Planter boxes are constructed planters that capture, temporarily store, and filter storm water runoff, typically from rooftop areas. The vegetation, ponding areas, and soil media in the Planter boxes remove contaminants and retain storm water flows from small drainage areas before directing the treated storm water to an underdrain system. Typically, planter boxes are completely contained systems; that can be part of a disconnection strategy allowing roof downspouts may be directed to vegetated planter boxes to store and filter stormwater. This results in a reduction of runoff and infiltration. Planter boxes provide “green space” in tightly confined urban areas that provide a soil/plant mixture suitable for stormwater capture and treatment.
Depending upon the type of box selected, evapotranspiration will be increased and otherwise decreased infiltration and groundwater recharge may be replenished. Water quality may benefit, depending upon how much runoff is directed into the ground and prevented from worsening erosive stream flows. Perhaps just as important are the positive aesthetic effects as Planter boxes offer considerable flexibility and can be incorporated into small spaces, enhancing natural aesthetics of the landscape.
Planter boxes can take any number of different configurations and be made out of a variety of different materials, although most are constructed from wood. Construction specifications are critical in order to make sure that an appropriate volume of runoff from smaller storms “feeds” the carefully selected vegetation types in the boxes (however carefully selected species might be, many can be expected to require additional watering during dry spells). Obviously, large volumes of runoff should not be directed into Planter Boxes which are undersized.
Planter boxes are effective for removing
| Drainage Area | Soil Infiltration Rate | Water Table Separation | Depth to bedrock | Facility slope | Inflow rate |
| < 0.35 ac | N/A | N/A | N/A | N/A | Mulch: 1 cfs, Grass: 3 cfs |
| Task | Frequency | Maintenance Notes |
| Pruning | 1–2 times/year | Nutrients in runoff often cause bioretention vegetation to flourish. |
| Mowing | 2–12 times/year | Frequency depends on location and desired aesthetic appeal. |
| Mulching | 1–2 times/year | Recommend maintaining a 1"–3" uniform mulch layer. |
| Mulch removal | 1 time/2–3 years | Mulch accumulation reduces available water storage volume. Removal of mulch also increases surface infiltration rate of fill soil. |
| Watering | 1 time/2–3 days for first 1–2 months; sporadically after establishment | If drought conditions exist, watering after the initial year may be required. |
| Fertilization | 1 time initially | One time spot fertilization for first year may be required to establish vegetation. |
| Remove and replace dead plants | 1 time/year | Within the first year, 10% of plants can die. Survival rates increase with time. |
| Inlet inspection | Once after first rain of the season, then monthly during the rainy season | Check for sediment accumulation to ensure that flow into the planter box is as designed. Remove any accumulated sediment. |
| Outlet inspection | Once after first rain of the season, then monthly during the rainy season | Check or erosion at the outlet and remove any accumulated mulch or sediment. |
| Miscellaneous upkeep | 12 times/year | Tasks include trash collection, plant health, spot weeding, removing invasive species, and removing mulch from the overflow device. |
| BMP | Retrofit ($/sq ft) | &M ($/sq ft/yr) | Retrofit with Underdrain ($/sq ft) | New Construction ($/sq ft) | New Construction with Underdrain ($/sq ft) |
| Planter Box | 42.6 | 1.6 | 42.6 | 2.6 | 42.6 |
