Stormwater retention is a hot-button issue among landscape architects. It’s something that all designers need to consider and can pose challenges on specific sites as well as in larger ecological systems. As landscape architects, we strive to implement creative practices to mitigate stormwater issues.
The planted retention/infiltration practice is one familiar to us all; however, this practice comes with its own unique set of care and maintenance issues. Enter the new guide from Cornell University’s Urban Horticulture Institute (UHI): “Woody Shrubs for Stormwater Retention Practices: Northeast and Mid-Atlantic Regions.” This guide by authors Ethan M. Dropkin and Nina Bassuk of Cornell University includes helpful information about issues associated with stormwater, various mitigation practices, and an extensive plant list.
In the past, designers have tended to select wet site-tolerant plants for these installations; however, while bioswale soils may be wet for brief periods, they are more often very dry between rainfall events. The authors tested several plants for their wet and dry tolerance and developed a bulletin describing many woody plants that are well-adapted to these conditions of alternately wet and dry soils.
The authors focus on woody shrubs because utilizing shrubs decreases the need for additional seasonal maintenance of cutting back dead herbaceous foliage while successfully adding aesthetic and functional vegetation to stormwater retention practices. Generally, when creating a planted stormwater practice, the urge is to use herbaceous plants either solely, or in greater proportion to woody plants. This guide indicates that the opposite may in fact be more ideal. Due to the overall reduced annual maintenance issues and costs associated with woody shrubs as opposed to herbaceous plants, these authors recommend using solely shrubs (or at least a majority of woody plants) in such installations.
Some of the plants listed were part of a three-month study in Ithaca, NY conducted by the authors. Their study focused on testing the flood and drought tolerances of six shrub species: Amorpha fruticosa, Hippophae rhamnoides, Salix arenaria, Salix purpurea, Shepherdia argentea, and Spiraea tomentosa. The plants were established in large containers of moistened perlite, with each container holding one specimen of all six species. Establishment lasted for about two months. After this point, the containers were broken into four treatment groups:
- Treatment 1: containers were flooded for seven days, allowed to drain for 4 days, and then flooded for another seven days. This flooding and draining sequence was repeated until the end of the experiment.
- Treatment 2: Containers were flooded for three days, allowed to drain for 4 days, then again flooded for three days. This sequence was repeated for the entire experiment.
- Treatment 3: Containers were kept well watered but allowed to drain continuously. Moisture levels were kept equivalent with a moist, well-drained situation.
- Treatment 4: Containers were allowed to dry down for a month and a half until they were re-watered and left to drain freely. In this treatment, the container was watered only once at the beginning of the experiment and once during the course of the experiment.
This treatment schedule was implemented over a three-month period. The end result of this experiment was that all species were successful in all treatments. As a result, all species showed a tolerance of both long-term flooding and drought, which made them promising candidates for use in stormwater practices. The shrubs from this experiment can be found in the guide along with a variety of others with tested or observed tolerances to the soil moisture extremes experienced in stormwater retention practices.
All of the plants covered come with extensive information about their cultural needs and available cultivars. In addition, these plants were specifically chosen for their ability to thrive in situations that experience both temporary inundation and prolonged drought. In short, rather than recommending plants based on their tolerance to wet sites, this guide focuses on species that can thrive along a varied moisture continuum.
In addition to plants, the guide also includes site assessment information (and a checklist) as well as design considerations and general maintenance guidelines. Because of its accessible style and color pictures, the information included might be particularly useful for explaining proposed designs to clients.
Overall, the main value of the guide comes from the extensive, carefully vetted plant list which should be an asset to any professional seeking to create successful planted stormwater retention practices. The guide is available free from the UHI website and a high-resolution version can be accessed here.
About the Urban Horticulture Institute
The Urban Horticulture Institute at Cornell University began in 1980 with the explicit mission of improving the quality of urban life by enhancing the functions of plants within the urban ecosystem. The Institute program integrates plant stress physiology, horticultural science, plant ecology, and soil science and applies them to four broad areas of inquiry. They are:
- The selection, evaluation and propagation of superior plants
- Developing strategies for improving degraded urban soils
- Developing improved transplant technologies to insure the successful establishment of plants in the urban environment
- Working with municipalities to assess and manage their urban tree resources while applying appropriate technologies
Many free resources are available from the UHI website.
About the Authors
Ethan M. Dropkin is a recent graduate of Cornell University with a Masters in Landscape Architecture and a Masters of Professional Studies in Horticulture. Mr. Dropkin works part-time for Cornell while job-hunting in the New York City area.
Nina Bassuk has been a professor and program leader of the Urban Horticulture Institute at Cornell University for the past 30 years. She is co-author of Trees in the Urban Landscape (Wiley), a text for landscape architects and horticultural practitioners on establishing trees in disturbed and urban landscapes. In addition, Dr. Bassuk has authored over 100 papers on the physiological problems of plants growing in urban environments, including improved plant selections for difficult sites, soil modification including the development of CU-Structural Soil™, and improved transplanting technology. She works closely with municipalities to help implement best practices in urban forestry management.
by Ethan M. Dropkin, Student ASLA, and Nina Bassuk
Thank you, Ethan and Prof. Bassuk for sharing info on this topic – particularly emphasis on the fact that, contrary to common misunderstanding & misperceptions about bioretention/filtration facilities and so-called “rain gardens”, these SWM facilities are designed & engineered to drain quickly into the substrate, and therefore the plant community that is specified MUST be tolerant of drought conditions.
I urge you to change the graphic of the “General Bioswale Dimensions” as the profile for such facilities has evolved considerably in the past decade and can be misleading for those seeking current info about bioretention/filtration facilities. The use of geotextile fabric between the planting media and stone base was discontinued due to clogging issues that resulted in failure of the bioretention/filtration system. The system profile has since evolved to a pea gravel “choker” layer instead of the geotextile fabric, and the planting media to a “clean” (clay & silt-free) sand/compost/soil mix of rather exacting standards to avoid failure due to development of fragipani. For those regions where it is available, I have been specifying a soil-less expanded slate horticultural media manufactured by Carolina Stalite known as “PermaTill” bioretention for the planting media as it solves many of the issues associated with soil-based profiles while providing ideal conditions for healthy plant growth. Also of note, key components of a well designed facility includes grass/turf pre-filtration strip to trap sediments and “trash” (including road de-icing/traction grit) from entering the basin/swale and 2-3″ mulch layer on top of the planting media to capture heavy metals as well as moderate temperature and moisture.
Stalite Environmental recently published, ‘Alternative Granular Media for Stormwater Bioretention and Filtration Applications’ that cites current research on the topic, including listing of plant materials were specified and performed well in various bioretention/filtration projects. (see http://www.permatill.com or http://www.StaliteEnvironmental.com for info)
With best wishes and gratitude for the opportunity to contribute to outreach education on a key area of concern to the allied professions and general welfare.