Get to Know the Garden’s Green Roof

Next time you’re near the Garden’s Diane H. and Joseph S. Steinberg Visitor Center, look up! A 9,600-square-foot green roof meadow thrives atop the building, with a palette of over 28,000 plants including four species of grasses and four perennial wildflowers, all native to North America. Planted in dramatic sweeps across the roof, this simple yet dynamic palette evokes a midwestern prairie and offers a lively progression of color and texture throughout the seasons.

This beautiful rooftop planting isn’t just for looks, though. Living roofs like this one have a host of benefits, including improved stormwater management, added wildlife habitat, lower energy needs and associated costs, reduced noise levels, lower levels of greenhouse gas emissions, improved air quality, and reduced urban heat island effect. Being on display at a botanic garden also provides opportunities for climate education.

The Visitor Center itself and the three-acre site that surrounds it feature several other green building elements, too. The curved building nestles into the adjacent hillside gardens and has earned LEED (Leadership in Energy and Environmental Design) Gold certification from the United States Green Building Council.

Included features of the green design are 28 geothermal wells for energy-efficient heating and cooling, reclaimed building materials, earth-sheltered construction that insulates the north-facing wall, and floor-to-ceiling windows that face south to soak in the light and heat of the winter sun.

Most green building projects, including this one, are designed to help sustainably manage stormwater runoff, a major source of water pollution when sudden, heavy rains overburden combined sewer systems. This is accomplished through not only the green roof, but also several sunken rain gardens, or bioswales, and a stormwater channel running midway through the site.

Together, they mimic nature to collect, filter, and absorb stormwater runoff from the surrounding gardens and pavement. During major storms, overflow from the green roof and adjacent spaces is directed to the bioswales. Overflow from those is directed to the pond in the Japanese Hill-and-Pond Garden and then circulates through the rest of the Garden’s Water Conservation Project system. These green infrastructure elements were a natural fit for the Garden to showcase the essential role plants have not only in recycling water, but also in supporting wildlife, filtering the air, and moderating temperature through a process called evapotranspiration.

Opened in 2012, the Visitor Center site was designed by architecture firm Weiss/Manfredi and the landscape was designed by HM White Site Architects. The green roof system was designed by SIKA Sarnafil in partnership with landscape architecture firms Roofmeadow and New York Green Roofs.

Our prairie-style living roof is considered a semi-intensive system, one of several roof types based on the depth of the soil. Extensive green roofs are the shallowest, with a depth of two to five inches. These support small, shallow-rooted, drought-resistant plants such as sedum. They’re usually cheaper to install, lighter in weight, and require less irrigation and maintenance than other living roofs, but do not support much plant or visual diversity.

At the other end of the spectrum are intensive green roof systems with soil depths of eight to 30 or more inches, which can incorporate trees and large shrubs. These are the most expensive, but they support the widest variety of plants and can support garden-like spaces for humans and habitat for wildlife.

Semi-intensive systems like ours fall in between with a soil depth of five to eight inches. These support not only sedums, but also grasses, wildflowers, and sometimes small shrubs. They require irrigation and maintenance, and offer plenty of visual interest and biological diversity. It was a good choice for a botanic garden display since it’s not only practical, but also educational and beautiful.

As the gardener and curator of the Visitor Center gardens, I work with a small team from New York Green Roofs to maintain the planting. While on the sloped roof, we each wear a fall arrest harness with a shock-absorbing lanyard that clips into a cable running along the spine of the roof. Weeding takes up the bulk of our time, but we also check on the irrigation system, monitor for pests, inspect the drains and weatherproofing elements, take soil samples to assess nutrient levels, apply soil amendments as needed, and take notes and photographs for monthly reports. I climb up to check on, troubleshoot, and plan work on the roof in between these team maintenance visits.

The lightweight, porous growing media supports eight perennial prairie species native to the eastern and midwestern United States. The four grasses growing on the roof are June grass (Koeleria macrantha), prairie dropseed (Sporobolus heterolepis), little bluestem (Schizachyrium scoparium ‘The Blues’), and sideoats grama (Bouteloua curtipendula). There are also four wildflowers—butterfly weed (Asclepias tuberosa), purple prairie clover (Dalea purpurea), dwarf blazing star (Liatris microcephala), and beardtongue (Penstemon hirsutus).

These native midwestern prairie plants have some drought tolerance and preference for full sun and well-drained soils when growing in their natural environment. Of course, a green roof is by no means a natural environment for these plants to live–in some ways it is a very large, shallow container planting.