Extensive green roofs in Colorado: Plant species performance, growing media modifications, and species response to growing media dry down

Green roofs provide many benefits and are often used to help alleviate the negative effects of urbanization. In order to provide these benefits, green roofs should remain alive and viable. Therefore, a series of studies were performed to elucidate some performance characteristics of extensive (shallow) green roofs.;Plant area covered was examined for six plant species on an existing modular extensive green roof in semi-arid Colorado. Species evaluated were Antennaria parvifolia Nutt., Bouteloua gracilis (Kunth) Lag., Delosperma cooperi (Hook. f.) L. Bol., Eriogonum umbellatum Torr. aureum 'Psdowns', Opuntia fragilis Nutt. and Sedum lanceolatum Torr. Most reported methods for measuring plant area covered (plant cover) are subjective and not based on quantitative measurements. This study compared digital image analysis data (DIA) to manually collected converted two-dimensional data (C2D) for plants grown on an extensive green roof. For each plant in the study, digital images and manual two-dimensional measurements were taken on four dates (at six week intervals) in 2008 and on four dates (at six week intervals) in 2009. Using SigmaScan Pro 5.0 image analysis software, DIA was performed on these images. Additionally, comparisons between DIA data and final biomass, and C2D and final biomass, were performed. Plant cover increased for all six species during the 2008 growing season. However, E. umbellatum aureum 'Psdowns' had a low overwintering rate (12.5%) and was removed from analysis in 2009. In the spring of 2009, four of the five remaining species exhibited decreased plant cover due to winter dieback; the one exception was O. fragilis. In terms of plant cover, both quantification methods (C2D and DIA) revealed that B. gracilis and D. cooperi out performed A. parvifolia, O. fragilis and S. lanceolatum. Thus, five of the six species evaluated in this study are appropriate for use in extensive green roof applications. High levels of correlation were found between the DIA and C2D data sets ( r = 0.77) averaged over the five species on all eight data collection dates. The groundcover species (A. parvifolia, D. cooperi and S. lanceolatum) had a higher correlation on average (r = 0.83) than the upright (B. gracilis , r = 0.70) and decumbent (O. fragilis, r = 0.65) species. Additionally, DIA and final biomass correlations showed parallel trends with groundcovers averaging r = 0.83, upright r = 0.64 and decumbent r = 0.41. Therefore, using DIA to evaluate plant cover and biomass accumulation is especially appropriate for groundcover species.;Success of an extensive green roof is primarily dependent on plant species ability to survive the low moisture content of the growing media. Due to the well-drained nature of the growing media, plants adaptable to dry, porous soils are primarily used in extensive green roof applications. Although Sedum species have dominated the plant palette for extensive green roofs, there is growing interest in expanding the plant list for extensive green roof systems. In order to effectively select suitable plants, species need to be evaluated in terms of their response to gradual and prolonged dry down of the growing media. A study to determine the relative rates of dry down for fifteen species was conducted in greenhouse and outdoor trials. During dry downs that extended over five months, succulent and herbaceous species dried down at different rates. Although, not all succulent or herbaceous plants had consistent moisture contents during the initial 18 days of dry down. Despite differences in dry down, the succulent species, as a group, maintained viable foliage for over five times longer than the herbaceous species. The revival rates of the succulent species were nearly double those of the herbaceous species. Therefore, not only are succulent species more likely to be longer-lived during periods of drought, but these species are more likely to resume growth soon after water is made available. Based on these results, irrigation frequency is recommended for succulent species at a maximum of 28 day intervals and herbaceous species at maximum of 14 day intervals. (Abstract shortened by UMI.)...