Developing a rain garden is designing with plants for bioretention. 

Once the engineering (size and location) and slope issues are resolved, populating the planting area with plants appropriate to the conditions is critical.  From the top, the finished product will look like an attractive garden.  Under the surface, many important environmental benefits will be occurring due to the action of roots and drainage media on the volume and quality of the stormwater.

The rain garden performs two basic functions : Physical and Aesthetic; it may also provide a third: Habitat. 

Physical  -- The rain garden performs  an important environmental function and is part of a BMP/ “treatment train” approach to stormwater management. 

Plant information to Remember: 

• Plants must be able to handle extremes of wet and dry; bioretention rain garden areas are “extreme environments”
• Trees should be generally situated on the edge of bioretention rain garden areas and not within 25’ of pipes that underpin the area if it is an underdrained bioretention area
• Use plants that can tolerate the concentrations of contaminants that may find their way into the area. The plant lists provided are all hardy in this way.
• Provide a grassy filter around an area if possible
• Initial establishment will require higher maintenance than subsequent periods
• Schedule pipe clean-out maintenance and provide access points to keep the system working as designed
• Bioretention cells/rain gardens are designed to restore a site’s predevelopment hydrology. The hydrology model is forest hydrology where runoff is less than 1%.  The rain garden may be comprised of either woody or perennial plants which work towards that hydrology goal.

Aesthetic –The rain garden can provide an important aesthetic enhancement to a site.

rain gardens do not have to be only configured as “slices of nature”. The condition of the planting medium, the volume of the garden and its unground profile determine if the garden will meet stormwater BMP requirements. A fully planted garden with appropriate plants will provide enough root action to clean and reduce the volume of the water. The rain garden appearance can range from the naturalistic to a more formal form. 

When considering design, evaluate it according to some basic design principles such as balance of form, color and scale and four season interest.  Determine if the design, as drawn or thought of will achieve the desired effect (concept).  Edit as necessary and keep the design internally consistent.

Bioretention Sizing
http://www.lid-stormwater.net/bioretention/bio_sizing.htm