While green roof systems have proven to be highly effective in the evaporative cooling of buildings, reduction of roof top temperatures, protection of roof membranes from solar radiation degradation, reducing stormwater runoff, as well as beautification of the urban roof top landscapes throughout Europe and in several regions in North America, green roof systems have not been evaluated in the high elevation, semi-arid regions in the United States. Because of the risk of plant failure from incorrectly selected species, the paucity of information on green roofs in this region, and the large potential for environmental benefits, studies were conducted on various performance parameters on the green roof of the building that houses the EPA Region 8 Headquarters in Denver, Colorado. Green roofs are vegetated roof tops. Green roofs provide several benefits to urban environments, including reduction of stormwater run-off volumes and intensity, filtration of stormwater discharge, reduction of the urban heat island effect, temperature moderation within the building underlying the green roof, and beautification of urban roof top landscapes. In order to provide these benefits, the green roofs must receive sufficient amounts of water and nutrients to keep the plants alive. In the semi-arid, high elevation environment of the Front Range of Colorado, green roof plants have not been scientifically tested for long term survivability and adaptability. The low annual precipitation, short periods of snow cover, low average relative humidity, high solar radiation (due to high elevation above sea level, approximately 1.6 km), high wind velocities, and predominantly sunny days all add up to challenging growing conditions for many species of plants. Due to the porous and well-drained nature of the typical growing media used in extensive (shallow) green roof systems, plant species considered for use in such systems need to be evaluated for their response water requirements and survivability and growth habits over multiple years. Thus, relative rate of dry down of the moisture content of the media for plant species considered for use in such systems is an important characteristic to assess. In semi-arid regions, such knowledge will help to determine the need for irrigation and the frequency of irrigation events for these species.