High above ground level, invisible to many of us, green roofs are present in almost all climate zones mimicking nature, helping clean the air, cooling down temperatures, and keeping rainfall on-site, alleviating pressure on urban stormwater systems.
Green roofs, also known as vegetated roofs, are classified as either intensive or extensive.
Intensive roofs feature a variety of plants in a heavy and deep substrate layer of 10 inches or more.
Intensive green roofs may incorporate many sizes and types of plants found on ground level including shrubs and trees,
can include planters, benches and fountains, and are usually used as a rooftop amenity space.
Extensive roofs are lighter in weight due to shallower substrate (1-6 inches) and typically feature vigorous, low-growing, drought tolerant plant species such as sedums and mosses. Extensive green roofs are an economical stormwater management solution, easier to install, remove and repair, require less maintenance than intensive systems, and may be retrofitted on many existing buildings.
Green roofs, whether intensive or extensive, experience the same cycles of growth, flowering and dormancy that occur to plants on the ground level. Vegetated roofs that are properly designed for climates with four seasons, correctly installed and routinely maintained, have a greater probability of survival no matter the season.
Best autumn and winter practices for extensive vegetated roofs
Extensive vegetated roofs are a prevalent green infrastructure technology throughout North America, yet many questions arise as to their aesthetics, performance and survivability in winter climates. Some people question whether green roofs die in the winter or whether snow damages the vegetation.
Others want to know whether the roof can be walked on in winter months or if vegetation needs to be re-seeded in the spring.
Extensive systems predominantly use hardy sedum species that can withstand harsh rooftop conditions year-round. Sedum are drought tolerant and thrive in harsh conditions. The best suited rooftop plants are pre-grown outdoors locally, for several months or a year ensuring they are acclimatized to the local climatic conditions. Ensuring that the vegetation is mature upon installation offers a higher probability of survival. In colder plant hardiness zones or in areas where there are extreme fluctuations in temperature, green roofs require a deeper depth of engineered growing medium to help mitigate the effects of intense temperature and moisture changes.
Additional growing medium acts as a thermal mass helping moderate the temperature and further hydrating the plants – helping reduce significant die-back in the winter or during hot summer droughts.
In Autumn, as winter approaches, green roofs enter the dormancy cycle. As rooftop conditions are harsher than at the ground level, the exposed vegetation on rooftops enter into the dormancy cycle earlier than at ground level.
Dormancy is a natural reaction to adverse environmental conditions. It can happen in the summer during periods of intense heat and drought, or in the fall in preparation for the coming cold winter months.
During dormancy, plants simply stop further growth and development to conserve energy. Dormancy is a plant’s defence mechanism to keep itself alive. The retreating plants are not dead.
Dormancy synchronizes with the environment and can be triggered by a temperature drop or sudden changes in climactic conditions, such as reduction in rainfall and water shortages. Green roofs lose their flowers and change colour creating a beautiful landscape of reds, bronze and browns, to deep purples.
During winter dormancy, coniferus sedum plants retreat to form a dense mat of glossy and fleshy leaves, while the leaves of deciduous sedum species completely fall off.
Many vegetated roofs are installed in the Autumn due to the construction schedule and desire to finish the project. In such cases, it is critical to have mature and well established plants as they have greater survivability on rooftops than plants that are immature seedlings.Pre-cultivated systems should have a minimum of 80% coverage upon installation to ensure there is blanket coverage and minimum erosion of growing medium. The pre-cultivated vegetation may have been disturbed during harvest, transport and installation. Therefore, after installation, it is imperative to keep the vegetated system hydrated during the establishment period for the plants to recover and take root and prepare for the coming winter.
SNOW, WIND & EXTREME TEMPERATURES
Once cold temperatures set in, the vegetated roof will go into dormancy and prepare for winter. The blanket of pre-cultivated vegetation shelters the growing medium from erosion from the strong, cold winds. Snow accumulation is ideal as it insulates the vegetation. A blanket of snow shelters the vegetated roof from strong winds and helps the plants retain moisture. If there is little or no snow accumulation, and the vegetated roof is subject to high winds, extreme fluctuations in temperature or a particularly severe ice storm, plant desiccation, or “winter burn” can occur. Desiccation a type of injury where the plant dries out and dies. This may expose areas of growing medium that may be subjected to erosion by high winds. If such damage occurs to the vegetated roof, a straightforward procedure for repair can be followed the following spring. This repair procedure also applies to other vegetated roof repairs due to excessive foot traffic or due to construction materials left on the plants forextended periods of time.
The repair is done in the spring using a variety of sedum clippings sprinkled, where needed, to replace the sections that have been damaged or eroded. Succulent plants such as sedums are easy to propagate. Many can be rooted from a single leaf, others will root quickly from a stem cutting.For areas that suffered erosion, additional engineered growing medium should be added, as required. After the initial planting of clippings, frequent, light irrigation is required to keep the vegetated roof moist and help ensure the plants take root.
FALL & SPRING MAINTENANCE
Year-round maintenance is key to ensuring long-term survivability as well as the stormwater performance of the vegetated roofs. One last maintenance check should be done in the autumn as part of the yearly routine. Vegetated roofs are also known for their ability to protect the roof membrane, however, not taking proper preventative measures can also lead to unforeseen problems.
Autumn maintenance includes the removal of debris and inspection of all drainage paths.
In Autumn, it is best to remove fallen leaves and twigs from the green roof. Thick layers of leaves and twigs can also stick together to form an impenetrable mat and may lock in too much moisture, potentially causing rot or damage to the plants.
nlWeed, one last time, before winter sets in. In an effort weed control, make every effort to capture the seed pod intact and tuck it away for disposal to avoid seeds blowing away. Some weeds may be tolerated, therefore maintenance personnel should be familiar with green roof plants and the owner’s green roof aesthetic preference.
Keeping the vegetated roof hydrated into the Autumn months helps the vegetationavoid winter’s freeze-drying effect caused by low temperatures and high winds.
If there is an irrigation system, it needs to be winterized around October in nothern States as frost typically sets in soon after. Maintenance crews typically use an air compressor to blow out the water from the irrigation system.
On ground level, perennials tend to be cut back. On the rooftop, however, tall grasses or wildflowers are ideally cut back the following spring. This encourages re-seeding and the stalks give winter protection to bees and other fauna. Snow-covered stalks also offer a pleasing visual aesthetic.
Vegetated roofs should not be fertilized past August, especially with a slow release type fertilizer. Fertilization may stimulate tender growth and compromise the hardiness of the plant. Secondly, plants won’t have a chance to fully take up the nutrients and the combination of high temperatures may cause the plants to burn. Slow release fertilizer is usually applied once, ideally in the spring.
Assess the performance of the green roof in the autumn and prepare for next spring. It is a good idea to document your visit with photos and record recent weather conditions for future reference and any potential warranty claims.
In cold climates, particularly when temperatures drop below freezing, foot traffic over vegetation must be avoided as it can prove lethal, leaving foot-shaped patches of dead plants. The damage will be obvious the following spring and recovery may take several months or may not happen at all.If pedestrian traffic is unavoidable, protective measures must be taken to diminish the impact of damage. Materials like plywood sheets or insulation boards may be laid over the vegetation temporarily to help distribute the weight on the green roof, and removed at the end of each day to allow plants to recover.
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