What is a rain garden?
A rain garden is an attractive catch basin for storm water runoff. Ranging greatly in size, rain gardens contain native plants that aid in the removal of contaminants, nutrients, and excess water. Removing these contaminants and excess water will help prevent flooding, slow the aging of water bodies, and assist in keeping waters clean.
What do I need to create a rain garden?
A basin is created, ranging anywhere from 1-3 feet deep, that will absorb rain water within 48 hours. The basin should be partially filled with a composite of masonry sand, compost, and topsoil, all topped with mulch. A depression of 3 to 8 inches should be made into the surface of the basin to collect water. Native plants should be planted within the basin, and SOC WA has provided a list of native plant species appropriate for rain gardens. It is important to use native plants when constructing your gardens. For more information, see our Landscaping Practices page.
How do I construct a rain garden?
The most important decisions in constructing a rain garden are where to build, what size, what composite of soil to use, and which plants to select.
The rain garden should be built at least 10 feet away from any structure, preferably in lower lying areas. It is best to establish the garden in a location where most of your runoff travels to during heavy rainfall.
Ideally the garden should collect all the runoff from your property. The size of the garden should be approximately 25% of the runoff surface area from which the water comes. For instance, if you are collecting water from your roof, the rain garden should be approximately 1/4 the size of your roof surface area. Define the boundaries of the garden with sod, stone, or brick to prevent erosion and help retain water.
The best composite for a rain garden should contain 25% sand, 50% compost and 25% topsoil. Heavily compacted soils need to be loosened up to a 2-foot depth. In poorly draining soils, shallower basins and a gravel underbed, tiles, and/or an underdrain are suggested. Keep the bottom of the basin flat, along with the top, in order to spread out the water and speed up absorption. Placing mulch on the surface of a rain garden will help to absorb and retain water as well.
As mentioned above, the plants chosen should be native to the area. Deep rooted plants are better for decomposing storm water pollutants. Having diverse plantings is better for survival, and encourages bio diversity.
Birds, bees, butterflies, and insects are dependent on different species of plants for food and reproductive use, thus having a greater diversity of plants yields a greater selection for these opportunities. Plants should also be selected base upon the availability of sunlight. Species require differing amounts of shade for survival.
** Image taken from the Southeastern Oakland County Water Authority publication
"Rain Gardens for the Rouge River."
What maintenance is required with my rain garden?
Although little care is required after a garden is constructed, there are maintenance responsibilities required. Like with other gardens, weeding is suggested. Monitoring to ensure that drainage occurs within 48 hours after a heavy rainfall. Standing water is bad and encourages mosquito reproduction. In turn, this can have negative health repercussions. Mulch should be replaced or refreshed at least once a year.