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In 2006, construction of a new water treatment vault near the edge of the maple swamp provided an opportunity to experiment with native plants in a green roof habitat. In order for green roofs to provide “habitat islands” for native insects and ground-nesting birds, they should be planted with native species, but little is known about which eastern North American plants can survive the demanding roof conditions.
The goals for the WCBG green roof are to evaluate native plants for potential use on roofs, and to sustain a diverse community of native plants without supplemental water or nutrients in standard green roof growing conditions (e.g. a maximum depth of 6” of a growing medium composed mostly of expanded shale). Of the 28 species originally planted in April 2006, all native to eastern North America, most individuals of all but one species survived the first two growing seasons, including the extended drought of late summer 2007, and some species are spreading (Fragaria virginiana) or seeding in in large numbers (Sedum nevii). No additional plantings are planned.
The plants are monitored each spring and fall. Tap-rooted weeds are pulled, but no other management applied. Species persisting on the roof as of fall 2007 are a mix of forbs (herbaceous flowing plants that are not grasses, sedges or rushes), grasses, low-growing shrubs and a fern, quite a diverse community. Individuals of many of the species on the roof are also planted in the adjoining garden, for comparison of growth and survivorship under the contrasting growing conditions.