On September 12, 2011, the National September 11 Memorial opened to the public. The outdoor memorial includes two reflecting pools in the footprints of the World Trade Center's twin towers, inscriptions of the names of the people who died at the site and a beautiful collection of trees. Gardeners watching coverage of the 10th anniversary of the 9/11 attacks may have wondered about those trees. How will they cope with the harsh urban environment of lower Manhattan?
The designers of the memorial believe in the importance of the trees both to the people who work and live in the area today and to visitors who come to pay respects to the fallen. They have taken every precaution to ensure the trees will thrive.
Veteran landscape architect Peter Walker was tapped to provide a greenscape by Michael Arad, the designer who came up with the plan for the reflecting pools, or "voids." Initially Arad's plan showed a stone plaza surrounding the voids, but the memorial's organizers asked him to find a landscape designer to change this. The Berkeley, Calif.–based Walker, now 79, came up with the idea to surround the voids with trees that would add texture, bring things down to a human scale and lend a sense of life and comfort.
Walker selected swamp white oak for the design, which includes 412 individual trees. (The original plan also included sweet gum, but these were edited out because their bright red fall color seemed too harsh.) The oaks are spaced so that as they reach their expected mature height of 60 feet, their crowns will knit together to form a dense overstory sheltering the entire plaza, except for the voids, which will remain open to the sky.
A city tree has to contend with detrimental factors including air pollution and high temperatures caused by the urban "heat-island" effect, in which buildings and paved surfaces store heat during the day then release it at night. Paved surfaces lessen rainfall reaching a street tree's roots. The soil around the tree is not fed by decaying matter as it is in the forest. In short, the city is a much different environment than any tree's natural habitat.
Some tree species adapt better than others to urban conditions. Swamp white oak, or Quercus bicolor—native to the Midwest, parts of the Mid-Atlantic and southern New England—is one of them. This pest- and disease-resistant tree tolerates dry, moist or wet soils, as well as salt, soil compaction and buffeting winds. It is also easier to transplant than other oak species because it does not have a taproot. All of these factors made swamp white oak a prime candidate for the 9/11 Memorial. It's an attractive, solid tree, with deep green summer leaves, yellow fall foliage and a rounded canopy. As an oak, it is also identifiable to many, with its lobed leaves and thick acorns. This lends an important air of familiarity, comfort and remembrance to the design.
Growing the trees
With the tree species selected, the memorial's designers next had to source the trees and plan for their installation and long-term care. Bartlett Tree Experts was selected to raise the trees, and they have contracted to care for transplanted trees at the memorial site for at least the next two years. Bartlett grew the trees in large containers for five years at one of their New Jersey locations, so that they could be transplanted with their entire root system in tact, avoiding typical transplant stress.
The plaza in which the trees are planted was designed to channel rainwater into drainage troughs that feed into underground cisterns. The water then feeds into an underground drip irrigation system that waters the trees. The plaza also includes underground aeration pipes to supply air to the roots, and the tree roots are braced to prevent them from buckling the paving. To further ensure their success, each tree has a monitoring system within its root zone that sends reports on conditions and the tree's overall health and growth. Bartlett Tree is also growing about 40 swamp white oaks for planting at the site of the United Flight 93 crash in Shanksville, Pa.
Progress to date
So far, almost half of the swamp white oaks destined for the memorial have been transplanted, in time for the official dedication ceremony, 10-year observance and public opening. The remainder will continue to be moved from New Jersey in batches. Today they stand about 30 feet tall, or half their expected eventual height, and have been trained and pruned to look uniform in canopy shape.
Eventually, one more special tree will join the swamp white oaks at the 9/11 Memorial. It is a Callery pear (Pyrus calleryana) that's come to be known as the Survivor Tree. It originally grew at the World Trade Center and though badly damaged, it was determined to be still alive after the attacks. It was moved to a Bronx, N.Y., park where it has been carefully tended for the past 10 years. When moved, it was essentially an 8-foot -tall stump, but today it stands over 30 feet tall. It continues to improve and the hope is that it will one day be moved back to Ground Zero.
Photo taken mid-August, 2011. Image attribution.
Read inspiring accounts of the special connection between people and trees in Keepers of the Trees.
Find help and inspiration for memorial tree planting in How to Plant a Tree: A Simple Celebration of Trees & Tree-Planting Ceremonies.