Rain Gardens: What They Are and Why They're So Important

A Beautiful Way to Help Prevent Storm Water Runoff

Overhead of a rain garden

DigiPub / Getty Images

Rain gardens are often misunderstood, with many misconceptions around what they are and why you would want one. Maybe you've heard rain gardens are important for the environment, or you've heard that they are gardens that belong in soggy areas in your yard.

It's true that rain gardens are eco-friendly, but there's so much more to this great garden project that would make you want one in your own yard.

What Is a Rain Garden?

A rain garden is a landscaped, shallow depression planted with native plants that captures, temporarily holds, and filters storm water back into the ground preventing it from running off and ultimately reaching nearby streams, rivers, and ponds.

A rain garden is, at its core, a garden that collects rainwater, holds it for a limited amount of time, and filters it before slowly releasing the water into the ground. It collects rainwater from impervious surfaces like roofs, pavements, driveways, patios, parking lots, or waterlogged yards, allowing the water to slowly seep back into the ground, preventing runoff from reaching local waterways.

Installing rain gardens in our communities are essential for the health of our waterways. As our green spaces become overdeveloped, rain gardens will become vital. They play a huge role in guarding our waterways from nonpoint source pollutants contained in runoff water.

What Are Nonpoint Source Pollutants?

Nonpoint source pollutants (NPS) are pollutants that are picked up and carried to waterways by runoff (rainwater, melting snow) flowing above or below ground. Examples of NPS include:

  • motor oil
  • gasoline
  • fertilizer
  • chemicals
  • herbicides and pesticides
  • pet waste

Characteristics of a Rain Garden

A rain garden is your individual way to practice environmental stewardship. These specialized gardens have features that make them distinct from your typical flower bed. The primary purpose of a rain garden is to reduce runoff, so it should be placed in a location enabling the garden to collect water.

Select only native plants (shrubs, perennials, and annuals) for your rain garden. Native plants control erosion by stabilizing soils and increase the recharge rate of water and will also filter any contaminants in the runoff before it enters the groundwater.

The importance of using native plants can be seen in the comparison between Big bluestem grass (Andropogon gerardii) and Kentucky bluegrass (Poa pratensis) roots. Blue big stem roots can grow up to 12 feet deep, whereas Kentucky bluegrass roots will grow 1 ½ to 2 feet deep, allowing for far less water filtration.

When installing a rain garden, you are not creating a pond; ideally, the garden should drain within 24 hours after a storm.   

How Does a Rain Garden Work?

The principle of how rain gardens work is pretty simple. You have seen water run downhill before and have seen a puddle on a lawn slowly disappear. A rain garden creates a channel to an artificial depression or serves as an artificial depression, where water will be able to collect and enter the ground slowly.

The rate at which water is absorbed is controlled by estimating the amount of runoff entering the rain garden. The amount of runoff determines the depth of the depression and the size of the rain garden. Absorption will be aided by adding native plants with deep root systems or amending the soil if it is not ideal.

Perform a simple water percolation test to determine your site’s soil drainage rate before you start working.

Tip

How to conduct a water absorption test:

  1. Dig a hole at your rain garden site, 12 inches deep and six inches wide.
  2. Fill the hole with water and then let it stand until the hole is empty.
  3. Refill the hole so that water is one inch from the top. Push a pencil into the side of the hole to show the starting water level. Note the time and use a ruler to measure how deep the water is.
  4. Measure how deep the water is with a ruler every hour for at least four hours. 
  5. Calculate how many inches of water drained per hour.  

Planning Ahead

Creating a functional, aesthetically pleasing rain garden won’t require you to hire a landscaper or environmental scientist. With the help of a few assistants (friends, family members, neighbors), you can finish the project over a weekend, but it will require some planning.

Budget

Expect to spend around five dollars per square foot on the project. Most of your budget will be spent on plant material. Shop at local nurseries specializing in native plants or grow your own from seed to save money. Look for local plant sales that offer native plants for sale; many extension services hold spring plant sales that will offer plants perfect for rain gardens. By installing the rain garden yourself, you will save a good deal of money compared to paying someone to install it for you.

Site Selection

The most important consideration when planning a rain garden is site selection. Locating your rain garden in a spot that receives little to no water or too much water defeats its purpose. You want to choose a location between two impervious surfaces, for instance, a roof, a lawn, and a street. Examine where the rainwater pours off the roof via downspouts, watch how it flows to the lawn, and see how where the runoff water enters the street.

Plant selection and placement

To design and lay out your garden, determine its depth and size, estimate the runoff amount, and use the soil percolation test mentioned above. Plant placement must allow for water uptake and optimum plant health.

On the very edge of the garden, plants must be able to tolerate dry conditions. Plants installed along the slope should be able to thrive in conditions that are sometimes dry or wet. In the deepest section of the rain garden, known as the base, place plants that can handle wet soil for an extended time.

Before putting shovel to earth, call your utility companies to safely identify underground utility lines. Make this call before you dig to prevent damage to property and injury to you and your loved ones. Identify the area of your proposed garden site with string or spray paint before the utility workers arrive.

Warning

Before beginning any project that requires you to dig, call your utility companies first identify the utilities in the area. The utility companies will use spray paint or colored flags to alert you where the utility lines are and to avoid digging in those locations..