4.4.7 Maintenance

Bioretention areas require periodic plant, soil, and mulch layer maintenance to ensure optimum infiltration, storage, and pollutant removal capabilities. Providing more frequent and well-timed maintenance (e.g., weeding prior to seed dispersal) during the first three years will ensure greater success and reduce future maintenance of bioretention areas. For more detailed maintenance guidelines, see Appendix G: Maintenance of LID Facilities. In general, bioretention maintenance requirements are typical landscape care procedures and include:
  • Watering: Plants should be selected to be drought tolerant and not require watering after establishment (2-3 years). In more arid environments, watering may be required during prolonged dry periods after plants are established.
  • Erosion control: Inspect flow entrances, ponding area, and surface overflow areas periodically, and replace soil, plant material, and/or mulch layer in areas if erosion has occurred. Properly designed facilities with appropriate flow velocities should not have erosion problems except perhaps in extreme events. If erosion problems occur, the following should be re-evaluated and adjusted as needed: (1) amount of drainage area contributing flows to the facility (2) flow velocities and gradients within the cell; and (3) flow dissipation and erosion protection strategies in the pretreatment area and flow entrance. If sediment is deposited in the bioretention area, immediately determine the source within the contributing area, stabilize, and remove excess surface deposits.
  • Sediment removal: Follow the maintenance plan schedule for visual inspection and remove sediment if the volume of the ponding area has been compromised.
  • Plant material: Depending on safety (pedestrian obstruction or site distances) and aesthetic requirements, occasional pruning and removing dead plant material may be necessary. Replace all dead plants, and if specific plants have a high mortality rate, assess the cause and replace with appropriate species. Periodic weeding is necessary until plants are established and adequately shade and capture the site from weed establishment.
  • Weeding: Invasive or nuisance plants should be removed regularly and not allowed to accumulate and exclude planted species. At a minimum, schedule weeding with inspections to coincide with important horticultural cycles (e.g., prior to major weed varieties dispersing seeds). Weeding should be done manually and without herbicide applications. The weeding schedule should become less frequent if the appropriate plant species and planting density are used and the selected plants grow to capture the site and exclude undesirable weeds.
  • Nutrients and pesticides: The soil mix and plants are selected for optimum fertility, plant establishment, and growth. Nutrient and pesticide inputs should not be required and may degrade the pollutant processing capability of the bioretention area as well as contribute pollutant loads to receiving waters. By design, bioretention areas are located in areas where P and N levels may be elevated and these should not be limiting nutrients. If in question, have soil analyzed for fertility.
  • Mulch: Replace mulch annually in bioretention areas where heavy metal deposition is high (e.g., contributing areas that include gas stations, ports, and roads with high traffic loads). In residential settings or other areas where metal or other pollutant loads are not anticipated to be high, replace or add mulch as needed (likely 3-5 years) to maintain a 2 to 3-inch depth.
  • Soil: Soil mixes for bioretention facilities are designed to maintain long-term fertility and pollutant processing capability. Estimates from metal attenuation research suggest that metal accumulation should not present an environmental concern for at least 20 years in bioretention systems. Replacing mulch in bioretention facilities where heavy metal deposition is likely provides an additional level of protection for prolonged performance. If in question, have soil analyzed for fertility and pollutant levels.


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