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Drainage Area

• limited to less than 2 to 3 acres
• preferably less than 1 acre

Ponding Depth

• maximum 6" recommended for soils with an infiltration rate of at least 2"/hr
• maximum 3 to 4" recommended for soils with low infiltration rates or high hydraulic loadings (combine with a smaller drainage area)
• ponding depth may be increased if using sandy soils and underdrains to increase filtration
• any pooled water should be drawn down within 4-6 hours after a storm event

Plants

• must be able to tolerate
1. expected pollutant loadings
2. highly variable soil moisture conditions
3. ponding water fluctuations
• cannot be an invasive species
• the use of native species is recommended
• minimum recommended caliper size for trees is 1"
• a minimum of 3 species of trees and 3 species of shrubs is recommended to insure diversity
• avoid species that require regular maintenance

Soil

• homogeneous mix of
• 50% construction sand
• 20-30% topsoil with less than 5% maximum clay content
• 20-30% organic leaf compost
• pH between 5.5 and 6.5
• recommended minimum depth of 2 to 2.5 feet without large tree plantings
• recommended depth of 4 to 4.5 feet with large trees
• soil infiltration rate should exceed 1.5"/hr

Mulch

• maximum 2 to 3 inches deep
• should be fresh, not aged
• apply uniformly, do not pile around the base of trees
• do NOT use grass clippings

Groundwater

• depth below the facility invert at least 2'

Slope

• if greater than 15%, technique not recommended or use a weep-garden design (see The Bioretention Manual)

Pollutant Concerns

• common homeowner pollutants include lawn fertilizers and driveway oils
• for the primary pollutants of concern, (nitrogen and phosphorus), the removal efficiency increases with depth suggesting that deeper cells reaching approximately 2 to 3 feet should be used (see Watershed Benefits)

Underdrain

• ***required for all rain gardens in residential areas
• build with a cleanout well that is accessible by the homeowner
• do not locate within the groundwater zone of saturation
• must have a hydraulic capacity greater than the planting soil infiltration rate
• may outfall to a suitable location such as a common space area, stream valley, drainage swale, roadside open-section, or existing enclosed drainage system

Surface Overflow

• typically not a design problem in residential settings due to
1. small drainage areas
2. surrounding grass provides a naturally safe, non-erosive surface for overflow
3. design is off-line, already utilizing an overland flow path

House Considerations

• if using on a lot with a basement, make sure that
1. there is a minimum 25 foot setback from the home's foundation
2. the rain garden's invert is lower than the proposed basement floor elevation
3. the rain garden is located downgrade from the home

Lot Considerations

• ensure that the rain garden is constructed entirely within lot boundaries
• maintain a minimum of 2 feet between rain garden and property lines
• if one lot's rain garden is planned to treat cross-lot drainage, a surface easement must be provided and no more than 2 lots' drainage can be involved
• to maintain typical use of the property, locate rain gardens near the perimeters and edges of the lots, away from traveled areas
• private rain gardens cannot be located in public right of ways without first obtaining permission from the Department of Public Works and Transportation
• do not plan a rain garden where wooded areas must be cleared to make room for the facility

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¹ The Bioretention Manual, Prince George's County Department of Environmental Resources Programs and Planning Division, Maryland, 2001.