Environmental Services
Rain Gardens
New EPA video! Reduce Runoff: Slow it Down, Spread it out, Soak it in Learn about raingardens, rain barrels and more!
View video of two raingarden projects at The Arlington condo in Arlington.
What is a Rain Garden?
A "rain garden" is a man-made depression in the ground that is used as a landscape tool to improve water quality. The rain garden forms a "bioretention area" by collecting water runoff and storing it, permitting it be filtered and slowly absorbed by the soil. The bioretention concept is based on the hydrologic function of forest habitat, in which the forest produces a spongy litter layer that soaks up water and allows it to slowly penetrate the soil layer. The rain garden should be strategically located to intercept water runoff.
Benefits of Rain Garden
Rain gardens help filter nutrients from rain water running off your driveway or roof, improving water quality. The first flush of rain water is ponded in the depression of the rain garden, and contains the highest concentration of materials washed off impervious surfaces such as roofs, roads, and parking lots. The water-loving plants in the rain garden also take up and use the rain water, reducing problems with excess water or ponding in your yard.
Many people worry about rain gardens causing mosquitoes. This is not a problem because rain gardens do not retain water long enough for mosquito reproduction. Standing water almost always soaks away within a few hours and usually within a matter of minutes. Mosquitoes require a number of days in standing water for reproduction. If water does remain for a matter of days in your rain garden, then your soil is possibly very clayey and/or very compacted. You may be able to remedy this problem by loosening and adding humus or mulch in the upper 6 to 18 inches.
Rain Garden Project
Residents at The Arlington condominium complex in south Arlington installed a rain garden in 2005 in an area that was known to be frequently soggy. The area is behind one of the condominium buildings, and receives runoff down a hill from an adjacent complex. The area is somewhat shady and not much grass or plants were growing prior to the project. View a second raingarden installed at this complex.
View of the area where the rain garden was installed.
The volunteers began the project by testing the drainage in the area for the proposed rain garden. One volunteer dug a one foot hole, filled it with water, and monitored it to see how long it took to drain. Ideally, it should drain within 6-12 hours. They also secured a truckload of mulch for amending the soil.
To start the garden, the team loosened the top layer of soil using pickaxes and pitchforks. Then, they removed about 8 inches of soil, using some of it to build a berm in the front of the rain garden. The made the berm with sloping sides and a flat top, approximately 18 inches across.
A load of mulch was secured for the project.
Volunteers loosening the top layer of soil.
After removing the top 8 inches of soil from the area, they loosened the soil along the bottom of the rain garden as well. Once the team had excavated the garden as deeply as possible (with the time and resources available), the group mixed the remaining soil and mulch together and refilled the excavated area.
A berm was created in the front of the rain garden.
The volunteers mixed the soil and mulch extensively, and then planted a variety of water loving shrubs and plants in the garden. The garden has weathered some heavy rainfalls this spring very well.
These images show the rain garden during a heavy rain storm of greater than 2 inches of rain.
This shows the rain garden 24 hours after a record rainfall on Friday, May 20, 2005. You can see that the rain garden has drained completely.
View additional rain garden projects in Arlington County.
Rain Garden Resources
Check out this new Raingarden Design and Construction Manual! (about PDF format)
Virginia Dept. of Forestry has created a Raingarden Technical Guide (about PDF format)
Find out if there are any upcoming Raingarden workshops and view presentations from previous workshops on the Northern Virginia Regional Commission raingarden web site.
Check out this handy manual: Raingardens - a how-to manual for homeowners (about PDF format)
Montgomery County has an excellent web site called Rainscapes on rain gardens and rain barrels.
Here is an excellent brochure on Rain Barrels and Rain Gardens from the Center for Watershed Protection (about PDF format).
Another excellent Rain Garden brochure (about PDF format).
This raingarden manual from the Northeast Ohio Public Education committee is useful.
Here is a general web site about Rain Gardens.
"You and Your Land – A Homeowner’s Guide to the Potomac River Watershed."
(about PDF format).
