![]() Plants that utilize CAM open their stomata (pores used in photosynthesis) at night, taking up carbon dioxide, and keep their stomata closed during the day while they photosynthesize the carbon dioxide. Succulents are one type of plant that utilizes crassulacean acid metabolism (CAM). Succulents, such as sedum, are often chosen for extensive green roofs because they withstand harsh conditions and minimize water loss. Plants for extensive green roofs should have low growth height, rapid growth/spreading, and fibrous roots that have high drought tolerance. Low growth height, water retention, and a hearty nature make these plants ideal for green roofs. Various plants used for extensive green roofs. Although no green roof is completely maintenance free, with regular attention it should last a long time. While most green roofs do not need irrigation systems, a temporary irrigation system or watering plan should be in place to nurture the plants until they are established or when there is a drought. Thus, the roof should be weeded periodically to remove invasive species and to prolong the life of the plants and growing medium. Weeds and other plant seeds are likely to be deposited on the roof by birds, insects, and the wind. Regardless of the plants selected, a green roof still requires maintenance that a traditional roof does not. Therefore, it is important to choose plants that will not substantially increase in height when planning a green roof on a historic building. ![]() Vegetation growing above the roofline may negatively impact the historic character of the building. Some plants that are low to the ground when first installed on a green roof may grow to be quite tall over time and, thus, visible from public view. ![]() The size and expected height of vegetation when mature should also be taken into account. Plantings that will grow taller than the roof parapet may negatively impact the historic character of the building. In the middle of the block are plantings that are highly visible from the street. All of these factors, including initial and continuing maintenance costs must guide plant selection. If the roof is to be used year round, consideration should be given to balancing plants that bloom, that are green all year, or that turn brown during the winter. A landscape architect or accredited Green Roof Professional should also be involved in plant selection and the roof design, especially if the roof is to be open to the public. ![]() Equally important in plant selection is the purpose of the green roof and the reason for installing it, whether it is storm water management, energy conservation, heat island reduction, aesthetics, or a combination of these factors. Soil depth and whether the roof will have initial or continued irrigation will also need to be factored in. It is always best to choose hardy, low–maintenance plants that can survive mainly on rainwater and require no special irrigation or nutritional needs rather than high–maintenance specimens. If using native species, indigenous plants should be selected that have worked well on green roofs and ensure they are watered and maintained according to their specific needs. It is also important to be aware that a separate microclimate with strong winds and extreme temperatures can sometimes exist at roof level of high rise buildings. Climate clearly plays a major role in choosing the best plants for a green roof. ![]() The plants chosen must be appropriate for the roof type-extensive, intensive, or semi-intensive. Along with the makeup and depth of the growing medium, selecting the right plants is crucial to the success of a green roof. ![]()
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