Using Trees in Stormwater Management Plans

Published: May 1, 2011

A swamp white oak planted in porous pavement in a parking lot at Missouri Botanical Garden.

Over the past ten years, there has been a dramatic increase in interest in methods for reducing or eliminating urban and suburban stormwater runoff. This has been stimulated by the implementation of "Phase II" regulations of the Clean Water Act published in 1999 and requiring smaller towns to develop stormwater management plans. These plans rely on accepted "Best Management Practices" (BMPs) that can be implemented to reduce or slow the movement of stormwater from properties. These BMPs include a wide range of approaches, including rain gardens, wetlands, bioswales, porous pavement, green roofs and many other interesting and creative ideas. Although many BMPs utilize plants because of their ability to slow down water and transpire it to the atmosphere, trees are often not featured. This is partially due to the general perception that tree roots disrupt engineered features like the berms used in rain gardens and bioswales. However, there is ample evidence that trees can significantly reduce stormwater runoff if used appropriately.

A study conducted in Fayetteville, Arkansas revealed that increasing tree canopy cover in the city from 27% to 40% reduced stormwater runoff by 31%. This translated to an estimated savings of $92 million due to reduced need for engineered retention structures. Trees can slow water movement in many ways. Tree leaves intercept some of the precipitation in a rainfall event and reduce the velocity of the droplets. This, in turn reduces splashing and soil compaction, keeping the soil more porous. Decomposing leaf litter creates a spongy layer at the soil surface, holding the water until tree roots can take it up. Deep tree roots also help to channel water deeper into the soil where it can be stored for future use by the trees.

The St. Louis Metropolitan Sewer District, in cooperation with the Missouri Department of Conservation, Missouri Botanical Garden, Shaw Nature Reserve, Missouri Department of Agriculture and Grow Native published an excellent guide entitled; "Landscape Guide for Stormwater Best Management Practice Design" (http://www.shawnature.org/documents/pdf/MSD.pdf). The guide gives lists of native Missouri plants recommended for planting in various practices. Many tree species are listed as suitable for planting on pond margins or slopes.

An interesting approach being tried in various parts of the US is diversion of rainwater into streetside bioswales through curb cuts. In some cases trees are planted in a synthetic "structural soil" consisting of large rocks with a small amount of soil and hygroscopic gel. The structural soil (patented by Cornell University) can be compacted to support streets and sidewalks but still contains large spaces where tree roots can grow. Thus a tree has access to a large root growth zone that can also serve as a temporary water reservoir. This method can reduce the peak flow of stormwater from a rainfall event by up to 80%.

As small and mid-sized Missouri towns struggle to comply with Phase II stormwater regulations, we will see many more interesting and creative ways to use plants in stormwater management plans. Hopefully trees will be included in most of the plantings.