Green roof on a green building

Gardening has reached new heights at The North Carolina Arboretum — about 40 feet off the ground, on top of the arboretum’s recently completed Operations Center. The new horticultural venture (and exhibit) is a “green roof” — sometimes known as a vegetated green roof. It’s one of the center’s many features that meet the state’s new “High Performance Building Guidelines.”

In addition to simply improving the view, a green roof helps the environment by:

* reducing storm-water runoff (a major pollution problem in urban areas) by absorbing rainfall and allowing it to evaporate into the air or be absorbed by the plants on the roof;

* cooling air temperatures through biological processes such as transpiration, when plants release moisture into the air;

* extending roof life by reducing maximum surface temperatures; and

* providing thermal and noise insulation.

When is a green roof different than a rooftop garden? A green roof is completely covered with a shallow, lightweight soil that supports masses of drought-tolerant plants; it can be viewed and enjoyed from the ground. In contrast, a rooftop garden is designed more like a garden, to be enjoyed by people on the roof; it usually features deeper soils that support perennials, shrubs and trees.

Green roofs, such as the one at the arboretum, are generally composed of a series of layered materials, including waterproof and root-barrier membranes, filter fabric, a lightweight, well-drained growing medium, and, on top, plants. The arboretum’s roof is bordered with limestone gravel to allow access for roof maintenance and to buffer rainwater as it passes through. Also due to the roof’s slight slope (3:12), a retention grid has been used to hold the soil in place.

For the purpose of comparing overall plant growth and water runoff, two different soil mixes were used on each half of the roof. One from Carolina Stalite Company in Salisbury is 80 percent expanded slate aggregate, 10 percent USGA root-zone sand and 10 percent composted yard waste. The other soil mix, from Erth Products, in Peachtree City, Ga., is 80 percent expanded clay aggregate, 10 percent river sand, 10 percent composted peanut hulls and municipal solid waste.

The roof faces north and will be subject to extreme environmental conditions, ranging from hot, dry and windy to cold, wet and frozen. A site this challenging calls for tough, low maintenance plants appropriate for hardiness zone six. We considered options from a native-plant palette and, in particular, species commonly seen on regional rock outcrops. These would be quite suitable for a site and soil situation such as this, but due to the construction and production needs, we followed the recommendation of Ed Snodgrass of Emory Knoll Farms, plant supplier, and opted for a plant mix comprised primarily of sedums, knowing that in coming seasons more native plants could be added. Ed explained that, over time, grasses will naturally seed themselves, cautioning us that if we want to wind up with 10 percent grasses in the mix, not to plant any.

Sedums are tough and well-known survivors in the plant world; they take advantage of the slightest bit of available moisture by growing what Ed calls “rainroots” within two hours after a rain. Then when the conditions turn dry, these same roots die off, becoming organic matter that, over time, helps create a growing environment supportive of other plants.

In mid-May, more than 8,000 plants were planted on the roof. We used a mix of sedums, chives and ice plants, in a random meadow-type planting to offer year-round interest with extended flowering, fall colors, plus some evergreen presence during winter months. During the planting process, we had to make a few adjustments: The roof’s shallow soil depth (4 inches total) required us to use 2-inch (big thumb) plugs; plants this small are easily lost, so we planted each type in groups of about six; and to protect the underlying roofing fabrics, fingers, not trowels, were used to create the planting holes.

The summer rains have been wonderful, helping the plants get established; already you can see significant plant growth … and even a few weeds!

The green building

Besides the green roof, some of the Operation Center’s other environmental high-performance features include:

* Roof-mounted solar panels preheat the center’s hot water, reducing heating costs. These solar panels will also be used to power a charging station for electric vehicles. (The arboretum currently has two electric vehicles and plans to add more alternative-fuel vehicles in the future.)

* North-facing, operable, clerestory windows in both office and workshop areas provide natural airflow via a “chimney effect,” reducing the need for cooling during spring and fall transitional months. They also provide light for workers inside, reducing the need for artificial lighting.

* A geothermal heat pump reduces the center’s heating and cooling costs by cycling HVAC water through a series of nine, 380-foot deep underground wells, either preheating or precooling it (depending on the season) to a nearly-constant 55 degrees.

* The workshop area will offer natural air-cooling features: “Air curtains” produce a blast of air at workshop area doorways, providing extra insulation and protecting interior air quality.

* A cistern for collection of rainwater runoff from the roof reduces the arboretum’s water needs.

* An oil/water separator filters pollutants from water used to wash vehicles and equipment before the water is then recycled for irrigation.

* Building lights are controlled by photo sensors and timers to reduce “on” time.

* Water-conservation methods, such as waterless urinals, are projected to reduce annual water use by 40,000 gallons.

The North Carolina Arboretum’s Operations Center is now home to the arboretum’s landscape and maintenance staff, as well as equipment storage and repair and maintenance shops. This $1.6 million project was funded through the N.C. Higher Education Improvement bond, a $3.1 billion capital program approved by state voters in November 2000 for capital projects for the University of North Carolina system. It is the first of four bond projects slated for the Arboretum. A $25,000 grant, awarded by the Creel Foundation, funded the green roof.

The Arboretum’s Operations Center is one of 15 state-constructed facilities included in the N.C. Office of Energy’s pilot program following the Triangle J “High Performance Building Guidelines.” These guidelines, adapted from the U.S. Green Building Council, require the facility to meet criteria for innovative design, energy efficiency and environmentally responsible practices.

The “High Performance Building Guidelines” were originally designed for government buildings 20,000 square feet or larger. Regional studies show that the typical commercial building is about 10,000 square feet, the size of our Operations Center. We proposed using the center as a prototype that would relevant to the area business community. If these techniques prove successful, the state may adopt them as a mandatory standard for all its buildings.

We are excited about the safe and efficient workspace this facility is providing our staff and hope you will join us as we look forward, and upward, to the new green roof feature of The North Carolina Arboretum’s new Operations Center.

Since this building is a work area, the Operations Center will not be open to the public on a daily basis. Visitors will, however, have an opportunity to tour this facility on Sunday, Oct. 3 from 1:30-3:30 p.m., and Tuesday, Oct. 5 from 4-6 p.m. For more information, please call 665-2492.

Arboretum Green roof plant list

Chives — Allium schoenoprasum

Dragon’s Blood Sedum — Sedum spurium

Jelly Bean Sedum — Sedum album ‘Coral Carpet’

John Creech Sedum — Sedum spurium ‘John Creech’

Hokkaido Island Sedum — Sedum cauticola ‘Lidakense’

Little Evergreen sedum — Sedum hybridum ‘Immergruenchen’

Rocky Mountain Sedum — Sedum stenopetalum

Shale Barrens Sedum — Sedum ternatum ‘Larinem Park’

Six Sided Sedum — Sedum sexangulare

Yellow Ice plant — Delasperma nubiegenum

[Alison Arnold is director of horticulture at The North Carolina Arboretum. She can be reached at: aarnold@ncarboretum.org.]