Extensive Green Roof Species and Soilless Media Evaluations In Semi-Arid Colorado

In the high elevation, semi-arid climate of Colorado, green roofs have not been scientifically tested. This research examined alternative plant species, soilless media blends and plant interactions on an existing, modular-extensive (shallow, 10 cm deep) green roof in Denver, Colorado. Six species were evaluated for potential use in green roof systems. Antennaria parvifolia, Bouteloua gracilis, Eriogonum umbellatum, Opuntia fragilis, and Sedum lanceolatum are natives of shallow, rocky, well-drained locations in Colorado and Delosperma cooperi is native to similar conditions in South Africa. Media blends evaluated include Green Grid® media (Weston Solutions, Inc.) and Green Grid® media plus various percentages of ZeoPro™ H-Plus (ZeoponiX, Inc.) to increase water and nutrient holding capacity. Media blends were evaluated for plant species growth performance and water holding capacity. Plant species used in the media study included Sedum acre, S. album, S. spurium ‘Dragons Blood’ and S. spurium ‘John Creech’; these species were already in use on the existing green roof. The plant interaction study utilized the six alternative species plus Allium cernuum and Sempervivum species in mixed plantings in trays that contained the Green Grid® media or a 1:1 mixture of the Green Grid® media and ZeoPro™ H-Plus. All experiments were in a randomized complete block design (trays for experiments on the green roof were mixed in with the existing trays). The alternative species study had species as a whole-plot effect and date as a repeated measures effect, while the soilless media study had media as a whole-plot effect, species as a split plot effect and date as a repeated measures effect.  Data collected included photographs to measure plant area covered, three dimentional plant size measured by hand, and soil moisture. Soil moisture was measured with a ML2x ThetaProbe (Delta-T Devices Ltd). Data was analyzed using the STAT/GLIMMIX procedure in SAS and multiple comparisons of alternative species and/or media at each of two dates. While all of the alternative species tested survived, results showed D. cooperi being the largest in size at the end of the growing season. Incorporating a percentage of ZeoPro™ H-Plus into the soilless media enhanced the growth of the Sedum species.