What is a Raingarden?

A raingarden is a shallow depression which captures and filters water. It may hold water or filter it right away, depending on the plants, size of the garden, and other factors.   Plants selected for a raingarden are specially selected for their ability to trap pollutants and filter water before it returns to the water cycle. They are species that prefer wet soils or prefer to have “wet feet.” These plants have deep root systems which reach far into the soil, creating “tubes’ or pipes underground.

When rain falls on a rain garden, the water seeps slowly into the ground, and is filtered naturally by the soil before re-entering our waterways. Rain absorbed within a rain garden will filter pollutants otherwise delivered to treatment plants or directly to streams. Studies report greater than 90% of copper, lead and zinc, 50% of nitrogen and 65% of phosphorus can be filtered in a rain garden. (American Rivers)

Why are Rain Gardens Important?

Human activities and development on land have dramatically changed how water returns to streams, lakes and rivers after it falls from the sky as precipitation. As we develop the land, we add roads, houses, parking lots, sidewalks, and driveways. In cities or other heavily developed areas, water rushes across hard surfaces before returning to our waterways.

In contrast, when rain falls in a forested or natural environment, about 50% of that water seeps into the ground, 40% evaporates or is taken up by plants and only about 10% runs off the surface into lakes and streams.

In areas such as cities and large commercial areas, where 75 to 100% of the surface is impervious, the majority of rainfall becomes runoff, and there is very little infiltration. There just isn’t anywhere for the water to filter to.. Runoff carries chemicals and other pollutants as it drains to storm sewers and eventually into our waterways such as streams, creeks, or rivers.

When rain falls through the air, it collects pollutants such as gaseous emissions and tiny particulate pollution. When it falls to the ground, here in Minneapolis, it usually lands on “impervious surfaces:” parking lots, sidewalks, streets and even rooftops. The rainwater now picks up pollutants such as road salts, litter, gas and other chemicals.

Parking lots, sidewalks, streets, and rooftops are all known as “impervious” surfaces—surfaces which water and other liquids can not permeate. Because the water can’t permeate, or soak in, it has to rush to the lowest point. Often, in our urban environment, that “lowest point” is a street gutter and storm drain—which drain directly to the Mississippi River.

The more impervious surfaces that are present in a community, the more polluted rainwater runoff travels straight into the river.