Green roofing the way to go in the Vancouver project
Main image: Photo courtesy Vitaroofs
When dealing with extreme weather such as cold winters or drought, green (vegetated) roofs are ideal as these shallow-rooted plant materials can withstand extreme climatic conditions on top of buildings. These assemblies have numerous potential benefits, from absorbing rainwater run-off and providing insulation, to mitigating the urban heat island effect and providing green space.
Choosing the ideal plants
Intensive vegetated roof systems are thicker and can support a wider variety of plants but are heavier and need more maintenance than extensive assemblies, which are covered in a light layer of vegetation. It is thus essential to ensure that the right plants are used.
The ideal plants for vegetated roofs tend to be hardier species that can be grown almost anywhere. They do not require a specific combination of soil material, need little water and some even thrive out of stones.
Native plant material
Green roofing consultants advise using plant material native to the project’s locale. This helps to avoid unnecessary plant death by way of establishment and/or if the flora is being transported from far away. Native plant species have also already been adjusted to that location’s climatic conditions, making it easier for them to get established quickly.
While design and construction professionals don’t need to be botanists or horticulturalists, having a background understanding of the subject can be invaluable when working with others to ensure the ideal green roof is being specified.
Implementing essential knowledge
The critical knowledge of rain capture and storage plays an important role in plant survivability on a non-irrigated roof as well. Manufacturers often need to provide a survivability certificate on the type of plants being used on a particular roof and their watering requirements, with information if they are drought-tolerant and can survive in heavy rainfall.
Plant selection, survivability and environmental issues
Green roofs are planted with sedums such as Sedum album, S. acre, S. kamtschaticum, S. rupestre and S. spurium. Most sedums are crassulacean acid metabolism (CAM) plants, which means they survive through a photosynthesis method that has carbon dioxide (CO₂) fixed by an enzyme such as phosphoenolpyruvate carboxylase (PEPC) during the night to form malic acid that is stored in the plant cell vacuoles, keeping the stomata closed the next day.
In other words, CAM plants are characterised by their water use efficiency, making them ideal for rooftop environments where they are unlikely to be frequently tended to by a landscaping team.
It has long been established that sedums do not like excessive water, marking them as self-sufficient, drought-tolerant plant species. On a sloped roof, irrigation requirements are critical because the system dries out faster than on a flat surface. Transferring mature and well-established plants on a roof is more beneficial for survivability than planting young seedlings.
Vegetative roofing providers are mainly interested in how to successfully grow plants to ensure a smooth transition from the field to the roof.
Physiological aspects of drought adaptations
Plant response to drought stress can be classified in three categories:
Drought escape (avoidance): Plants that “escape” drought successfully reproduce before the onset of severe stress.
Dehydration avoidance (postponement): Dehydration avoidance can be defined as where a genotype has a higher relative water content than another genotype when both are subjected to the same stress.
Dehydration tolerance: Dehydration tolerance is a plant’s ability to maintain its function when the relative water content decreases. It can also be defined as the critical relative water at which the plant function stops or starts, such as leaf death.
Survivability of plants in the project
It is important to take note of the plants’ survivability. It is critical to ensure that a green roof will not be saturated with water during the winter. Plants that are acclimatised to cold temperatures will have enough reserves to revive their normal growth features in the spring.
Plants need energy for their growth and development to complete their lifecycle promptly. The remaining green parts, flowers and seeds in winter plants survive by producing sugars, which will be used as reserves when under the snow cover to keep them growing to return to their normal growing activities in the spring.
People working for the green roof industry need to have adequate knowledge to understand the effects of cold and drought on the mechanisms of the growth and development of sedum plants.