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Perennials With a View

Perennials With a View

  • White Stonecrop (Sedum album)

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    White Stonecrop (Sedum album)

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    Simon Eugster

    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)

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    Prickly Pear (Opuntia humifusa)

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    Ted Bodner@USDA-NRCS Plants Database/James H. Miller and Karl V. Miller

    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)

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    Nodding Onion (Allium cernuum)

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    Walter Siegmund

    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)

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    Hardy Ice Plant (Delosperma cooperi)

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    Andrew Massyn

    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.)

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    Hens and Chicks (Sempervivum spp.)

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    Derek Ramsey

    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)

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    Middendorf Stonecrop (Sedum middendorffianum)

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    Pavel Kacl

    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)

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    Prairie Dropseed (Sporobolus heterolepis)

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    Robin Carlson/Chicago Botanic Garden

    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)

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    Creeping Phlox (Phlox subulata)

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    Robin Carlson/Chicago Botanic Garden

    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)

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    Stonecrop (Sedum kamtschaticum)

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    Stan Shebs

    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)

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    Ornamental Onion (Allium senescens)

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    Adamantios

    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)

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    Mint (Mentha)

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    Amit Bansal

    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)

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    Two-Row Stonecrop (Sedum spurium)

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    Rob Hille

    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.

Plant selection depends on a few factors. First, a project team should determine what type of green roof is best suited to its project: intensive or extensive. Intensive green roofs have deeper planting media depths and are similar to traditional landscaping. Extensive green roofs, designed to boost building performance and environmental sustainability, use shallower depths and require less maintenance.

Next, teams should consider their roof’s microclimate, which is determined by several factors, including average temperatures, wind levels, sun intensity, and rainfall. The U.S. Department of Agriculture publishes its Plant Hardiness Zone Map (planthardiness.ars.usda.gov) based on the average annual minimum winter temperatures, divided into 10-degree zones; lower-numbered zones are colder.

However, depending on roof height, the conditions on top of buildings may be very different from conditions on grade. Rooftops may require plants suited to a different zone than the landscaping planted several stories below on ground level does. Green Roofs for Healthy Cities offers a database of green-roof professionals (greenroofs.org/index.php/find-greenroofprofessional), who can help sort out the technicalities of selecting plants for rooftops.