This Arbor Day, we’re highlighting the importance of trees to the sustainable design of the 9/11 Memorial plaza.
The American Society of Landscape Architects describes the 9/11 Memorial as a “massive green roof—a fully constructed ecology—that operates on top of multiple structures.” This “green roof” effect is made by the more than 400 swamp white oaks that populate the plaza, which were initially chosen for the site due to their durability and vibrant leaf color.
The swamp white oak was chosen because it is “a hardy urban tree,” not easily susceptible to blight, disease, or insect infestation, and has a high tolerance for variance in temperature—which is important, given the reflective surfaces of neighboring World Trade Center buildings can create “hot spots” on the plaza, said Edward Sidor, senior vice president for buildings and grounds at the 9/11 Memorial & Museum.
A uniform monoculture of trees was a key feature of landscape architect Peter Walker’s design for the 9/11 Memorial grounds. Horticulturalists and 9/11 Memorial buildings and grounds staff spent two months traveling from nursery to nursery from Connecticut to Virginia selecting swamp white oaks in the in a 3- to 3-½-caliper range. The trees were all dug and transported to Halka Nursery in New Jersey, where a Memorial plaza nursery of 479 trees was created.
In Walker’s vision, over time the sturdy branches would be pruned to create an even canopy that could reach heights of 60 feet in conditions like those on the plaza to create a cathedral-like effect. “As the canopies of the trees start to touch and grown into each other,” Sidor says visitors will be able to “walk under the cathedral of branches and then come to the opening and light of the North and South Pools.”
Read more about the design of the 9/11 Memorial plaza.
By 9/11 Memorial Staff