Rain Gardens: A Sustainable Stormwater Solution

This article was written by guest author, Heather Prince. Heather is a professional horticulturist and garden communicator based in the Chicagoland area.

Water, water everywhere? Sometimes a sustainable solution to that wet spot in the corner is to build a lively, dynamic garden area that not only brings us the joy of flowers, but welcomes pollinators and slows stormwater runoff. A rain garden might be just what you’re looking for.

What is a rain garden?

A rain garden is a shallow depression that purposefully holds water from a recent rainfall from a rain event, sump pump discharge, or swale so that it slowly seeps back into the water table instead of entering the municipal storm system. Rain gardens are only deep enough to move and hold rainfall, which means they are typically 6 to 8 inches deep. This shallow bowl can be a variety of shapes and sizes and is planted with appropriate plants that will thrive in its unique conditions. Native plants often take a role, but there are many options.

Benefits of a rain garden

Rain gardens are an eco-friendly choice and can offer many benefits:

• Keep stormwater on property and reduce pressure on municipal water systems.
• Allows plants to hold polluted water and in some cases, pull pollutants into plant tissues.
• Creates a garden space full of beneficial plants for you to enjoy.
• Attracts pollinators, butterflies, birds, frogs, toads, dragonflies, and more!

Getting started

First, check with your town, village, or homeowners association to determine if there are any ordinances or rules in place for rain gardens. Many municipalities are now offering rebates or assistance for sustainable stormwater solutions like rain gardens, so it’s a good idea to check and see if you might be eligible.

When considering a rain garden, take a walk through your property to determine the best site. Choose an area that is:

• A natural depression
• Where water tends to flow
• Around a storm grate to slow rainfall
• A desired spot for a garden

Rain gardens are typically not a good solution for a large area that floods and has standing water longer than a few days. They are meant to hold and slow rain and regular flooding often requires an engineered solution.

Because the goal is to move and contain water in a rainfall event, there are some things to consider:

• Gardens should be at least 10 feet from the foundation of a structure, so it doesn’t seep into a basement or crawl space.
• Do not place directly over a septic system as it can interfere with its drainage.
• Avoid big trees as digging in tree roots can be problematic for you and the tree. Some species of trees like oaks and beeches will suffer when their roots are disturbed. Plus, it can be difficult to dig planting holes amongst mature tree roots.
• Plan on holding a one to two-inch rainfall.

Basics of installation

Once you’ve figured out where you’d like your new rain garden, there is a bit of engineering involved. Fortunately, water doesn’t require much slope to move, but you should have an idea of how much volume you’ll be dealing with. Determine the source of water for the rain garden.

• House or garage roof
• Driveway or patio
• Sump pump outflow
• Drainage swale or culvert
• Roof, driveway, or patio - determine the square footage of the area being addressed
• Sump pump or swale – have an idea of velocity and frequency of outflow

Think of a rain garden as a shallow dish with three zones for plants.

• Bottom zone: this is the wettest area and may be consistently moist. Plants that love wet feet will thrive here. You may have to water this zone in times of extended drought.
• Middle zone: this area can go from wet to dry, depending on the volume of water in the garden. Plants that are flexible in their water requirements will be happy here.
• Upper or grade zone: this area is at the same grade as the larger area or yard. It tends to be average to dry, although may see extra water in an extreme rain event. Plants that like drier soils will grow here.

A rain garden can be sunny or shady depending on your property and where you’d like it to be. There are a wide range of plants that will thrive in rain gardens, plus it might give you the opportunity to plant some species you might not otherwise be able to enjoy. There are also wet tolerant trees and shrubs if your plan allows.

The Wisconsin Department of Natural Resources Rain Garden Guide provides detailed instructions and plant suggestions. It’s a terrific resource for determining how big your rain garden should be to meet your needs. Check out your local botanic garden or state extension service for more information, detailed plans, and plant suggestions appropriate to your area.