White Stonecrop (Sedum album) Good for: Year-round color Zone: 5 to 8 Height: 2 to 6 inches Sun: Full Blooms: Varies Tolerant of shallow planting media and intense sunlight, sedums are frequently used on green roofs. The hardy, low-growing plants come in many varieties, says Jennifer Bousselot, a former researcher at Iowa State and Colorado State universities. “Depending on the cultivar, the winter colors are endless—ranging from orange to pink to yellow.”
Prickly Pear (Opuntia humifusa) Good for: Color and low maintenance Zone: 2 to 10 Height: 8 inches Sun: Full Blooms: May to July No “plant it and forget it” species exists, Bousselot says, but the prickly pear comes close. Its drawbacks include pedestrian-unfriendly spines and slow growth, but it can endure frigid climates. Native and widespread in the eastern U.S., it produces waxy yellow flowers that are followed by edible fruit.
Nodding Onion (Allium cernuum) Good for: Year-round color Zone: 4 to 8 Height: 1 to 3 feet Sun: Full Blooms: June to August For use in milder climates, this evergreen plant adds height and interest to roof gardens, Bousselot says. “It has grasslike but thick leaves with a beautiful flower. Its flower head dries and creates excellent winter interest.” Once abundant on the Chicago River banks, the plant features edible leaves, bulbs, and bulblets.
Hardy Ice plant (Delosperma cooperi)Good for: Year-round color Zone: 7 to 10 Height: 3 to 6 inches Sun: Full Blooms: June to September Though it requires a well-drained substrate, this fast-growing plant has succulent foliage that turns purple in the winter, Bousselot says. Suitable as a ground cover, it produces fuchsia flowers from late spring until the first frost. However, as a native of Southern Africa, it is not reliably winter hardy north of zone 7.
Hens and Chicks (Sempervivum spp.) Good for: Low-maintenance Zone: 3 to 8 Height: 3 to 6 inches Sun: Full This plant lives up to its Latin name sempervivum, which means to live forever. Though hens and chicks can take longer to establish than sedums, the payoff is worth it once the plants take root. Not only are the evergreen succulents drought resistant and low-maintenance, but they also provide color, producing purple-red flowers in midsummer.
Middendorf Stonecrop (Sedum middendorffianum) Good for: Roofs without irrigation Zone: 3 to 9 Height: 8 to 10 inches Sun: Full Blooms: Summer Well suited for roofs with limited additional load capacity, this colorful sedum tolerates soil depths as shallow as 1 to 3 inches, says Kristin Getter, a floriculture outreach specialist at Michigan State University (MSU). White Stonecrop is another option, but it does not withstand hot summers, Getter says.
Prairie Dropseed (Sporobolus Heterolepis) Good for: Kid-friendly, high-traffic areas Zone: 3 to 9 Height: 2 to 3 feet Sun: Full Blooms: August to October This plant, native to Chicago, produces pink flowers with brown tints in the late summer. Its foliage turns gold and orange in the fall and fades to light bronze in the winter. “The grass … has a strong fragrance in late summer into early fall that has been likened to popcorn,” says Chicago Botanic Garden horticulturist Emily Shelton.
Creeping Phlox (Phlox subulata) Good for: Kid-friendly, high-traffic areas Zone: 3 to 9 Height: 6 feet Sun: Full sun to partial shade The Chicago Botanic Garden is examining the durability of this semi-evergreen ground cover, which grows into a dense mat, Shelton says. “It can handle some foot traffic and is a carpet of blooms in the spring”—good news for a plant that must withstand frigid winters, frequent handling by students, and up to 1 million annual visitors to the garden’s Daniel F. and Ada L. Rice Plant Conservation Science Center.
Stonecrop (Sedum kamtschaticum) Good for: Storing carbon Zone: 4 to 9 Height: 6 to 12 inches Sun: Full sun to partial shade Blooms: Early summer All plants store carbon, but some are more effective than others, says MSU horticulture professor Brad Rowe. Carbon storage potential is directly related to biomass—for example, a tree will store more than a perennial. Stonecrop, Rowe says, is a larger rooftop plant that can be planted in shallow soil. It produces long-lasting, half-inch yellow flowers.
Ornamental Onion (Allium senescens) Good for: High-salinity environments Zone: 4 to 8 Height: 6 to 8 inches Sun: Full sun to partial shade Blooms: Mid to late summer In coastal applications and on roofs in which de-icers are used, plants that can tolerate high salinity are a must. In his research, Rowe has found ornamental onions to be very salt tolerant. With blue-green leaves that smell like onion when bruised, the plant grows in clumps and produces lilac-pink flowers.
Mint (Mentha) Good for: Bulk food source Zone: 3 to 10 Height: 1 to 4 feet Sun: Full sun to partial shade For the Ledge Kitchen & Drinks restaurant in Dorchester, Mass., Recover Green Roofs worked with Green City Growers to create a rooftop kitchen garden, whose abundance of produce includes mint—traditional, chocolate, pineapple, spearmint, and peppermint. The fast-growing, continually harvestable plant is “easy to apply to a menu,” said Recover project manager Brendan Shea.
Two-Row Stonecrop (Sedum spurium) Good for: Storing carbon Zone: 4 to 9 Height: 2 to 6 inches Sun: Full sun to partial shade Blooms: Summer Rowe also pointed to this sedum variety as another excellent carbon-storage plant that can grow in shallow planting soils. The semi-evergreen sedum produces white or purplish star-shaped flowers in the summer. Its foliage turns burgundy in the fall. Like Stonecrop, the plant also provides good ground cover.