Rain gardens are a stormwater management solution that’s simple enough to tackle as a DIY project: dig a shallow basin to intercept runoff from a downspout or paved area, plant it with ornamental grasses and flowers, and pat yourself on the back for doing your small part to prevent flash flooding, streambank erosion, and water pollution!  However, I’ve noticed a few things that can go wrong.  Here are some tips to avoid them!

These tips also apply to the bioretention cells you’ll see in public right-of-ways, which are similar, but have an underdrain and an engineered soil mix.

Standing Water

A rain garden may fill up with water after a heavy rain, but it should soak into the ground within 24 hours.  This will prevent mosquitos from breeding and allow a wider variety of plants to thrive.  To achieve this, you either need to find a spot with well-drained soil, or you need to amend the soil with sand and compost.  It’s important to test the infiltration rate of the soil: a 6 inch deep rain garden would need soil that can infiltrate at least 0.25 inches of water per hour.

This bioretention cell has turned into a cattail marsh, with standing water for extended periods in the spring.  I tested the soil for a school project and determined that the soil had too much clay content and had been compacted during construction.

Erosion and Debris

There are guidelines for how big to make a rain garden to handle the runoff from a given area.  However, if space is limited, an undersized rain garden is better than no rain garden.  The trick is to manage the overflow.

This rain garden treats the runoff from a church parking lot.  Once the water level gets high enough, it enters a pipe that connects with the storm sewer.  This pipe was installed in response to concerns about soil and mulch washing onto the sidewalk when water overtopped the berm.  We installed a mesh barrier to keep woodchips from clogging the outlet.  I’ve heard that shredded hardwood mulch is less prone to floating than wood chips and bark nuggets.

One other issue with this rain garden was erosion as water made its way from the pavement to the low point.  To solve this problem, I regraded the basin so it had a level bottom (using a line level, rake, and shovel), and put some stones and gravel at the edge of the pavement to absorb the force of the water.    

Dead plants and weeds

There are many lovely species of native plants that can thrive in a rain garden, once established, but as seedlings they may struggle to withstand alternating periods of flooding and drought.  To improve their odds of success, you can redirect the downspout or plug the opening to the rain garden for a few months, irrigate, and protect them from rabbits and deer.  If that’s not possible, expect to replace a few plants.

Just like any other garden, a rain garden can get overrun with weeds, especially when the desirable plants are just getting started.  Sturdy labels or ID guides can help whoever is responsible for maintenance know what to pull and what to keep, but for a situation like this (a highly visible commercial right-of-way managed by the city) simpler planting plans are probably better.

I hope this article helps you avoid some common pitfalls but doesn’t discourage you from trying.  I planted a rain garden in my backyard last spring and have already enjoyed some blooms and monarch butterflies!

For more information, I highly recommend the Iowa Rain Garden Design and Installation Guide, available for free as a PDF.