Designing and Planting Rain Gardens

Designing and planting a rain garden is very similar to creating other perennial gardens, with a few of the following exceptions:

Location

Rain gardens must be located to intercept runoff from impervious areas. They can be placed anywhere good soils with adequate drainage rates exist. It is best to keep rain gardens away from building foundations, utilities, water wells, and septic systems where applicable.

Size

Rain gardens sited for single-family homes are typically 5 to 10 percent the size of the impervious surface generating the runoff entering the garden. Measure the square footage of the impervious area (length x width); then multiply this by 0.07 (7 percent).

Determine a length and width of the rain garden that best fits the site. For example, a 2,000-square-foot roof, when multiplied by 7 percent, would call for a rain garden 140 square feet in size, or 14 feet long by 10 feet wide.

Garden Depth

A typical rain garden is between four and eight inches deep. A rain garden less than four inches deep will need too much surface area to provide enough water storage to infiltrate larger storms. Storm water runoff should spread evenly across the entire rain garden, to increase the opportunity for infiltration.

Soil Amendments

To prepare for a rain garden, remove soil to create a depressional area. Blend in soil, sand, and compost mixture to enhance infiltration by creating deep, loose soil to absorb water and pollutants. Special consideration may be needed for clay soil. The most common cause of rain garden failure is soil compaction.

Plant Selection

While rain gardens are a highly functional way to help protect water quality, they can also be an attractive part of your yard and neighborhood. Choose native plants based on site considerations for light, moisture, and soil. Vary plant structure, height, and flower color for seasonal appeal and butterfly habitat. Mowed grass borders are recommended around the garden.

Young plants, or plugs, are best for rain gardens because they are easier to establish and maintain. When laying plants out, randomly clump individual species in groups of 3 to 5 plants to provide bolder color. Be sure to repeat these individual groupings to create repetition and cohesion in a planting. It is a good idea to place plant labels next to each individual grouping. This will help identify the young native plants from weeds as you maintain the garden.

It is important to water rain gardens regularly throughout the first season. Once established, they may require additional watering during drought or extended dry periods, especially in the eastern part of Montana. A shredded wood mulch is an important part of a rain garden. Mulch helps retain moisture and discourages weed seeds from germinating.

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Last Modified: 07/05/2007