I had a chance to look over the 45,000-sq-foot (4,050-sq-meter) green roof that PECO Energy (the local utility) installed on its headquarters building in central Philadelphia last week. Frankly, I thought that green roofs were mostly "tree hugger" hype, but this visit made me a believer. The roof receives 1.5 million gallons (5.7 million liters) of precipitation during the year, only sheds 0.5 million gallons (1.9 million liters), and it sheds those half-million gallons over a protracted period that doesn't tax the city's stormwater system with excess runoff following heavy rainfall events.
The Cira Center, Philadelphia's most unusual skyscraper, a half-mile away
The roof has many other advantages. It will last 2-3 times longer than a conventional roof, it is far cooler than a conventional flat tar roof, it absorbs air pollutants, and it actually provides some habitat for birds and insects; we watched honeybees gathering pollen. The water management angle really captured my interest, though.
Philadelphia Museum of Art across the green roof
The major part of the roof is planted with 12 species of orpines or stonecrops (Sedum spp.) (alas, no native species). At this time of year, they are coloring-up beautifully. The roof is three years old, and the designer knows and expects that some of the stonecrops will be better competitors than others, so the species diversity will decline over time, but right now they make an especially appealing mosaic.The company also included deeper planting boxes along the edge of the roof where the building's structure could support heavier soil. The boxes were planted with a mixture of attractive native perennials and native grasses. Little bluestem grass (Schizachyrium scoparium) has "escaped" the confines of its boxes, and spread its seed into the main part of the roof. It will be interesting to see if it persists over time in the shallow, droughty, low-density "soil" in which the Sedums are growing.