Page 45 - 7166 - Rain Garden Handbook

Build
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If a Berm is Used to Hold
Water in the Rain Garden

• Water flowing into the rain garden can erode the berm if the soil is
loose and unprotected, so pack the berm with firm foot pressure,
mulch, and plant fast-growing groundcovers and low shrubs. Refer
to the Plant List in Appendix A.
• Before the soil is placed remove all vegetation and rough up
the ground under the berm so the fill bonds well to the ground
surface. As the berm is constructed, be sure to firmly compact the
soil after every few inches is added.
• The berm should be at least 6 inches above the maximum
ponding depth and at least twice as wide as it is high.
• Where the overflow extends through the berm, cover the overflow
area with rock to protect it from erosion (see page 42).

The Berm Has to Be Water Tight

A sandy soil that drains well isn’t the best material for a berm around the rain garden. The
berm needs to hold water after heavy rainfall or frequent storms. Consider bringing in soil
with higher clay or silt content that is less well-draining to construct the berm. Before the
soil is placed, remove all vegetation and rough up the ground under the berm. As the berm
is constructed, be sure to firmly compact the soil after every few inches added, but do not
compact the bottom of the rain garden.

David Hymel