Not all plants thrive on roofs, where conditions can be harsh. Look for varieties suited to your hardiness zone. Choose drought-tolerant plants, but not those with deep root systems. Here are a handful that thrive in most climates, especially high-altitude, dry climates such as Montana.
• Basket of Gold (Alyssum montanum ‘Mountain Gold’)
• Cacti (Escobaria leei ‘Lee’s dwarf snowball’)
• Hens and chicks (Sempervivum spp.)
• Ice plant (Delosperma)
• Native grasses (Buffalo grass, bluestem, blue grama, perennial ryegrass, blue fescue)
• Creeping phlox (Phlox stolonifera)
• Sedum (Sedum spp.): ‘Dragon’s Blood’, ‘Tri-colored’, ‘Golden Carpet’, ‘Angelina’, ‘Stonecrop’)
• Thymes (Thymus vulgaris): ‘Wooly’, ‘Pink Chintz’, ‘Creeping Pink’, ‘Elfin’)
• Snow-in-Summer (Cerastium tomentosum)
Source: Green Roof Plants: A Resource and Planting Guide by Edmund and Lucie Snodgrass (Timber Press, 2006)