Selecting Plants for a Green Roof
There are many factors to consider during the plant selection process for a green roof system. The type and purpose of the green roof is especially important. The desired visual look and level of maintenance which will be required must be considered. The plants selected should be tolerant to the weather conditions of the specific geographical area.
Extensive green roofs are usually limited to moss, grasses, herbs, and sedums due to the low media depth. Perennials with low growth height are often selected as they have shallow roots. Sedums have fibrous roots that retain water, survive in a drought, and hold it in place during wind or rain. Alpine plants also have been selected as they are very tolerant to hostile weather conditions and exposed in a similar way to plants on a rooftop. Drought tolerant plants are typically more resistant to weather conditions and fit well with the standard low maintenance requirements of an extensive green roof system.
Intensive green roofs permit a larger variety of plants due to their increased media depth. The increased media depths of intensive green roofs allow for deeper root growth and provide more nutrients and water. Depending on the media depth, intensive green roofs can support plants, shrubs, and trees. A broader plant variety increases the amount of maintenance required. Meadow moss, grass, and wildflower perennial vegetation is often chosen as they are fairly tolerant to most weather conditions. Intensive green roofs which support large varieties of vegetation often require irrigation systems to supplement natural water supplies.
Weather conditions are often magnified on a roof top as a roof’s exposure level is high. It is important to consider a plant’s ability to tolerate weather fluctuations, humidity, temperature, rainfall, frost, sun/shade, and drought.
During the plant selection process it is important to consider the desired visual appeal of the green roof. The appearance of a green roof will vary from season to season. During the winter or periods with little rainfall, certain vegetation which may have been bright and colourful during the summer season may appear grey and dull. Evergreen vegetation can be included in a green roof system which stays green year-round.
Weight loads are important when considering trees and shrubbery for a green roof system. A retaining wall can be implemented in an intensive system to increase the media depth surrounding a tree. Trees are more likely to tip and be susceptible to wind pressure than other vegetation. The size of a tree’s leaves and crown determine how susceptible it is to wind. A tree usually has to be secured into the green roof system through cabling around the root ball in order to provide extra support; this is referred to as anchoring.
Determining factors:
- Weather/Climate conditions: Temperature, humidity, and rainfall. Tolerance to drought, frost, temperature fluctuations, sun/shade.
- Visual appeal during all seasons.
- Maintenance requirements (irrigation, weeding, nutrient supply, perennial vs. replanting).
To discuss the green roofing solutions provided by Godfrey Roofing Inc, please request a free no-obligation estimate or visit the Ottawa green roofing section for more detailed information.
