Chicago Botanic Gardens has published an in-depth evaluation of which plants make the best green roof plantings for Zone 5 American green roofs.
Using both American native and exotic species in an original design by Oehme, van Sweden Landscape Architecture (OVS), CBG assessed flowering perennials, grasses and succulents for their ability to withstand the difficult environment in its 16,000 square foot (1500m²) green roof gardens. The 216 different plants were grown and monitored over a 5 year period.
Green roofs typically create conditions of both temperature and moisture extremes. The plants were assessed for their ability to adapt to the green roof growing medium, resist pests and diseases, survive a harsh Zone 5b winter, provide ornamental qualities, reseed but not become weedy, and longevity.
Nine plants species and cultivars were given the thumbs up of a 5 star rating, including several Phlox, Antennaria, Calamintha and Juniperus.
The full report makes most interesting reading and has many observations relevant to a wide variety of climates and countries. See more Chicago Botanic Gardens An Evaluation Study of Plants for Use on Green Roofs.