africa and Mozambique that has become a popular houseplant around the world. Some people keep it as a bonsai. Just like the Christmas kalanchoe, C. ovata is low maintenance and easy to care for.
In English, C. ovata is known as Jade plant, Friendship tree, Lucky plant and Money plant. Using the name Money plant (or Money tree) can cause confusion, since this English name is also used for the Pachira aquatica tree.
The Jade plant requires little water in the summer and even less during the winter. It grows best in rich, well-draining soil. Over-watering will cause problems, especially if the soil doesn’t drain well. This is a succulent plant that is very good at storing and preserving water. Both branches and leaves are thick, and the leaves stay green year round. The leaves are of a rich jade green color, hence the name Jade plant. If a red tinge developed on the edges of the leaves, it means that they have been exposed to very high levels of sunlight.
The flowers are pink or white. A period of cooler and drier conditions, preferably also with long nights, can be required to coax the plant into flowering.
Propagating this plant is easy, since you can use:
- Leaves that have dropped from the plant
Propagation attempts with clippings or leaves tend to have a higher success rate than cuttings. Leaves can be placed directly on top of (or gently pushed down into) the soil. You can expect new roots to form after four weeks.
In the wild, the most common way for the Jade plant to form new plants is to detach branches; the branches fall to the ground where they take root and form new plants.
Pests & Diseases
The Jade plant is generally resilient to pests and diseases, but just like many other succulents it is susceptible to mealybugs / scale insects, and an attack can cause the plant to be deformed.
Many Crassulaceae species are unsuitable for vegetated roofs in temperate climates, but the genus Sedum is a notable exceptions. In this genera you can find plenty of species that prefer temperate conditions over subtropical ones (especially if the subtropical climate is humid). It’s important to research the individual species or cultivars you are interested in, because not all Sedum plants are suitable for a green roof in a temperate climate, especially not a cold temperate climate.
Examples of notable green roofs planted with Sedum plants:
- The living roof on Ford’s Dearborn Truck Plant in Michigan, USA. This roof features 450,000 square feet (42,000 m2) of Sedum plants.
- The roof of the Jacob K. Javits Convention Center in Hell’s Kitchen, New York City, have 292,000 square feet (27,100 m2) of Sedum.
- Rolls-Royce Motor Cars’ plant in Goodwood, England have a 242,000 square feet (22,500 m2) roof complex planted with Sedum plants.
- The headquarter for Nintendo of America – located in Redmond, Washington State, USA – is covered in approximately 75,000 square feet (7,000 m2) of Sedum. The fourth floor of this building is part conference room space and part walkable living roof.
The genus Sedum contains several hundred named species.
In the wild, Sedum plants are chiefly found in the northern hemisphere, but their range does extend into the southern hemisphere in Africa and South America.
Sedum plants are leaf succulents and can store an amazing amount of water in their leaves. There is a lot of variation within the genus, from annual herbs to perennial shrubs, which should be taken into account when you chose species for your green roof. Some are creeping while others grow more upright.
Examples of well known Sedum species from Europe are White stonecrop, Corsican stonecrop, Spanish stonecrop, Goldmoss sedum, and Stone orpine.
Sedum as food
Many Sedum plants can be included in a living roof kitchen garden since they are edible, but it is important to research each species because some, such as Sedum rubrotinctum, is toxic to humans.
Stone orphin (S. reflexum) is good in salads, when you want to add something with a slightly astringent sour taste.
In North America, S. divergens was included in the diet of First Nations people, especially the Haida and the Nisga’a, at the point of European contact.