How to Install a Rain Garden

Note: This post describes the rain garden demonstration project which was installed in Scottsbluff in July 2010 with the help of the UNL Extension Stormwater Team.  The garden is located on the corner of 19th Street and Avenue B.

Step 1 Choosing the Site

Rain gardens are designed to catch runoff from roofs, driveways, streets, sidewalks, or other areas of the lawn.  This was an excellent site for a rain garden because of the downspout that drains into the area.  Minimum work was needed to channel the runoff into the rain garden.

Step 2 Checking Infiltration Rate of Soil

To check the infiltration rate of the soil, we dug a hole about 10 inches deep and filled it with water.  Then we waited to see how much water would infiltrate into the soil.  For a site to be suitable for a rain garden, the soil must drain at least 6 inches in 24 hour.

Step 3 Digging Out the Garden

We dug out an area for our garden, using soil from the bottom of the rain garden to build up a berm on the perimeter of the garden.  This gives the runoff an area to pool while it soaks into the ground.  Our site was very compacted from construction, so we dug a little deeper to break up the soil that was compacted so that runoff could easily drain into the sandy soil below.

Step 4 Soil Amendments

Our rain garden was on a recent construction site.  This meant that the soil we were working with was compacted fill material, which did not drain very well and had little nutrients.  To compensate for this, we amended the soil by layering sand and compost and tilling it into the soil.  We also used a small amount of fertilizer.

Step 5 Final Grading

Using rakes and shovels, we made final adjustments to the garden.  We then used a string level to make sure that the inlet and the outlet were at the same height.  This assured that water will pool over the whole garden instead of just one end of it.  The bottom of the garden is about 6 inches below the level of the inlet and outlet.

Step 6 Building the Inlet and Outlet

In order to slow down runoff coming from the downspout, we used large river rock (3-6”) to build the inlet.  This keeps the runoff from eroding the area where it enters the garden.  We also used rock for the overflow outlet on the other end of the garden.  This assures that in a large storm event the excess runoff will not wash out the berm around the garden.

Step 7 Planting

We used a variety of native plants in our rain garden.  Native plants are better able to tolerate both wet and dry conditions, so once established, they will not need to be watered as much, but they will still be able to tolerate ponded water for a few hours after a storm event.

Step 8 Mulching

Mulch serves several purposes.  By covering the bare ground, it helps retain soil moisture, supresses weed growth, and also provides an attractive appearance until the plants grow and fill in.  During a rain event, the mulch will probably float to the surface of the water, but since the garden is level, most of it should stay in place.

Step 9 Maintenance

The rain garden will need maintenance until the plants are established and self-sustaining.  In the first year, plants will be watered, weeds will be pulled, and mulch will be replaced and redistributed.  Within about three years, maintenance will be reduced to cutting back the garden once a year, removing sediment from the garden, and pulling invasive weeds.

The Results:

The rain garden has functioned very well in rain events, collecting all of the runoff from the downspout that is directed to it.  It drains within just a few hours of each storm.

Plants are becoming more established and filling in.  This photo was taken in June 2011.  Come back to our website to see updates on how the garden looks!

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