Stormwater retention practices: woody shrubs to the rescue

Stormwater retention practices: woody shrubs to the rescue

Professor Nina Bassuk discusses stormwater and bioswales and introduces her new guide for professionals seeking to create successful planted stormwater retention practices.

A healthy ecosystem of plants and soils provides many environmental benefits such as energy conservation, erosion control, carbon sequestration, air pollution reduction, the creation of habitats for pollinators and the reduction of storm water runoff, amongst others. These are called ‘ecosystem services’ and their value can be quantified. Climate scientists are predicting that large scale rainfall events may become more numerous with extended dry periods in between. As a result, many cities are concerned about increasing stormwater runoff.

What is Stormwater?

Stormwater is rain or snowmelt which flows over the ground and does not directly infiltrate into the soil. Historically, stormwater runoff only occurred during large storm events when the rate of rainfall or snowmelt was greater than the rate at which water could be absorbed into the soil. With the advent of wide-scale development and urbanisation, the area of impervious surface in the built environment increases in artificial impervious surfaces like roads, roofs, sidewalks and parking lots that have created a corresponding increase in stormwater runoff.

In addition to an increase in impervious surfaces, soils, which can become compacted due to human disturbance, also experience a significant drop in their ability to absorb runoff during storm events. Increases in stormwater tax the capacity of our sewage treatment systems, especially in cities that have combined stormwater and sanitary sewers. This can create water pollution as well as other stormwater related issues.

Stormwater Issues

Stormwater runoff mitigation is a hot-button issue among landscape architects and engineers; it is something that all designers need to consider and can pose challenges on sites as well as in larger ecological systems. Increased volumes of stormwater runoff caused by an increase in impervious surface area and compacted soils have resulted in a variety of issues including:

  • Sedimentation of water sources, which reduces light penetration of the water column, warms water by absorbing solar radiation, and negatively homogenises stream bottom habitats
  • Streambank erosion
  • Excess nutrient and organic carbon loading resulting in anoxic (low-oxygen) water conditions, which is detrimental to aquatic life
  • Bacterial contamination of water sources, especially in conditions where sanitary sewers combine with stormwater
  • Hydrocarbon pollution of rivers, lakes
    and oceans
  • Pesticide poisoning of aquatic habitats when excess pesticides are washed into water systems
  • Chloride contamination of freshwater systems from de-icing salts used in winter
  • Thermal impacts from warmer stormwater heating aquatic habitats of cool water species
  • Terrestrial trash and debris collecting in aquatic systems


This is a term used by many municipalities and regulatory bodies. Filter strips, vegetated swales, and rain gardens could all be considered bioswales if constructed correctly. Bioswales are essentially stormwater runoff conveyance and infiltration systems that slow, direct, clean and help infiltrate runoff. Plants assist with stormwater infiltration by trapping and taking up water while providing ecological value such as creating habitat and reducing urban heat island effects.

The planted bioswale practice is becoming more familiar worldwide. However, this comes with its own unique set of care and maintenance issues. The new guide from the Cornel Urban Horticulture Institute (Woody Shrubs for Stormwater Retention Practices: Northeast and Mid-Atlantic Regions) 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 a brief period, they are more often very dry between rainfall events. The authors therefore tested several plants for their wet and dry tolerance and developed a guide describing many woody plants that would be adapted to these conditions of alternatively wet and dry soils.

Most stormwater treatment planting guidelines suggest the use of native plants exclusively. However, due to the unique moisture/drought tolerances required for successful growth in these sorts of practices, it would be imprudent to exclude non-native plants. Because of the growth limitations associated with these planting areas, as long as a plant can succeed on site, and is not invasive, any plant that can grow here should be considered.

To promote plantings with reduced maintenance costs, the value of woody plants, specifically shrubs, should be considered. While herbaceous plants may establish more quickly and fill a site, they require at least yearly pruning post-establishment to remove dead foliage and seasonal die-back. Woody plants however – due to their generally slower growth rate and more permanent growth habit – require far less pruning.

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.

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 PDF.

Professor Nina Bassuk
Urban Horticulture Institute
Cornell University
+1 607 255 4586
Tweet @CornellCALS

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