Iowa Stormwater Education Program

Rain Gardens

The secret to most Rainscaping practices, including rain gardens, is good quality soil. If an area of good soil with adequate infiltration rates exists on a property, it may be an ideal place to locate a rain garden. A rain garden is defined as a landscaped depression that captures runoff from a property and allows it to drain slowly, over a period of 12-24 hours. This allows time for plants to use the water and for pollutants to be filtered as the water seeps into the soil. To manage the water falling on your property, including your rooftop and driveway, work to infiltrate the runoff generated by a 1.25" rainfall.  A 1.25" rain equates to 0.78 gallons of water per square foot of drainage area.

When deciding upon the size of garden to build, determine the approximate area of the impervious surfaces (i.e. roof, driveway) and multiply it by 7 - 20% to calculate the size of the rain garden.

Keep in mind the following guidelines for a successful rain garden:

  • Soil infiltration rates should be greater than 0.5 inches per hour - any less and the water will pond for too long. The soil may need to be amended if this is the case.
  • Located them at least 10 feet from building foundations. 
  • The bottom of the rain garden should be level from side to side and front to back - no slope.
  • There should be at least 6" of ponding depth, allowing the runoff time to infiltrate into the ground.
  • Plant native plants with deep roots and water-loving plants to help maintain soil quality and good infiltration / percolation rates.

To learn more, please refer to the Iowa Rain Garden Manual.


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