n. A garden that uses sandy soil to help filter stormwater runoff from nearby roofs, roads, and other hard surfaces.

Example Citations:
In Melbourne, Australia, a five-year scheme is establishing 10,000 “raingardens” — flower and vegetable beds underlain with sandy soil to help water filter away.
—Geoffrey Lean, “How Suds can stop the floods,” The Telegraph, January 3, 2014

The Nine Mile Creek Watershed District is offering money to residents, schools, nonprofit groups, associations, businesses and others to pay for projects that protect and improve water and natural resources in the Nine Mile Creek Watershed District. ... Eligible projects include raingardens, pervious pavers, cisterns, native habitat restoration and shoreline or streambank restoration projects, according to a news release.
—“Watershed offers more than $100,000 in grants,” Eden Prairie News, February 19, 2014

Earliest Citation:
Stormwater from the hospital and Lincoln Road will flow into a landscaped stormwater pond before flowing into Henderson Creek. Stormwater from the new car parks and some of the existing car parks will flow into swales and raingardens before entering Henderson Creek. All this is designed to cleanse the water from sediments and pollutants and to prevent flooding further downstream.
—“Waitakere Hospital — a new Eco Hospital for our community” (PDF),, Waitakere City Council, December 15, 2001

This entry highlights raingarden's emergence as a single word (although note that an outfit named "Digital Raingardens" seems to have been around since the late 1990s; no idea if this refers to the garden type or is just a name picked for its juxtapositional poetry). The phrase rain garden is a bit older, dating to at least 1995:

Rain Gardens are an alternative storm water management practice being applied as a pilot project at Somerset. The Gardens are a combination of grasses, shrubs, and trees that serve as ground cover, a middle story, and a canopy in simulation of a forest environment. The shallow, landscaped gardens manage stormwater through bioretention, combining physical, biological, and chemical processes to maximize pollutant removal. The settling of sediments in shallow pool areas, the natural processes of plants and microbes, and chemical reactions occurring in the soil allow the gardens to absorb and purify stormwater runoff.
—“Nonpoint Source News-Notes” (PDF), Environmental Protection Agency, August 1, 1995

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