The 2010 ASHS Annual Conference

Green roofing helps offset metropolitan regions’ many negative environmental impacts. Vegetation, growing media, drainage and additional layers work in concert to reduce stormwater runoff, cool ambient air temperatures, sequester carbon, and a number of other environmental benefits. Several experiments across the U.S. have investigated which growing media and plant species perform best on green roofs, but little information exists about which growing media and plant species work best in California. The goals of this study were to compare (1) three growing media and (2) four plant groups under extensive green roof conditions (substrate depth < 20 cm) on the Central Coast of California. Both studies evaluated performance of the treatments in terms of plant growth (percent cover).

The growing media experiment investigated three media, placed on three 2.5 x 2.5 m platforms each divided into nine sections in a Latin Square statistical design (nine replications). 16 plugs, equally divided among four Sedum species, were placed in an equidistant 4 x 4 grid on the nine experimental units of each platform. Growth was measured weekly, using a 100-square evaluation frame placed above each section, and counting how many squares were filled with vegetation. Measurements continued for six months to determine which growing medium supported the fastest plant growth. Medium # 3 (large particle size, 15% organic matter) had a significantly higher percent cover than medium # 1 (lightweight foam) and # 2 (small particle size, low organic matter) after eight weeks. Medium # 3 had 81% cover after six months, while medium # 1 and # 2 supported 61% and 72% cover, respectively.

The plant species experiment used one growing medium to compare growth rates of four plant groups. Groups were Sedum (Sedum album, Sedum rupestre, Sedum rubrotinctum, Sedum Spurium), grasses (Buchaloe dactyloides, Boutelouea gracilis, Sporabolus airoides, Festuca glauca), sedges (Carex pragracilis, Carex divulsa, Carex glauca, Carex testaceae), and taxa of interest (Fragaria chiloensis, Delosperma cooperii, Dymondia margaretea, Achillea millefolium). Plant groups were randomly placed on four platforms 2.5 x 2.5 m in a randomized complete block design (four replications). Growth was measured weekly, using a 100-square evaluation frame, for six months. After measurements were completed, the taxa of interest and the sedges had an average of 88% and 69% cover, respectively, while the Sedum and grass groups had 59% and 61% cover, respectively.