Expanding the Plant Palette for Green Roofs

摘要

Fifty-four percent of the world's population lives in urban areas and this number continues to grow (United Nations, 2014). Dense development along with the accompanying increase in impervious surfaces can have harmful effects on humans, wildlife, and the earth. Green infrastructure such as green roofs can improve urban areas by introducing plant life to otherwise barren rooftops, thus reducing the negative impact humans may have on the environment. While green roofs are a historically old practice, they are relatively new to North America. Because of their novelty, we are still developing best management practices and discovering what plants are best suited to rooftop environments. If the green roof industry is to grow, the list of suitable plant species must be expanded and this information must be disseminated to green roof practitioners and the general public. Because climate differs from one region to the next, data on plant performance for each climatic region is needed to properly specify green roofs within that region. A rooftop experiment was conducted on the Molecular Plant Sciences Building at Michigan State University in East Lansing, Michigan, USA, to test the suitability of several succulents, herbaceous perennials, and grasses in varying depths of green roof substrate over a seven-year period. Forty-five percent of the species were no longer preset on the roof by the end of the study. Still, several species emerged as successful, as defined by their ability to establish themselves on a green roof and long term survival. A reference guide to green roof plants was also compiled based on research from 11 studies conducted in the Great Lakes Region that tested plant performance. The result is a database of 80 plant species that have been successfully grown on green roofs in the region and lists traits such as plant growth habit, size, hardiness, drought tolerance, wildlife attractiveness, and other attributes. The reference guide is mostly composed of herbaceous perennials (65%) and succulents (17%). Based on the 11 studies used to compile the plant database, the average minimum depth required for survival of all species tested was 13 cm. Required minimum depth was less for succulent species alone. In addition to plant performance, I also explain why certain groups of plants performed well and how plant selection influences the desired benefits of the green roof. This reference guide can be used by green roof professionals in the Great Lakes region and in other parts of the world with a similar climate (Koppen climate types 'Dfa' (hot summer humid continental) or 'Dfb' (warm summer humid continental)).