“Constitution Gardens will become a biodiversity hotspot on the National Mall,” said Adam Greenspan, FASLA, design partner at PWP Landscape Architecture. “We will create a garden, based in nature, while respecting the historic design.” Constitution Gardens in Washington, D.C. opened in 1976 to commemorate the bicentennial of the American revolution. Concepts outlined in the McMillan Plan and designs by Dan Kiley and SOM shaped the landscape.
But over the past forty years, the gardens fell into disrepair and became a pass-through site on the way to other more popular destinations on the mall. Now, the second phase of a three-phase plan to revitalize the garden has been approved by the Commission of Fine Arts and National Capital Planning Commission. The design by PWP Landscape Architecture and Rogers Partners Architects will create a “new ecological landscape” designed for people and hundreds of plant and animal species.
Phase one of the project involved picking up and moving the historic Lockkeeper’s House, the oldest structure on the National Mall, a few blocks away. The restored House is now a visitor center.
Phase two, which is expected to begin later this year, will redesign the 6.75-acre lake at the heart of the landscape, and create 2.5 acres of new meadows and woodlands that will together function as a natural system.
“The current concrete-lined lake is ecologically dysfunctional,” Greenspan said. “We are rebuilding the lake as a healthy living system.”
PWP will deepen and widen the lake and replace the concrete bottom with clay. A diverse range of aquatic plants will help clean the water and ensure the lake becomes a habitat for fish, frogs, and birds.
PWP envisions such a healthy lake that fly-casting will be possible from a new lake ring, a circular pathway. The interior of the ring will also be a spot for model boating.
The landscape surrounding the lake will be designed to act as part of the water-cleaning system. New soils will be brought in to replace the highly compacted existing soils. Nearly half of the lawns, which don’t add any ecological value, will be replaced with native meadows and woodlands. In these new woods, 478 trees will be planted.
The trees and meadows will feature 124 species. “We are planting a highly diverse palette that will shift and change over time.”
With new soils, meadows, and trees, all the stormwater that hits the site will be captured and filtered, and then circulate into the lake. “The bioinfiltration system, above and below the surface, will leverage plants, soils, bacteria, and animals to clean the water before it enters the lake,” Greenspan said.
The system is expected filter more than nine million gallons of water a day.
An upcoming phase three of the project will include a new pavilion with event spaces; expanded woodlands; and new connections to surrounding streets.
Greenspan argues that the new soil is critical to the success of the project. In 2011, they found half of the original trees on site had failed to thrive and been removed. And since another survey in 2014, another 30-40 percent have gone. “The site is currently inhospitable to plant life.”
“We need healthy soils to create a healthy tree canopy, which can then provide shade to cool the landscape.” The new trees will also shade the lake, chilling the water and adding to the site’s overall cooling effect in D.C.’s increasingly hot summers.
While there is an embodied carbon cost to trucking in acres of new soils, “this is not a place where we could use the very degraded, compacted soil, which is mostly rubble.”
There are trade-offs. The carbon emissions released from soil construction enables the increase in biodiversity and long-term carbon storage and climate resilience of the site.
This article was originally published on The Dirt.
