Article and photos by Laura Knosp
One of the common misconceptions about landscape architecture is that its primary role is placing plants around buildings—or, as we sometimes joke, “putting parsley around the pig.” In truth, the foundational underpinning of the profession is water management, which influences nearly every aspect of our work.
At a macro scale, water shapes ecosystems and cultural identities and connects people to a sense of place. At the scale of parks and gardens, it offers aesthetic and experiential value, grounding people in the natural world. Across all scales, water is paradoxically both a vital resource to conserve and also a powerful force to defend against—extremes that are becoming more critical in an era of climate volatility and environmental instability.
Landscape architects must continuously navigate this paradox: designing spaces that celebrate the beauty and cultural significance of water, while also building resilience to its extremes. In response, the profession is expanding its toolkit, from rain gardens to revived wetlands, learning from natural systems and historical technologies to restore balance to a disrupted water cycle.
In 2019, we were commissioned by New England Botanic Garden at Tower Hill in Boylston, Massachusetts, to redesign their visitor entry experience, including major accessibility upgrades and a significant expansion of the parking area, which more than doubled its capacity. While necessary, this expansion substantially increased the impervious surface on the site. Complicating matters, NEBG lies within the resource protection zone of a nearby public drinking water reservoir, and state guidelines prohibit the use of permeable paving in this area.
To address the increased runoff, we worked closely with civil engineers to develop an integrated stormwater management system that included rain gardens, vegetated swales, filtration catch basins, underground infiltration chambers, and detention basins. These elements work together to filter, slow, and infiltrate runoff from the expanded parking areas, meeting regulatory standards and protecting water quality.
In simple terms, rain gardens are depressions in the ground that help to manage stormwater through natural processes. They help slow runoff, promote infiltration, filter pollutants, and reduce dependence on traditional stormwater infrastructure. At NEBG, we implemented two types of rain gardens: filtration and infiltration, each serving distinct functions.
In the center of the new parking lot, we took advantage of the sloping topography to create a linear rain garden approximately 10 feet wide and 100 feet long, running between two bays of parking. Due to the rain garden’s close proximity to vehicles and its placement within the overall system, state regulations required it to function strictly as a filtration garden rather than an infiltration feature.
This filtration garden serves as a first line of defense, capturing and treating contaminants commonly found in parking lot runoff—such as Total Suspended Solids (TSS), phosphorus, nitrogen, and hydrocarbons. It can also temporarily hold up to 500 gallons of stormwater, slowing its release into the larger drainage system downstream.
The design of the rain garden is straightforward: A four-foot-deep trench with an impervious liner prevents infiltration into the groundwater. A perforated underdrain pipe sits in nine inches of free-draining gravel at the bottom of the trench. This pipe then connects to an outlet structure located at the lowest point of the garden. Above the gravel, thirty inches of engineered rain garden soil temporarily absorbs rainwater and provides filtration. Finally, the garden contains over 1,000 perennials, grasses, and shrubs specifically selected for their ability to thrive in both wet and dry conditions.
Select Planting List:
Shrubs
- Red chokeberry (Aronia arbutifolia)
- Buttonbush (Cephalanthus occidentalis)
- Summersweet or sweet pepperbush (Clethra alnifolia)
- Redtwig dogwood (Cornus sericea)
- Virginia sweetspire (Itea virginica)
- Fragrant sumac (Rhus aromatica)
Herbaceous Perennials
- Rose milkweed (Asclepias incarnata)
- Joe Pye weed (Eutrochium purpureum)
- Blazing star (Liatris spicata)
- Wild bergamot (Monarda fistulosa)
- Tall white beardtongue (Penstemon digitalis)
- New York aster (Symphyotrichum novi-belgii)
Ornamental Grasses and Sedges
- Fox sedge (Carex vulpinoidea)
- Switchgrass (Panicum virgatum)
- Little bluestem (Schizachyrium scoparium)
The majority of the parking lot at NEBG drains through a series of catch basins, vegetated swales, and the previously described filtration rain garden, eventually reaching a detention basin located about 25 feet downslope to the west. In a separate watershed on the east side of a small ridge, we installed a larger infiltration rain garden to capture runoff from an additional 50 parking spaces and an expanded garden maintenance facility. These areas collectively account for approximately 25,000 square feet of impervious surface, which required a rain garden roughly 50 feet wide by 150 feet long.
This larger garden shares a similar design to the filtration rain garden but differs in two key ways. First, a number of low-tech strategies act to filter the water before it reaches the rain garden. These include a wide grass swale along the parking lot edge to remove debris, a sediment forebay that allows small particles to settle out before water enters the rain garden, and a stone check dam that reduces water velocity, limiting erosion as runoff enters the system. Second, unlike the filtration garden, this rain garden is unlined, allowing infiltration into the underlying soils.
These gardens have now been established for several years, and their stewards have recently shared valuable insights regarding their condition. The smaller infiltration garden has been a clear success and is well-loved, though the combination of dense woody planting and steep side slopes has introduced some maintenance challenges, particularly regarding invasive species management.
The larger basin has presented more difficulties. During construction, the soil subgrade was over-compacted, limiting infiltration capacity, and although the garden was renovated afterward, it continues to hold water. Garden stewards remain optimistic that as new plantings are established and their root systems develop, soil conditions will improve, enhancing infiltration over time.
Photo courtesy of New England Botanic Garden
Takeaways from the NEBG project highlight the importance of protecting subgrade soils from compaction and selecting plantings with a realistic understanding of long-term maintenance—particularly in smaller, more constrained gardens. Feedback also serves as a valuable reminder that inherent to the word “garden” is the presence of a “gardener.” While rain gardens are nature-based solutions, they require ongoing stewardship to succeed!
Projects like the one at NEBG reflect the evolving role of landscape architects as critical problem-solvers in an era of climate uncertainty. As water becomes an increasingly complex design challenge, our responsibility to create resilient spaces has never been more urgent.
Laura Knosp is a Principal at Ryan Associates Landscape Architecture and Planning in Waltham, MA. Her work covers a variety of scales and project types, from large parks and mixed-use developments to residential and institutional works. She particularly enjoys working with multidisciplinary teams to create innovative designs.
Laura earned her Master’s Degree in Landscape Architecture from the Rhode Island School of Design, where her academic work focused on creating sustainable design solutions to cultural and environmental challenges. At The Evergreen State College in Olympia, WA, Laura completed her undergraduate studies with a dual concentration in Environmental Science and Fine Arts.