Norfolk Botanical Garden demonstrating conservation method

The small building where visitors buy tickets for boat rides at Norfolk Botanical Garden is now one of the garden displays, rather than simply an adjunct structure.

The building's cedar shingles are gone and in their place is a green roof, a living bed of succulents such as sedums and hens and chicks. The year-round roof will be a natural part of the growing season along with all the rest of the garden's plant beds.

Come spring and summer, the succulent roof will bloom and hens will produce chicks. Come fall, many will turn a rich red, pink or chartreuse, said Melissa Butler, herbaceous plants curator at the garden.

"They are really very pretty in fall," Butler added.

But unlike other plants in the garden, pretty is as pretty does for these succulents. They have a job to do.

With the new green roof on display, the garden hopes to educate the public about this environmentally sensitive roof that can reduce stormwater run-off as well as reduce heating and cooling costs, said Amy Dagnall, the garden's public relations manager.

"We want to show the public what can be done," Dagnall said, "and that there are companies right here that can do it."

The company that designed the garden's new roof is Building Logics in Virginia Beach. Phelps Construction in Suffolk then installed the roof with the help of the garden's horticulture staff, as well as students from the garden's Horticulture Enrichment and Learning Program.

A green roof is good for the environment because it replaces a roof's hard surface with plants that limit the amount of rainwater running off the roof, said Tom Sims, production manager for Building Logics.

"The green roof is meant to retain about an inch of water," Sims said, "and to filter and slow down excess water."

A green roof also lowers cooling and heating costs.

"Soil is one of the best insulators," Sims said, "so it will trap the heat."

Green roofs generally fluctuate about 7 degrees in temperature, whereas an asphalt roof can reach 140 degrees in summer.

The garden's green roof effort actually began last year when staff planted 10-1/2-by-22-inch garden trays with a variety of succulents, said Butler. They planted enough trays to cover the 750-square-foot roof. As the plants mature, they are growing into dense mats.

The trays are actually two trays, one fitted inside the other between a filtering fabric that allows for drainage. The soil is a 50/50 mix of compost and a lightweight mixture like Perma Till. The whole thing is called a "pod" in green roof trade parlance.

The pods were installed on the roof after the plywood base was covered first with waterproofing membranes, then a layer to serve as a barrier to plant roots, and finally an absorbent layer to hold water. The layer holds just the right amount of water but not enough to weigh down the roof, Sims explained.

A green roof should last 40 to 50 years, Sims continued, which more than makes up for the additional cost of installing one. Maintenance becomes less and less, the thicker the mats of succulents become, Butler said.

As birds and trees drop seeds onto the pods, garden horticulture staff, who are used to weeding flower beds on ground level, may also have to do a little roof weeding at first.

"Some of the more aggressive plants also may have to get reined in," Butler said, "but the main goal is not plant display, but green roof display."

Mary Reid Barrow, barrow1@cox.net