Up in the Air
Rooftop gardeners take heat and drought in stride. Here's what you can learn from them.
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It isn't enough that rooftop gardeners have to worry about the weight of potting soil and how many elevator trips it will take to schlep two bags of amendments, a flat of marigolds and a small crabapple tree. They also have heat, drought and drying winds to worry about. Whether your garden is 12 stories up or on a backyard patio, their advice can help you cope:
Make smart plant choices. "A rooftop garden is exposed. It's almost like a beach environment — hot, dry and windy," says Carmen DeVito, a landscape designer in Brooklyn, N.Y. "So I use a plant palette like I would at a beach house." In her rooftop designs, she uses bayberry (Myrica pensylvanica, a common coastal plant), roses (especially 'The Fairy'), herbs, grasses (Miscanthus sinensis 'Yakujima'), succulents, yarrow, pines (like Pinus mugo), dwarf junipers, cotoneaster and Russian olive.
Provide some shade if you can. The ambient temperature of an urban setting builds up as heat is absorbed and reflected from a jungle of hard, concrete surfaces. If you can, grow vines up a trellis, add a patio umbrella or even build a pergola to reduce the amount of sun shining on your garden floor. "Or you can construct shade with metal posts and sailcloth," says Joan Grabel, a landscape designer in Studio City, Calif.
Provide for consistent water and good drainage.
If you don't have an irrigation system, remember the rule of watering — water well whenever you water. It takes time for potting soil to fully absorb water, so water slowly or rewater about 30 minutes later.
Rooftop (or balcony or deck) gardeners have to consider how much weight the structure can support. If you're planning to do some significant gardening or hardscaping, says DeVito, "work with the building engineer and review the building codes to see what's allowed. Codes limit what types of materials can be used." If you're renting, you probably won't be able to (or want to) provide any structural reinforcements.
Turning rooftops as green as possible only makes good environmental sense, says David Yocca, landscape architect and planning director of the Conservation Design Forum in Elmhurst, Ill., a firm that designs ecoroofs. The ecoroofs are lined with special materials, topped with a lightweight blend of soil, compost and perlite, and planted with anything from small grasses to eight-foot conifers. Irrigation systems on timers dispense water on a regular basis.
"It's good from an ecological and human health standpoint," Yocca says. "It's a way to manage [runoff] in a positive way and it minimizes chemicals."
Plus, plants evapotranspire, Yocca explains, and that "creates natural air conditioning, so you don't have an urban heat island. An ecoroof protects the building against heat loss in winter and heat gain in summer." Since plants convert carbon dioxide into oxygen, ecoroofs also improve air quality. In several cities, ecoroof advocates like Yocca are working to have green roofs become common practice. In Europe, they're already fairly common; in Switzerland new buildings are required to have on their roofs the same amount of vegetation they replaced. Get more information on ecoroofs.