Rain Garden 101

A rain garden is a depressed area in the landscape that collects water from roof downspouts, asphalt or sump pump discharge and allows it to soak into the ground rather than enter the storm sewer system. Less runoff from your home means less channel erosion and fewer suspended solids and pollution in the waterways.

When planted with native grasses and flowering perennials, rain gardens can be a cost effective, low maintenance and beautiful way to reduce and filter runoff from your property. Create butterfly habitat, prevent flooding and make your lawn more attractive by installing a rain garden at your own home!

Calculate Your Run-off

At home, rooftops, driveways, patios and other impervious surfaces prevent precipitation from infiltrating into the ground, so it instead flows off of these hard surfaces and becomes run-off. This run-off picks up pollutants off the ground on its way to storm drains, and eventually our surface waters. Rain gardens are one tool to help capture this run-off on site. Use the following calculator to see how much run-off your home produces (and how much you could capture with green infrastructure)!

For reference, Lansing, Michigan receives an average of 32 inches of rain fall per year.

(Credit: This Old House: How to Build a Rain Garden to Filter Runoff)

BUILD YOUR OWN

FIND A SITE THAT CAN HOLD WATER

Although an existing low-lying area might seem like a natural for a rain garden, you need a place that isn't overly soggy already. Stay at least 10 feet from the house and at least 50 feet from a septic system or slopes greater than 15 percent. Call 811 to make sure underground utilities aren't in the way. Once you have a tentative site, test the soil's percolation rate. Dig a hole 2 feet deep and time how long it takes for 8 to 12 inches of water to disappear. For example, if 8 inches drains in 12 hours, the rate is 8 inches divided by 12 hours, or 0.67 inches per hour. A rate higher than 0.5 is great—your rain garden needs to be just 18 inches deep. If the rate is lower than 0.5 you'll have to dig 30 inches deep. If the percolation rate is less than 0.1, the site isn't suitable for a rain garden.

DETERMINE SIZE AND SHAPE
Your local extension office may have information to help you size a rain garden to suit rainfall patterns typical in your area. The ideal size might be smaller than you expect. On well-draining soil, a rain garden just one-tenth the size of a roof handles 99 percent of its gutter water. Ovals, kidneys, and teardrops often look best, but rain gardens can also be long and skinny. Use a garden hose to test possible shapes. Once you settle on a design, decide where the water will flow in and where any overflow will exit. Mark the shape with spray paint. On your lawn, mark 18 inches farther out for sod removal, since grass has a way of creeping into planting beds.

GRAB YOUR SHOVEL

Remove grass and soil down to approximately 4-8 inches deep. Create a flat bottom so that water will percolate down evenly. If the rain garden is on a slope, you can pile some of the excavated soil into a berm on the low side to retain the water. For stability, stomp the berm soil down well and make the base at least 2 feet wide and the top at least 1 foot wide. The peak of the berm should be at least 6 inches higher than the water level when the rain garden is full.

RUN PIPING

Dig a trench for a pipe that will carry water from one or more gutter downspouts to the rain garden. Install the piping. Rigid piping with smooth walls is the most durable, but corrugated tubing is easier to work with; get the kind without perforations. Extend the piping into the rain garden basin by a foot or so. Line the area underneath with stones to prevent erosion. You can also place stones over and beside the pipe to hide it and to keep corrugated tubing from curling up. When all the piping is in place, fill in the rest of the trench with excavated soil. *You can alternatively keep the piping above ground, as long as it is graded down towards the rain garden.

ADD SOIL

Fill the excavated area with rain-garden soil. Slope the sides gently. If the soil you excavated is relatively free of clay, you can use a mixture of 65 percent native soil to 35 percent compost, or 2 scoops of soil for each scoop of compost. If you dug out clay soil, refill with a mixture of 60 percent screened sand and 40 percent compost.

ADD PLANTS

Group plants in zones, based on how well they tolerate having "wet feet." Plants that thrive in the wettest environment go in the center of the rain garden; that area tends to stay wet the longest after a storm. Put plants that can handle standing water on the sloping sides, and those that are suited to drier conditions on the edges.

MULCH

Once the plants are in the ground, cover the inside of the rain garden with a 3-inch layer of mulch to keep the soil moist and to prevent weed seeds from sprouting. Until a rain garden's plants are established, even drought-tolerant plants require supplemental watering to survive dry seasons. Check the mulch depth annually and replenish as necessary. Rain gardens don't require fertilizers beyond the compost used in the soil mix. Weed and prune to keep the rain garden looking its best.