Lesson 2: How to build a rain gardens in your back yard

By Marla Barhoum | Jun 19, 2013
Courtesy of: Snohomish Conservation District With the help of Snohomish Conservation District staff and WSU Master Gardeners, local girl scouts planted the rain garden at Columbia Elementary last year.

The following is the second of three mini lesson plans the Beacon will run about rain gardens and other Low Impact Development projects residents can install in their yards to help protect our local streams and Puget Sound.

City staff created the lessons as part of a watershed-based Stormwater Strategies Plan developed in partnership with the state Department of Ecology, city of Everett, Paine Field Airport, Mukilteo School District and the Snohomish Conservation District.

For more information, go to www.ci.mukilteo.wa.us. -Ed.

In the last lesson, we learned about polluted runoff and its harmful effects on our local water supply. The good news is there’s something you can do to help – build a rain garden!

Rain gardens are shallow landscaped depressions in the ground that can be shaped and sized to fit any yard.

They are made with special soil mixes that allow water from our roofs and driveways to soak in rapidly and support healthy plant growth. They can also be landscaped with a variety of plants to beautify the area.

The primary function of rain gardens is to keep rainwater close to where it falls. It mimics the natural forested ecosystem that existed before our city was developed and helps balance our local water cycle.

A rain garden acts like a native forest by collecting, absorbing and filtering stormwater runoff from hard surfaces such as rooftops, driveways, patios and other areas that don’t allow water to soak directly into the ground.

This runoff travels to the nearest storm drain; collecting pollutants along its way. This is the exact point where clean rainwater becomes contaminated.

By corralling the direct runoff from the hard surfaces on your property, you can stop polluted runoff in its tracks.

The city of Mukilteo has installed rain gardens and other low impact development (LID) techniques for managing polluted runoff at City Hall, Lighthouse Park and Rosehill Community Center.

These small scale drainage features (such as rain gardens, bioswales and permeable pavement) are functional and beautiful and can be applied to existing residential and business sites.

Building Rain Gardens takes some planning, but it’s worth the effort. Take time to research and consult with rain garden experts, Master Gardener groups or local natural resource agencies when building your own rain garden.

Make sure you have all the correct information you need, before you start building your rain garden.

• Measure the square feet (area) of the roof or pavement the water collects from

• Determine the infiltration capacity in your yard (how quickly the soils drain)

• Calculate the best overall size and depth for your rain garden

Instead of growing fruits and vegetables, native plants thrive in rain gardens. The magic happens deep under the earth which is why rain gardens are so successful.

A native plant’s deep root system and a healthy soil mix provides natural filters that absorb pollutants and cleans the water as it moves underground to our local waterways. The best time to build a rain garden is in the summer, when the soil is dry.

Last year, fifth graders at Columbia Elementary explored the human impacts on our water cycle by exploring their local watershed, examining storm drains and collecting rain data.

Then, local Girl Scout troops built a rain garden on the Columbia campus, which now treats the water that once pooled in the main parking lot.

You can see this community rain garden in action as it is visible from the parking lot, just off Harbour Pointe Boulevard.

Currently, the fifth grade Leadership Team at Columbia Elementary collects rain data at their school to upload to a national network (CoCoRahS, Community Collaborative Rain Hail and Snow, www.cocorahs.org) and Girl Scout troops from around Mukilteo maintain the new rain garden.

In the final segment of this series, you will learn what supplies are needed and estimate costs to build, plant and maintain a rain garden, as well as connect with local rain garden experts and agencies in our community.

Family activities

Track the rainfall in your back yard. Place a measuring cup outside for one week. Make a chart by labeling Monday through Sunday across the top of the page and divide each day of the week with long lines.

Write how much rain falls each day of the week. What was the total amount of rain for the week? Did the final results surprise you?

Collect your own rain data along with our own Columbia Elementary and other families and school groups around the country by joining the Community Collaborative Rain Hail and Snow data collection network.

Become a citizen scientist and share your rain data online at http://www.cocorahs.org/. Because every drop counts!

Visit a rain garden

Visit one of the public working rain gardens or bioswales in Mukilteo at Columbia Elementary, Mukilteo City Hall, Rosehill Community Center and Lighthouse Park.

Upon arrival, take some time to sketch the garden. What stands out to you about the design? Record your observations? Did anything surprise you? If you were to build your own rain garden, what would you change or add?

Calculate the infiltration rate of your soils

To measure the rate that water infiltrates at the garden site:

1. Dig a 1.5 – 2 foot deep, narrow hole

2. Place yardstick (or other measuring tool) at bottom of hole, sticking up vertically

3. Fill hole 1 foot deep with water

4. Measure how long it takes for water to drain completely. If the weather is dry, measure three times and use the last measurement

5. The Infiltration Rate equals the Depth of the water in the hole in inches divided by the number of Minutes it takes for the water to drain completely divided by 60

IR = D ÷ M ÷ 60

6. Assess the infiltration rate:

Fast = 0.5 inches/hour or more

Slow = 0.1 – 0.5 inches/hour (consider 2–3 foot rain garden depth)

Very slow = less than 0.1 inches /hour (consider another location)

What is the right size for the rain garden?

The rain garden must be large enough to collect and absorb almost all the rain water falling on the hard surfaces around it.

Size depends on: 1) square feet (area) of roof or pavement it collects from; 2) the infiltration capacity of the soils and 3) depth of rain garden.

Watch out for places where rain gardens don't work!

Conditions that are unsuitable for rain gardens:

• Depth to groundwater less than 3 feet below surface;

• Distance from steep slopes (slopes steeper than 15 percent);

• Low spots that already accumulate water and don’t drain;

• Septic drain field within 50 feet; and

• Building foundation within 10 feet.


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