Once you have determined the size, depth, and shape of your rain garden, it's time to build!

Excavation, Backfilling

If the rain garden is located on a slope of less than 5%:

1. Excavate the soil to the desired depth.

2. If your native soil has excellent or well-draining soil, set the excavated soil aside to use for backfill.

3. Make sure the rain garden has a flat bottom by setting a level on top of a 2x4 set across the bottom of the excavation.

4. Use approx 2/3 of the native soil mixed with 1/3 compost to backfill the rain garden. If your native soil is poor-draining, it may be best not to use it at all, but instead to backfill with a purchased soil mixture that is 60% screened sand and 40% compost.

5. Add the soil mixture approx. 6 inches at a time, lightly compacting each layer before adding another. Leave 6 inches above the top layer of soil for the ponding area.

If the rain garden site has a slope of 5% or more:

1. Follow all the steps above, but set aside some of the excavated soil to be used to build a berm on the downhill side before you backfill the rain garden.

2. The berm will need to be as tall as the string height on the low stake (found when you determined the degree of slope).

3. The berm will also need to have a wide base and sloping sides with its soil well-compacted, so that water cannot seep through it, and extend 6 inches above the top soil layer for the ponding area.

Routing Water into the Rain Garden

There are several options for routing water into your rain garden, and most depend on the source of the runoff and the degree of slope. No matter which option you choose for the runoff to enter the rain garden, place washed drain rock all around the inlet to guard against erosion.

• If the slope is 2% or less, runoff can be routed into the rain garden though a shallow swale, with gently-sloping sides, that is protected from erosion with drain rock or vegetation.

• If greater than 2%, use the method above and add small "check dams" every 5-10 ft. to slow the water.

• It may be best to use a solid pipe to direct downspout water either partway or the entire route into the rain garden, so that it cannot reach your home's foundation even in a heavy downpour.

Creating the Outlet

During most rainfall events, the rain garden will absorb all the runoff that flows into it. However, an outlet is needed in all rain gardens in case a large storm delivers runoff more quickly into the rain garden than it can store and infiltrate.

• The overflow should be located just below the topmost point of the ponding area.
• It should be lined with rock to slow down the exiting water, and extend away from the rain garden for four feet, where it either dissipates into the landscape or into a storm drain.