Greening on the rooftops

Interest in rooftop gardening is skyrocketing—or should that be skyscraping? Urbanites longing for an outdoor space to dig in the dirt have chosen to go up. The urban ecosystem (and everyone in it) benefits from rooftop gardening.

Rooftop gardening (including green roofs) reduces storm water runoff, combats the heat island effect, acts as a biofilter and provides a wildlife habitat. In addition, rooftop gardening can save money by prolonging the life of the roof, and perhaps qualifying for a tax subsidy. (Both Chicago and New York offer tax incentives for rooftop gardening.)

Big-city benefits. Storm-water runoff is a big problem in urban areas. During heavy downpours the sewer systems become overwhelmed, which leads to flooded streets and basements. Rooftop gardens act like sponges and absorb lots of rainwater. Of course, green roofs hold the most water, but even rooftop container gardens help mitigate the volume of runoff.

Another urban problem is the heat island effect. The immense volume of concrete, steel and asphalt absorbs and releases radiant heat, which can raise temperatures to unbearable levels. Cities can be 10 to 20 degrees warmer than similarly situated rural areas. Plants cool the air by transpiration (release of water vapor through leaf stomata). All plants are like mini air conditioners in summer. It's the reason lush, green grass always feels cool between the toes.

Plants absorb the sun’s energy and convert it into growth, flowers and fruit. This is in stark contrast to concrete and asphalt, which absorb the sun’s energy only to release it as convective heat. Gardens not only keep the rooftop and surrounding areas cooler but also the building underneath. Green roofs are excellent insulators and have been shown to reduce annual energy consumption (both heating and cooling) by up to 15 percent.

Plants are also great filters. The leaves remove pollutants from the air, while the roots filter water and soil. Their ability to hold and bind pollutants while releasing fresh air makes plants the perfect addition to urban areas. Rooftop gardens convert some of the dirtiest air into pure oxygen to the benefit of all city dwellers.

Wildlife appreciates rooftop gardens. They are oases in what can appear to be a lifeless urban desert. The flowers of rooftop gardens provide nectar for bees, butterflies and hummingbirds, all of which have a tough time finding food in a city. Robins, finches, dragonflies and mosquito-catching spiders also visit rooftop gardens.

Interest in rooftop gardens is skyrocketing. Or should that be skyscraping?Economically, rooftop gardening can extend the life of a roof. Green roofs are more durable and estimated to last approximately 40 years, compared to around 10 years for typical roofing. Establishing a green roof costs more initially, but is much cheaper in the long run. In addition, because of the numerous environmental benefits, many cities offer tax incentives to rooftop gardeners.

Plant picks. When offering plants for patios and balconies, consider those from habitats that most closely match your conditions, such as exposure to wind and sun, limited water, shallow soils, large temperature fluctuations, etc. Plants from scrubland, screes, mountainsides, steppes and deserts often make great rooftop, patio and balcony plants because they are adapted to the harsh conditions.

A common thread among these tough plants is foliage that’s hairy, waxy, hard, small, thin and/or leathery to help them tolerate drought. Deep taproots or extensive branching roots also promote drought resistance.

To help consumers make the right choice, retailers should have a section especially for container and rooftop plants. Trumpet the durability and beauty of hardy succulents (Jovibarba, Sempervivum, etc), half-hardy rain lilies (Zephyranthes), baskets-of-gold (Aurinia), and Mediterranean herbs. These plants are easy to cultivate and combine well with seasonal annuals, like moss rose and calibrachoa.

Hardy succulents, in particular, should be featured as rooftop and balcony art. Decorative ceramic bowls with colorful rocks/sand and hardy succulents remain attractive all season. They should literally be displayed like ornaments on a mock balcony or rooftop in the garden center.

TOP 10 ROOFDECK PLANTS If your customers are capable of some maintenance (mostly watering), then thesea re some of the more ornamental and productive plants for rooftops:
Amelanchier alnifolia 'Regent' Aurinia saxatile (basket of gold) Columbine (Aquilegia cultivars) Gaillardia 'Frenzy'	Hyssop (Hyssopus officinalis)  Lilium cultivars Ornamental peppers Portulaca grandiflora Rain lily Rosa Ring of Fire
TOP 10 LOW-MAINTENANCE ROOFDECK PLANTS
For customers that can't water at all, consult this list. Even these tough plants require adequate containers with well-drained soil. Agave (frost-tender species and cultivars) Aloe (frost-tender species and cultivars) Cercidium Palo Verde (frost tender) Hesperaloe (Red yucca species and cultivars, some frost tender) Hylotelephium cauticola (upright sedum and cultivars) Jovibarba (beard-of-Jove) Sedum (S. album, middendorf, kamschaticum, rupestre, acre and cultivars) Sempervivum (hen and chicks) Tulipa tarda Yucca