Rain Gardens-Nature's Water Filters

By creating a rain garden, you can help protect the watershed from storm water pollution and you get to enjoy the beauty of a low-maintenance garden filled with native plants.

Safe drinking water from local drinking water systems or private wells originates in ground water, streams, rivers, springs or lakes - this is called a watershed. Protection of all water sources in the watershed requires the combined efforts of municipalities, water systems managers, conservation agencies, farmers and individuals.

A rain garden is a living system that protects our watershed from pollutants. When it rains or when snow melts water-borne contaminants - mud, silt, fertilizers, pesticides, salt, auto fluids and other potential pollutants - wash into storm drain systems from roads, driveways, parking areas, walks, farm land and rooftops.

Collecting storm water and keeping it in the area where it originates prevents runoff from carrying mud or silt and other pollutants into waterways through storm drains and downspout systems. Instead, it collects in an on-site shallow bowl-shaped depression that filters out debris and pollutants naturally and lets plants and local groundwater reabsorb the water locally.

Rain gardens are not back yard ponds. They're garden features that collect runoff water during a storm then let your garden pull it back into the soil naturally over the course of a day or two. Rain gardens act like sponges that fill up and then dry out. Because rain gardens don't hold water indefinitely like a backyard pond, there's no long-standing water source to permit mosquitoes to breed.

Rain gardens don't need a water circulation system or pump; rain gardens capture and hold storm water runoff just long enough for it to be absorbed back into the soil of the garden and the ground around the rain garden collection area. Plants living in the shallow depression area and on the berms absorb the collected storm water within a few days.

Rain gardens come in all sizes and range from large bio-retention gardens for commercial areas, business parks and urban parks to simple homeowner gardens and suburban yards. All types of rain gardens benefit the environment while they add beauty to the landscape. They create a simple self-managing solution to protect the watershed and they enhance the areas where they're created.

High volume bio-retention rain gardens use piping systems that send water from the basin areas into underground tanks that remove pollutants and eventually discharge the filtered storm water out to traditional storm water systems or swale areas. These higher capacity systems are generally used in larger commercial developments and parking lots or in areas located near wetlands or high quality water sources. Home rain gardens don't need to use complex underground storage equipment. Home rain gardens are small scale natural garden systems that are simple to construct and virtually maintenance-free.

Rain water is natural; storm water is not, and it contributes up to 70% of the pollution in regional watersheds. The pollutants travel to local streams, rivers and lakes by storm water through direct drainage runoff from land, paved surfaces and storm drain systems.

Close to half of the pollution in storm water runoff comes from yards and gardens. By adding a rain garden to your landscape plan, you help preserve water quality and beautify your yard with a low-maintenance garden area that practically takes care of itself while filtering runoff from your property.

Our entire yard is a series of rain gardens. We designed and built a connected system of rain gardens because we have a sloped lot with the highest point located at our driveway and the lowest spot leading to a meadow with a creek running through it. Our goal was to collect our storm water runoff and prevent it from reaching the creek. The added benefits are we've nurtured many native plant species and have attracted birds, toads, frogs, crayfish, butterflies and dragonflies to our garden.

Rain Garden Location
Water flows down hill (down grade). The place to locate your rain garden is down grade from gutter spouts or adjacent to the paved areas of your driveway or parking lot. You can also create a shallow channel, swale or dry stream bed leading from downspouts or paved areas to your rain garden. Try to choose a spot at least 10 feet away from any buildings.

Estimate the size of your rain garden garden by figuring one third the square footage of your roof. This is the recommendation for standard, loamy soil, but if you have sandy soil your garden can be smaller or if your soil is heavy with high clay content it'll need to be a bit larger.

Rain Garden Design
The basin area or depression of the rain garden should be about 8 inches below the level of the lawn or garden area that supply the runoff water. If you have an area of the garden or lawn where water already tends to pond after storms or snow melt, this area is a good choice. Nature tries to take care of itself, so you can take advantage of that by using the natural contours and creating a planned collection area for your storm water runoff to filter and dissipate naturally.

Soil Considerations
Sandy, fast-draining soil is ideal for rain gardens but you can amend other soil types to work well and absorb water. The goal is to collect the rain water from a storm in the bowl, then let it slowly drain into the soil within a day or so. If you have soil with high clay content, you'll need to increase the dimensions and add sand and organic matter to lighten/loosen the soil texture and make it more absorptive. Rain gardens should not hold water for more than a few days - ideally, they should dry out within 24 to 48 hours after the end of a storm.

Work with Nature and Land Contours
If you've chosen a low spot in your yard for your rain garden, a location where water naturally collects after a rain storm or during spring thaws, size won't matter as much. You can work with the depression nature wants to use for filtering storm water. Dig down into the soil 6-8 inches within the center of the depression and shape the sides around the edges into raised edges (berms) that gently slope into the center. Be sure to create a berm on the downhill side to contain the runoff and hold it in the rain garden instead of letting it run out and into storm drains, creeks or the street.

Add organic compost and mulch material to the center of the depression and plant the floor of the rain garden with native wildflowers, grasses, plants and shrubs based on recommendations for your area. Be sure to arrange the plants and flowers by height. You might need to dig a shallow channel (swale) from your roof drain spot or runoff area to the rain garden to help the water reach the water collection location.

You can line the swale area with river stones or plant it with grass. The swale will direct storm water into the rain garden center, and any stones or grass in the channel will slow down the runoff stream and help to filter the water as it travels to the collection area. You can also create a second or third collection garden down grade from the first, and let the spillover from each one follow a swale or channel to the next rain garden area.