Plant Diversity

Natural wetlands typically contain a wide diversity of plant life. Attempts to replicate that diversity in constructed wetlands designed for wastewater treatment have in general not been successful. The relatively high nutrient content of most wastewaters tends to favor the growth of cattails, reeds, etc., and these tend to crowd out the other less competitive species over time. Many of these constructed wetlands in the United States and Europe have been planted as a monoculture or at most with two or three plant species, and these have all survived and provided excellent wastewater treatment. The FWS wetland concept has greater potential for beneficial habitat values because the water surface is exposed and accessible to birds and animals. Further enhancement is possible via incorporation of deep open-water zones and the use of selected plantings to provide attractive food sources (e.g., sago pond weed and similar plants). Nesting islands can also be constructed within these deep water zones for further enhancement. These deep-water zones can also provide treatment benefits as they increase the hydraulic retention time (HRT) in the system and serve to redistribute the flow, if properly constructed. The portions of the FWS wetland designed specifically for treatment can be planted with a single species. Cattails and bulrush are often used but are at risk from muskrat and nutria damage; Phragmites offers significant advantages in this regard. A number of FWS and SSF wetlands in the southern United States were initially planted with attractive flowering species (e.g., Canna lily, iris) for esthetic reasons. These plants have soft tissues which decompose very quickly when the emergent portion dies back in the fall and after even a mild frost. The rapid decomposition has resulted in a measurable increase in biological oxygen demand (BOD) and nitrogen leaving the wetland system. In some cases, the system managers utilized an annual harvest for removal of these plants prior to the seasonal dieback or frosts. In most cases, the problems have been completely avoided by replacing these plants with the more resistant reeds, rushes, or cattails, which do not require an annual harvest. Use of soft-tissue flowering species is not recommended for future systems, except possibly as a border.

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