can be effective without the use of underdrains or replacement of native soils with the bioretention mix. Yet it also indicates that pre-installation infiltration rates should be at least .25 in/hour. Type D soils will require an underdrain paired with the sandy soil mix in order to drain properly. Rain gardens are often located near a building's roof drainpipe (with or without rainwater tanks). Most rain gardens are designed to be an endpoint of a building's or urban site's drainage system with a capacity to percolate all incoming water through a series of soil or gravel layers beneath the surface plantings. A French drain may be used to direct a portion of the rainwater to an overflow location for heavier rain events. If the bioretention site has additional runoff directed from downspouts leading from the roof of a building, or if the existing soil has a filtration rate faster than 5 inches per hour, the substrate of the rain garden should include a layer of gravel or sand beneath the topsoil to meet that increased infiltration load. If not originally designed to include a rain garden onsite, downpipes from the roof can be disconnected and diverted to a rain garden for retrofit stormwater management. This reduces the amount of water load on the conventional drainage system, and instead directs water for infiltration and treatment through bioretention features. By reducing peak stormwater discharge, rain gardens extend hydraulic lag time and somewhat mimic the natural water cycle displaced by urban development and allow for groundwater recharge. While rain gardens always allow for restored groundwater recharge, and reduced stormwater volumes, they may not improve pollution unless remediation materials are included in the design of the filtration layers. Vegetation Typical rain garden plants are herbaceous perennials and grasses, which are chosen for their porous root structure and high growth rate. Trees and shrubs can also be planted to cover larger areas on the bioretention site. Although specific plants are selected and designed for respective soils and climates, plants that can tolerate both saturated and dry soil are typically used for the rain garden. They need to be maintained for maximum efficiency, and be compatible with adjacent land uses. Native and adapted plants are commonly selected for rain gardens because they are more tolerant of the local climate, soil, and water conditions; have deep and variable root systems for enhanced water infiltration and drought tolerance;