Sustainable Urban Design: Educator’s Toolkit for Project-Based Learning, page 17 of 20

W.B. Saul students install a rain garden at their Philadelphia school
W.B. Saul students install a rain garden at their Philadelphia school. © Andrew Kahl/TNC

Green Stormwater Infrastructure: Rain Gardens

A rain garden filters and absorbs storm water from impervious surfaces, helping to prevent erosion, flooding, and pollution caused by runoff. Rain gardens are often located near buildings, capturing rainwater from downspouts that would otherwise be routed toward a storm drain and sewer. Rain gardens may resemble a regular garden, but they are specially designed to retain stormwater runoff and let it slowly infiltrate the soil. By incorporating native plants into rain gardens, you can increase habitat and support biodiversity!

If a rain garden seems out of scope, consider working with students to build a rain garden in a box, also called a Grattix; or a stormwater planter, and position it under a downspout. Students can even experiment to see if they can improve upon the design!

An infographic that describes how rain gardens naturally filter water
© Erika Simek Sloniker/TNC

Get Started

  1. Determine if a rain garden is the right project for the location. Rain Gardens can be installed in a variety of locations; at a minimum, the location needs to have well-draining soil, be located at least 10 ft. from a building away from utilities and on relatively flat or gently sloping area. The Right Place, Right Project guide by Seattle Public Utilities and King County can be useful in determining if a rain garden is the right project for your space.
  2. Test your soil by digging a test hole about 2 feet deep and 1-2 feet in diameter. Determine the soil texture as you dig. Sandy soil will be well draining, while sticky soil that forms a ball has a higher clay content and won't drain as well. Determine the soil drainage rate by filling your hole with 6 inches of water. Record how long it takes for this water to drain. Divide the inches of water by the hours to drain and you'll have your drainage rate.
  3. Determine how much rainfall your location receives.
  4. Determine the size of rain garden based on the amount of water drainage from the impervious surfaces you are trying to mitigate, the soil texture, and the amount of rainfall in your area. You can use an online calculator like the one found here by Three Rivers Rain Garden Alliance to give you a ballpark estimate. Explore rain garden size calculations in more detail on pages 16-21 of The Rain Garden Handbook for Western Washington Homeowners by Washington State University Extension.
  5. Determine the type of equipment you will need. Rain gardens often require shovels, hand trowels, gloves, curb saw, and sometimes an excavator!
  6. Identify who can help with implementation and funding. Many schools have successfully secured funding through their school club budget, PTA, grant-funding from outside sources, or asking their city’s stormwater department to help fund the project. Consult our financial support page for other options.
  7. More detailed information on how install and maintain a rain garden can be found in The Rain Garden Handbook for Western Washington Homeowners by Washington State University Extension. Though this handbook is geared toward WA State, it's a valuable resource for identifying critical projects steps from start to finish.

Common Challenges

  • An engineer stamped plan is often required before the school will approve.
  • Grounds and facilities staff are concerned about standing water, mosquitoes, and long-term maintenance. It may require a lot of research, meetings, and persistence to get their approval.
  • Ongoing annual maintenance is required to ensure the garden functions and looks nice.

Maintenance Schedule and Other Notes

Ongoing maintenance of the rain garden ensures that it functions properly and becomes a positive example for future rain garden projects. The following guides are great resources for rain garden maintenance and other topics:

The following list describes specific maintenance issues that need to be addressed with rain gardens.

  • Conveyance System: All pipes and drains (including trench drains) shall be cleaned as needed if sediment or other debris accumulates and reduces the conveyance capacity.
  • Debris removal: Check the inlet in the rain garden for garbage and debris throughout the year. Before the rainy season begins in September, check for debris to ensure that storm water drains freely into the rain garden.
  • Mulching: A 3” layer of medium bark mulch or arborist wood chips will help block weeds from sprouting on the soil surface, will help keep the soil moist, and will help prevent soil compaction from light foot traffic. Mulch should cover the rain garden surface in between all plantings and should be replenished if any gaps exist.
  • Watering: To ensure that plants get well established, plants will need deep watering during the drier months for the first two or three years. Remember to water after a dry period even when the planting beds are established.
  • Weeding: Weed the planting beds 3 - 5 times a year to keep invasive plants and weeds under control. Some plants may need to be deadheaded or lightly pruned.
  • Fertilizing: Do not apply fertilizers to your rain garden. The rain garden soil mix provides plenty of nutrients and your garden is planted with native plants that are adapted to this region, no fertilizing is needed.
  • Herbicides and Pesticides: Do not use herbicides or pesticides in your rain garden.
  • Erosion control: Check the rain garden for areas of exposed soil, particularly in the fall before the wet season begins, as well as in the winter. If erosion persists in the rain garden, too much water may be flowing into the garden too rapidly.
  • Plant replacement: Occasionally certain plants in the new rain garden will not thrive or become a problem in the rain garden. Make a note of which plants did not function well and why. Replace plants as needed.

Related Lessons and Classroom Integration

Additional Project Resources