PWP Landscape Architecture

Twitter share

National September 11 Memorial


New York, New York

Completion Date



National September 11 Memorial & Museum


Michael Arad, Davis Brody Bond (associate architects)


>Parks >Gardens >Civic Landscapes >Featured

The Memorial commemorates the victims of the attacks of September 11, 2001, at the World Trade Center, Shanksville, Pennsylvania, and the Pentagon as well as the World Trade Center attack of February 26, 1993. 




PWP joined Michael Arad in the final stage of the Memorial design competition with a mandate from the jury to humanize the scheme without diminishing the abstraction that had established it as a finalist. The broad scope of the trauma of 9/11 requires that the Memorial use a symbolic language understood by a diverse audience; this language is an integral part of “Reflecting Absence” by Michael Arad and Peter Walker.



Sacred Space

Visitors will leave the everyday life of the city and enter into a sacred zone defined by a dense forest of 416 oak trees. Above the limbed-up trunks, a canopy of leaves will provided welcome shade in the heat of the summer and seasonal color in the fall. In the winter the sun will cast shadows through a light tracery of bare branches, and in spring, the trees will express the renewal of nature.

The Voids

Using a language similar to Michael Heizer's North, East, South, West, the voids render absence visible. In this way, the overwhelming losses of September 11th are given permanent presence. Within the protected space of the forest, visitors will arrive at the two great voids with their thundering waterfalls. After viewing the victims' names on the bronze parapets of the voids, visitors will move back to the city through the trees and take comfort from the soothing, life-affirming forest. 

The Flat Plane

Through the trunks of the trees the flat plane of the park is visible in its entirety. The density of the trunks extend the apparent depth and size of the plane and at the same time soften the view of the buildings beyond. The horizontal surfaces of the plaza--stone, ground cover, lawn, and steel grating--are patterned to assert and reinforce the flatness of the constructed plane. PWP combined graphic techniques and technical solutions developed through the firm's long tradition of designing perceptually flat landscapes.


The one-acre voids, cut 30 feet into the site, are lined with waterfalls designed by Dan Euser  With the addition of lighting designed by Paul Marantz, the waterfalls are also visible at night.

The Forest

The Memorial grove resembles a “natural” forest, until visitors discover that the trees align to form arching corridors in one orientation. The form recalls the arches that architect Minoru Yamasaki placed at the bottom of the original towers. In this way, the grove expresses the shared patterns of nature and humanity.

The Glade

A grassy clearing within the grove is a quiet space away from the bustle of the plaza. Designed to accommodate ceremonies--specifically, the reading of victims' names annually on September 11th--the space also provides soft green park space on typical days. 

Scaling Space 

Within the Memorial grove, the varying distances between trees, the placement of benches, and the rhythm of ground-cover beds will create spaces with distinct scale, character, and qualities of light. 

Paving Design

PWP studied the paving pattern at full scale in chalk, ultimately arriving at 12” x 60” pavers and 3” x 15” cobbles. The varying density of shadows in the joints between the pavers and the cobbles creates a subtle banding pattern that breaks up the vast flat plane of the Memorial into human-scaled zones.


The Memorial Tree

Working in collaboration with arborist Paul Cowie, PWP selected swamp white oaks as the tree species for the Memorial. Considered more disease resistant than red or pin oaks, the swamp white provides the strength, longevity, and symbolic weight that PWP was seeking for the Memorial.

Digging and Boxing

PWP developed a tightly-controlled specification for advanced tree procurement and maintenance and selected each specimen for consistency in size and form, seeking straight-trunked oaks that would form sturdy columns when planted at the Memorial. To maximize transplant recovery time the trees were transplanted into boxes several years ahead of construction.

The Temporary Nursery 

The trees were transported by Environmental Design to a temporary nursery in New Jersey where they have acclimated to the local climate for several years. Here, Bartlett Tree Service pruned, irrigated, and fertilized the trees to generate a canopy consistent in form and height.

Tree Transportation and Installation

Since the trees were grown in over-sized boxes at a nearby location, tree transport and installation had virtually no impact on the root system. The trees were simply craned onto a flatbed truck, driven into the city, and craned into planting pits within the plaza. In contrast to the typical 'ball and burlap' tree-transplant process, which is limited to tight windows of time in the spring and fall, the Memorial trees were installed in batches through the seasons, in pace with plaza construction.


Model Building

The PWP design process involves studying projects in a variety of scales and media. For the Memorial, large models were integral to establishing the scale, alignments, and relationships within the plaza. These models were also used to describe the project to stakeholders, political leaders, and the press.


Full-scale mock-ups of each plaza component were integral to understanding details and proportions. 

Fountain Testing

Using full-scale mock-ups to study the performance of the water, Dan Euser developed a tapered, rounded weir that is both water-and-energy efficient as well as highly visible and beautiful. 

Selecting Materials

The plaza is built of relatively few elements and materials. A single pole, for instance, incorporates lighting and security. One type of granite is used for cobblestones, pavers, and benches. Planted ground coverings are limited to evergreen English ivies and turf grass. A single tree species is repeated throughout the Memorial grove. The limited palette is critical to the notable quietness of the plaza. PWP conducted wide searches and brought great care to the selection of each material.

Jurisdictional Complexity

Despite its apparent simplicity, the Memorial is a massive green roof—a fully constructed ecology—that operates on top of multiple structures including the PATH station and tracks, the Memorial Museum, a central chiller plant, parking, and additional infrastructure. Together with Davis Brody Bond, the associate architect for the project, PWP coordinated with multiple agencies and stakeholders to establish a consistent visitor experience that extends over multiple structures and through several jurisdictions.


Endurance and Performance

Throughout the design and construction process, sustainability of the plaza was considered in terms of both material endurance and landscape performance. 

Reusing Storm Water


The plaza surface-and-drainage infrastructure is designed to function as a large self-sustaining cistern. Water from rainfall and snow melt is channeled into large holding tanks, treated and stored and re-used to support the Memorial forest via a specialized drip-and-spray irrigation system.

Shade and Cooling

The Memorial grove is a dense planting of trees--a forest at the heart of the redeveloped World Trade Center. As they grow the trees will provide shaded space to increase comfort for visitors and reduce heat absorption on the plaza. The transpiration of the many leaves will cool the air throughout the district.

Access to Systems

The plaza is designed with a network of maintenance tunnels that provide access to site systems--irrigation, electrical, drainage. These tunnels will extend the life of the plaza by allowing maintenance crews to access, test, adjust, and repair systems with ease.

Generous, Healthy Soils

Substantial soil volumes, adequately irrigated, aerated, and drained, are the most critical element in the long-term success of the Memorial grove. An enormous volume of soil--40,000 tons in total—lies buried beneath the plaza to ensure that the oaks will grow and thrive to maturity. 


The WTC District

Advanced infrastructure planning was necessary to fit together all the buildings and streets in the district. In 2008, with towers 2, 3, and 4 in advanced stages of design—and well in advance of the establishment of the Port Authority's streets-and-sidewalks engineering team—Silverstein Properties commissioned PWP to establish a design framework for the public spaces of the entire district. Detailed elevations set the levels of all future streets and sidewalks, building lobbies, and the Memorial. Existing and proposed underground utilities and infrastructure, such as the existing subway box, were analyzed and negotiated to create a final surface design. 

An important example of this rebuilding effort is PWP's development of a complete design for Cortlandt Way as an important pedestrian connection aligned with the Memorial. Now under construction, Cortlandt Way, is built with finely detailed stone pavements, stone seat walls, and retail and dining terraces to serve the needs of office workers and visitors to the Memorial; the significant grade change leading down to the Memorial was designed to amplify the view through the trees and into the South void. Construction will be in phases related to the completion of towers 3 and 4. 


Overseeing Construction 

The first phase of the National 9/11 Memorial was dedicated on September 11. 2011. The Memorial opened to the public on September 12, 2011 on a reservations only basis. Construction continues on the remaining work. Through regular site visits and meetings with the contractor and our collaborators, PWP continues to advocate for a Memorial that will endure through the ages. 

Tree Installation 

The trees were craned into place by Kelco Landscaping after the surrounding plaza was constructed and prepared for them.

Plaza Paving

Pavers were hand set using a sand-set paving method. Like European streets, the pavers can be lifted out and re-used whenever repairs are required.

Memorial Glade

Sod was rolled out over an engineered (compaction-resistant) soil mix for the lawn areas within the Memorial Glade.

Finishing Touches

In the days leading up to the Phase One opening, workers rushed to finish the plaza. All adjustments and repairs were finalized, and the plaza was given a final cleaning in time for the opening.



CONSULTANTS Arborist: Paul Cowie Associates; Fountains: Dan Euser Waterarchitecture ; Lighting: Fisher Marantz Stone; Irrigation: Northern Designs; Soils: C.R. Dixon & Associates; Paving Engineer: Mark Smallridge and Associates; Signage and Wayfinding: C&G Partners; Structural Engineer: WSP Cantor Seinuk; Mechanical Electrical Plumbing: Jaros Baum & Bolles; Tree Procurement and Moving: Environmental Design; Vector Control: Global Environmental Options  

SUPPLIERS Pavement: Savema s.p.a.; Tree Planting: Kelco Landscaping