Anyone familiar with landscape architect John Howard’s award-winning work would expect to find the grounds of the Flower Showhouse as beautifully detailed as the house itself. But what might surprise some is just how far the landscaping extends—up to the rooftops of the home’s one-story wings. Viewed from most of the second-floor rooms in the central part of the house, the two flat roofs caught the attention of one of the home’s developers, Doug Beasley, who suggested that John work his magic on these empty spaces. “He recognized that there was a lot of flat roof that could be seen, so he wanted to do something that would make the tops of the roofs more attractive,” says John, who quickly got to work on creating two rooftop gardens.
Envisioning parterres of sorts, the landscape architect organized each roof into a trio of distinct yet aesthetically-connected garden spaces consisting of tidy, rectilinear layouts designed to complement the home’s architecture. “The architecture is Regency style, so we wanted to keep the lines very simple and graphic,” says John. In parterre-like fashion, each section of the garden consists of linear masses of plant material, which are surrounded by gravel paths. However, in a break from tradition, the rooftop gardens seen here do not feature boxwoods, shrubbery, or any other plants that Howard might typically use when planting parterres in the ground. Instead, he took a different approach.
“The biggest challenge you have with roof gardens is the harsh environment,” says John. “Too much sun or a lack of sun plus rain and harsh winter winds are the reality of it.” He adds that weight limits dictated by the roofs’ structure were also considered during the planning process. To help determine which plants might work best in this particular setting, John consulted XeroFlor, a company whose specialty is roofs covered in vegetation, also known as green roofs. Together, they came up with a plan of using a variety of sedums, a type of succulent which has the advantages of hardiness and low growth, making it an ideal plant to be used on rooftops. XeroFlor grew the sedums in lightweight modular trays that are embedded with root barriers, drainage mats, and growth material. Once established, the plants, which are meant to remain in their trays, were shipped to John, who was able to easily integrate the trays of sedums into his design by simply locking the trays together to create the desired patterns and placing them directly on the roofs.
Choosing Sedums for a Roof Garden
Although John used a cutting-edge way of cultivating the plants, he still relied on good old-fashioned gardening know-how when it came to ensuring that the gardens would provide year-round interest. “It’s a combination of about 15 or so sedum varieties, which do different things throughout the year,” he says. “Some are evergreen while others are deciduous, losing their foliage in the winter and re-sprouting every spring. And they do have bloom to them.” For example, there is Sedum spurium “Summer Glory”, which produces dark pink blooms during the summer, as well as Sedum hybridum “Czar’s Gold”, a perennial whose golden blooms herald the spring and summer seasons. Come fall, some of the varieties, such as Sedum sexangulare, will blaze with foliage in autumnal shades of red, copper, and bronze. But even with these sometimes vibrant accent colors, the gardens are anything but showy. Instead, the collective effect is like a refined tapestry in which each color harmonizes with the others.
Likewise, the sedum roof gardens’ finishing touches were carefully considered. For the gravel paths, John ruled out small stones, which might have allowed the plants’ roots to grow beyond their trays. Instead, he chose larger-sized black river pebbles to prevent roots from spreading and to provide dark, graphic borders around the masses of sedums. In terms of maintenance, other than the occasional manicure and the need to blow off any errant leaves during the fall season, the gardens’ main requirement is water, which John addressed by installing a convenient drip-irrigation system. On an ordinary roof, this might seem like an accident waiting to happen, but because these are membrane roofs, they act as a waterproof barrier. In addition, the two rooftop gardens provide an unexpected advantage that will likely prove beneficial during the winter and summer months: They serve as an extra layer of insulation, making the home more energy efficient.
A Guide to Rooftop Gardens
Landscape architect John Howard offers his tips on creating your own rooftop garden
What are the initial steps one should take when planning a rooftop garden?
First, hire an expert to conduct an engineering assessment of the load that the roof can take. Not all roofs are designed for the load and weight of plant material, especially if you’re planning to use large pots of soil. Tray systems, like those we used here, are lighter because they’re only a couple of inches tall. The second thing you should consider is the light or lack of light to determine what kinds of plants will work. And finally, figure out if you can get water to the space. If you can’t get a drip system, which is the most efficient way of doing it, you will need to plan to hand-water the garden.
Generally speaking, which plants seem to work well on rooftops?
Sedums, any kind of succulents, and grasses can work well on rooftops. If you have built-in planters or pots, you can do shrubs and trees as well, but you will need more soil depth for that. And again, you will have to consider whether the roof can take the weight of the pot, the weight of the soil, and the weight of wet soil.
What about designing the garden?
I really think it would be smart to hire a design professional who knows about green roof systems. When we create green roofs, we make the aesthetic decisions but rely on green roof specialists to detail them.