by Jeremy Person, PLA, ASLA, Brian Wethington, Donna Evans, and Irene Ogata, ASLA
For more than two decades landscape architects and stormwater professionals have been utilizing vegetated bioretention systems to help address complex stormwater and climate change-related issues. Bioretention systems use a combination of soil and plants to collect, detain, treat, and infiltrate runoff from roads, roofs, and other impervious surfaces. It is becoming apparent that plant health is one of the major drivers of increasing life-cycle costs, and that improper plant selection is partially to blame.
Landscape architects, horticulturalists, and designers are beginning to better define which characteristics make a plant ideal for use in bioretention. Understanding the site-specific needs for plants and identifying project goals allow designers to address performance issues up front and reduce long-term maintenance liabilities. The following three issues should be considered as early in a project as possible:
1. Project Goals and Facility Design
The two major goals for most bioretention projects are pollution reduction and flow control. Projects may serve one goal or both, and this may vary across a city or region. Bioretention facilities are designed for project-specific hydraulic regimes with controlled flooding and hydroperiods that affect plant viability. This affects plant selection in several ways:
- Hydroperiod: Understanding the flooding cycle of the facility, its frequency, and how it relates to the growing cycle of the plants is critical. Smaller plants often fail because they are routinely flooded during the growing season, depriving them of needed oxygen. Designers should prioritize plants that grow taller than the high-water level and take cues from native wetland plants that have evolved to tolerate similar hydroperiods.