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Guide to Installing a Rain Garden

April 14, 2017

Rain gardens allow excess water from rooftops, low-lying areas, and drainage ditches to be beneficially absorbed into the ground.  They make great use of the excess water by providing the water for plants, and allowing the runoff to be slowly absorbed into the ground instead of pooling or flooding and potentially causing damage and pollution.  They are a great way to display native plants of your area, and attract native butterflies, birds, and insects.  Rain gardens  improve water quality by filtering runoff and providing local flood control.  

There are a few things to consider when installing a rain garden; soil and drainage, and plant selection.


It is crucial that the soil in a rain garden is fit to absorb and filter runoff water. The soil must be permeable/porous enough to allow water to drain and filter properly within a 48 hour time frame.  If the preexisting soil isn't suitable, it can be replaced with a mix consisting of 50% sand, 30% compost, and 20% topsoil.  It is extremely important that the soil isn't compacted. Water won't flow properly if the ground is compacted. Rain gardens should also have an overflow drain or perforated pipe for extreme rain conditions  that could overwhelm the garden. Raingardens in Minnesota

Installing a Rain Garden


 Rain gardens are designed to collect and slowly absorb large amounts of water, but in between rainstorms, the rain garden will be dry. Because of this, the type of plants selected should be able to tolerate both dry and saturated soil.  Depending on the rain garden design, there will be low points that stay saturated and steeper slopes that will stay mostly dry. The placement of the plants should coordinate with their moisture preference. Native plants are encouraged as they will contribute to urban habitats for native butterflies, birds, and beneficial insects. Here is a short list of five plants that are staples in rain gardens.  The 30 Best Plants for Tough Sites

1. Blue Flag Iris
2. Swamp Milkweed
3. Prairie Blazing Star
4. Little Bluestem and
5. Redosier Dogwod

 Swamp MilkweedPrairie Blazing StarBlue Flag Iris


Rain gardens are typically used to collect, absorb, and filter runoff from impervious surfaces like rooftops, driveways, and patios.  Downspouts can be directed towards the rain garden, but shouldn't be placed within 10 feet of the house because of  potential water problems in the basement.  There are several online rain garden calculators to determine the appropriate size of  a rain garden.  

Step 1: Calculate the impervious surface (roof, driveway, etc.) that will be treated. 10'x15'=150 sq. ft.
Step 2:  Determine the depth of the rain garden and amount of rainwater that will be treated.  These figures vary but a typical rain garden might be 6 inches deep and treat 2 inches of rainfall. For this example, the rain garden is only 4 inches  deep and will treat a 1 inch rainfall event.
Step 3: Divide the impervious surface square footage by the depth and amount of water in the rain garden. 150/4=37.5
Step 4:  This particular rain garden will be 37.5 sq. ft. at 4 inches deep and will be able to handle up to 1 inch of rainfall in a 48 hour period.

Some counties or cities may offer grants to homeowners that are interested in installing a rain garden as well as other water stewardship practices.  Information on Funding. Check with your local council on the construction requirements and if there are any benefits that you would receive from doing so.