Central Wharf Plaza
Landscape Performance Benefits
- Reduces the average ground-level temperature of the plaza by 10.4°F with tree canopy cover that shades 94% of the site.
- Prevents 369,000 gallons of annual stormwater runoff from entering the city’s combined sewer system by infiltrating all runoff for up to a 25-year, 24-hour storm event.
- Sequesters over 3,600 lbs of carbon annually in the 25 oak trees.
- Increases the tree growth rate by 57% when compared to a typical urban oak by providing over 1,500 cubic feet of soil per tree. When the trees reach their projected mature size in about 33 years, they will sequester over 13,000 lbs of carbon and intercept almost 87,000 gallons of rainwater annually.
- Provides a pleasant connection between the Rose Fitzgerald Kennedy Greenway and the Inner Harbor waterfront with over 1,550 pedestrians observed passing through the site in 5.5 hours (~280 pedestrians per hour).
- Provides a place of respite and relaxation for 22% of plaza visitors who were observed spending an average of 12 minutes in the space
- Improved safety with the number of traffic accidents in streets surrounding the site falling from 6 reported in the years before construction to 1 after the new plaza was complete.
At a Glance
Reed Hilderbrand LLC
Former Land Use
244 Atlantic Avenue
Boston, Massachusetts 02110
This tiny plaza, shaded by 25 mixed-species oaks, connects Boston’s Rose Fitzgerald Kennedy Greenway with the waterfront of the Inner Harbor. Standing in stark contrast to wide-open, nearly treeless areas covering the Greenway, the plaza’s closely spaced large oaks create a micro-forest on the small site. The plaza is well-used by downtown workers, student groups visiting the New England Aquarium, and tourists and commuters walking to nearby ferries. But beyond its civic success, the project demonstrates the value of dedication to the establishment and stewardship of a high-performing urban tree canopy that provides numerous environmental benefits. Employing exemplary planting practices, the below-grade infrastructure investments support the project and the quality, density and scale of trees in the plaza.
- 25 closely spaced, 13-in DBH trees provide a pervasive canopy and extensive shade for the plaza. They are a mix of Red and Pin oaks, two native species that are wind and salt tolerant and relatively resistant to health problems.
- Pervious surfaces cover almost 10,000 sf, or approximately 75% of the site, in the form of dry-laid granite pavers with permeable joints, stonedust and planting soils. The pavers and stonedust allowstormwater to infiltrate while supporting heavy public use.
- 12 slot drains collect all excess runoff from the site and carry the stormwater directly to 13 trees for passive irrigation, with any water not taken up by tree roots continuing to percolate down through the soil. In addition, the drains convey air to the root zone through a perforated lattice below the surface.
- Flood bubblers provide an efficient deep root mechanical watering system for the trees. A rain sensor automatically adjusts irrigation based on the level of precipitation, while moisture sensors installed on select trees allow for periodic manual adjustment depending on soil moisture.
- The continuous layer of sand-based structural soil below the surface allows for an unobstructed root zone by providing additional support for seat walls and stairs with shallow spread footings, filters stormwater as it percolates down, and enables better distribution of moisture across the site.
- Mini-piles and spanning grade beams support the granite benches and catenary posts, reducing physical barriers throughout the root zone and allowing flexibility for utility configurations below grade.
- The pitched plane of the plaza, sloped at a 2% gradient, provides elevated views toward the water and a stepped gathering place at the active harbor edge where over 1.3 million Aquarium visitors and 335,000 ferry riders pass by annually.
- 290 linear feet of granite seat walls with wood insets and freestanding wood benches, provide a place of respite for tourists, neighbors and commuters.
- A system of LED lights hung from catenary wires threaded through the tree canopy provides efficient and subtle lighting, extending the useful hours of the plaza.
- All streets surrounding the plaza were narrowed and a separate vehicle drop-off area was added on the wharf/aquarium side. The new drop-off area allows vehicles to pull through when leaving instead of having to back up into the flow of traffic.
The design team began working from the belief that the site, a fully paved traffic island only 4,000 sf in size, would succeed only if the spatial result was strong and positive. Furthermore, summer shade was deemed the primary characteristic that would appeal to visitors, offering a contrast to the more open parks of the neighboring Rose Fitzgerald Kennedy Greenway. Set among busy streets along the waterfront of downtown Boston, the site’s location posed difficult conditions above and below grade. Close proximity to the harbor meant that plants would have to be carefully selected and arranged to withstand high winds and salt. Because the site was once part of the bay, the sub-soils and urban fill below the surface could not support normal spread footings without considerable settlement. In addition, the tangled network of underground utilities further contributed to the complexity of the site.
Working with city agencies, the team expanded the site to over 13,000 square feet - just enough scale to provide identity and spatial coherence. Many large caliper trees were installed to create a complete canopy with instant shade, while still allowing unhindered pedestrian circulation through the site. Red and Pin oaks, both native species, were selected primarily for their form, wind and salt tolerance, and relative resistance to health problems. Below the surface, a specific soil and infrastructure matrix was designed to accommodate settling sub-soils and complex utilities, while optimizing tree growth. Mini-piles and spanning grade beams were utilized to support the granite benches and catenary posts, while shallow spread footings and sand-based structural soil provide support for seat walls and stairs. These allow for a continuous layer of soil and unobstructed root zone for the trees and offer flexibility for managing utility configurations. To support the long-term function and appearance of the site, design team members sustain a continued relationship with the client and are an ongoing part of maintenance decisions. Should any trees need to be replaced in the future, the dry-laid paving allows for a quick turnaround and minimal impact to the site’s ground plane.
- The integrated planting system on the site contains several elements in its design beyond what is typically found in an urban plaza. These added features improve the conditions for root growth, increasing tree health, size and longevity. Compared to the 4.3% average annual mortality rate of a typical urban street tree, the trees on the site have a current annual mortality rate of only 1.6%. Based on these rates, in a typical urban plaza, all 25 trees would have to be replaced in 23 years, while in this plaza only 9 would have to be replaced over the same time frame, a savings of nearly $50,000.
- The key role that soil biology plays in maintaining the high performance of the plaza’s urban tree canopy became evident when the condition of several trees started declining immediately after installation due to the stresses of transplantation. An arborist with a specialty in soil biology was brought on to the project to help stabilize the trees. He suggested that the best way to accomplish this was to activate the latent biology of the manufactured soil on the site through compost tea and mycorrhizal applications. The applications would quickly recreate the trees’ native soil biology, helping to support good root growth and nutrient uptake, leading to robust, healthy trees. This has become part of the routine maintenance of the site with a single annual application used to adjust soil biology based on yearly testing results. As the site’s soil biology changes over time, an annual decision is made with the owner on whether or not to continue this process.
Granite Pavers: Cold Spring Granite
Granite Walls: A Lacroix et Fils Granite
Lighting: BK Lighting
Soils: Dexter and Harpell Inc.
Trees: Halka Nurseries Inc
Client: Frog Pond Foundation
Architect: Chan Krieger Sieniewicz
Landscape Architect: Reed Hilderbrand, LLC
Structural/MEP Engineer: ARUP
Civil Engineer: Vanasse Hangen Brustlin
Soil Scientist: Pine Swallow Environmental
Lighting Designer: LAM Partners
Specification Writer: Rico Associates
General Contractor, Construction Management: Turner Construction
Landscape Contractor: ValleyCrest
Role of the Landscape Architect
The landscape architect worked with the client, a private philanthropy dedicated to supporting urban open space initiatives, to develop and apply exemplary, high-performance urban design and planting practices. On behalf of the client, the landscape architect negotiated with city agencies to expand the project site from the original 4,000 sf traffic island to create a 13,100 sf plaza. Through collaboration with the design team of an urban designer/architect, soils engineer, soils biologist, arborist and structural, civil and geotechnical engineers, the landscape architect created an integrated and optimized infrastructure to support healthy trees and heavy public use.
Case Study Prepared By
Research Fellow: Victoria Chanse, PhD, Assistant Professor, University of Maryland
Research Assistant: Jen Salazar, PhD candidate, University of Maryland
Firm Liaisons: Eric Kramer, Principal and Ryan Wampler, Associate, Reed Hilderbrand, LLC
Chanse, Victoria, and Jennifer Salazar. “Central Wharf Plaza.” Landscape Performance Series. Landscape Architecture Foundation, 2012. https://doi.org/10.31353/cs0270