Woody Shrubs for Storm Water Retention
Planted stormwater retention and infiltration practices are important for reducing runoff and maximizing green space in urban areas. While a wide variety of herbaceous plants such as Soft Rush (Juncus effusus), Swamp Milkweed (Asclepias incarnata), and Joe-Pye Weed (Eutrochium spp., formerly Eupatorium spp.) are often successfully used in these spaces, they can present maintenance issues because of the need to annually cut back dead foliage and stems. Utilizing woody plants decreases the need for additional seasonal maintenance while successfully adding aesthetic and functional vegetation to stormwater retention practices.
Why Use Plants?
Plants are an integral part of all of the stormwater infiltration practices listed above. They play a variety of important roles in the success of these practices. Plants:
- provide infiltration channels for water to move through via their rootsassist in the removal of pollutants like heavy metals from retained stormwater, in conjunction with soil microorganisms
- help to hold soils together, reducing erosion by stormwater
- slow runoff which in turn may cause some suspended solids, sediment, etc. to settle out of the water
- add aesthetic value and educational potential
- improve air quality through uptake of gaseous pollutants
- reduce the urban heat island effect through evaporative cooling and shading
- can sequester atmospheric carbon
- increase biodiversity and create wildlife habitat
Given all of these benefits, it is critical to include plants in stormwater practices whenever possible.
What Plants are Best?
When selecting plants for stormwater infiltration, common sense would seem to dictate the use of wetland plants. However, due to the rate at which most of these practices allow water to infiltrate, the majority of planted stormwater practices will likely only be inundated for a few minutes after a small storm event, and up to a day or two for a larger event. Unlike most permanent or semi-permanent wetlands, these areas remain relatively dry most of the time. Because of this charateristic, plants that can handle both temporary inundation and relatively protracted drought are the best choices for a low-maintenance planting.
The location within the stormwater practice that plants are installed is also significant. Generally these practices will have high points around their edges, which are relatively level to the surrounding topography, and low points towards their center which will be several inches or feet (depending on the size of the practice and its intended water holding capacity) lower than the rim. As a result, the wettest parts of these plantings will be their lowest point. This report deals exclusively with plants appropriate for these low lying, periodically saturated areas. Soil moisture levels at the upper edge of retention and infiltration practices are generally average to dry. Therefore many plants which are appropriate for average to dry soils can be used there. Appropriate plant selections for these drier areas include (but are not limited to): Juniperus spp., Rosa spp., Cotinus spp., Callicarpa spp., etc. However, many of these plants might be unlikely to thrive in the periodically saturated conditions found at the lower elevations of these practices.
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, potentially only once every three years, reducing the overall amount of maintenance significantly.