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As in the case of any stormwater BMP, maintenance of bioretention facilities is an import component that ensures consistent performance and aesthetics. Maintenance agreements that set forth specific responsibilities, easements, and covenants should be in accordance with local requirements. RESIDENTIAL APPLICATIONS For bioretention located on private residential lots, encumbering each homeowner with easements, maintenance agreements, and enforcement mechanisms may not be practical. In many cases, municipalities have been hesitant (or refused) to allow bioretention (or rain gardens) on residential projects because they believe that their traditional maintenance requirements would need to be applied to each homeowner. In these cases, a creative approach and awareness of practical considerations is necessary. Bioretention or rain gardens installed by developers or builders are usually considered an amenity integrated into landscape packages. As such, the likelihood or improper maintenance or the potential of removal is not likely to be widespread. Therefore, maintenance issues can be addressed with community and landowner awareness programs including fact-sheets on the function and maintenance of bioretention. In cases where the individual homeowners are appropriately empowered, the municipality can end up with less of a burden on inspection and enforcement resources than with traditional stormwater management practices. The following items are suggested to be incorporated into a consistent maintenance program via landowner awareness programs or legally binding documentation, as appropriate. Weeding As with any garden, bioretention requires weeding of unwanted plant materials. Mulching helps to reduce weed growth and retain moisture in the soil. Weeding should be accomplished routinely as required. Watering If plants wilt during the heat of the day, but recover in the evening, watering is not necessary. The plants are simply conserving moisture. If they do not recover, watering is indicated. Another good rule of thumb is to stick a pencil or screwdriver about four inches into the soil. If the soil is moist at that depth, watering is not needed. If the soil is dry, and the shrubs or trees were planted within the last three years, watering is necessary. Fertilization In traditional, intensively cropped landscapes, soil fertility (and especially the level of available nitrogen) is considered the limiting factor to plant growth. By design, however, bioretention facilities are located in areas where nutrients (especially nitrogen) are significantly elevated above natural levels. Therefore, it is unlikely that soil fertility will be the limiting factor in plant growth, and thus fertilization would be unnecessary. Excess fertilization, (besides compromising the facility’s pollutant reduction effectiveness) leads to weak plant growth, promotes disease and pest outbreaks, and inhibits soil life. If soil fertility is in doubt, a simple soil test can resolve the question. If fertilization should become necessary, an organic fertilizer will provide nutrients as needed without disrupting soil life. Mulching The mulch materials placed in the facility will decompose and blend with the soil medium over time. Typically, mulch material should be re-applied once every six months. The depth of the mulch layer should be no more than 3". Mulch applied any deeper than three inches reduces proper oxygen and carbon dioxide cycling between the soil and the atmosphere, and keeps plant roots from making good contact with the soil. The mulch layer provides an important role in the bioretention physical properties for removing heavy metals from the system. Dividing & Replanting The properly designed facility should thrive and allow planting materials to expand and propagate, eventually becoming overcrowded. If this occurs, perennial plants should be divided in spring or fall. Plants that do not perform well, or die, should also be replaced. Trimming and Harvesting Current practice is to leave ornamental grasses and perennial seed heads standing to provide winter interest, wildlife forage, and homes for beneficial insects. Plants should not be cut back until spring when new growth commences, and even then it is only done for neatness, it does not impact growth. Plants may be pinched, pruned, sheared or dead-headed during the growing season to encourage more flowering, a bushier plant, or a fresh set of leaves. Diseased or damaged plant parts should be pruned as they occur, and if a plant is pest-infested, then perform cleanup in fall to deny the pest a winter home. Trees and shrubs may be pruned for shape or to maximize fruit production. Standing Water Problems Bioretention facilities are designed to have water standing for up to four hours. If this period is routinely exceeded, the facility may not be functioning properly. Should standing or pooling water become a maintenance burden, minor corrective action can usually correct it. Pooling water is usually caused by clogging or blockage of either the surface layer or fines obstructing the filter fabric used between the gravel bed/underdrain and the surrounding planting soil. The surface blockage problem may be corrected by removing the mulch layer and raking the surface. For blocked filter fabric, use lengths of small reinforcing bar (2’-3’ #4 rebar) to puncture the fabric with holes every 1' on center. If the soils themselves are causing the problem, punch holes in the soil or optionally, install a "sand window" at least 1 foot wide running vertically to the underdrain system elevation. In a worst case scenario, the entire facility may need to be re-installed. Trash and Debris Runoff flowing into bioretention facilities may carry trash and debris with it, particularly in commercial settings. Trash and debris should be removed regularly both to ensure that inlets do not become blocked and to keep the area from becoming unsightly. Pet Waste Pet waste should not be left to decay in bioretention facilities because of the danger of disease-causing organisms.
Some of the above information was derived from the Prince George's County Bioretention Manual. |
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