Building a Rain Garden – A Step-by-Step Guide

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Robert Pavlis

Rain gardens are popping up everywhere and many gardeners are adding them to their landscape. They are beautiful, low-maintenance, help give back to the community and support the health of local waterways. To understand the benefits and how to design a rain garden have a look at my previous post called: Rain Garden for Home Gardeners.

In this article I’ll show you a simple method for installing your own rain garden, also called a bioretention garden.

Building a Rain Garden - A Step-by-Step Guide
Building a Rain Garden – A Step-by-Step Guide, source: Rain Dog Designs

Selecting a Location for the Rain Garden

First, you need to choose a location. A popular spot is near a downspout where the ground slopes away from the house. Some gardeners even create a swale (small river bed) that leads from the downspout to the rain garden. It is usually covered by rocks to prevent soil from washing away. To be safe, rain gardens should be at least 10 feet (3 meters) away from any buildings to prevent water from seeping into the foundation. The garden should be located even farther away from any septic systems.

Request a utility locate to make sure it is safe to dig in your selected spot.

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Of course, creating a rain garden by your downspout might not be possible if your house is too close to another house, or if your downspout is near a walkway or property line. Another way to choose a location is to observe where water pools in your yard after it rains. A low, and relatively flat area in your yard would be an ideal spot. A steeply sloped area is harder to use because the water tends to flow out of the garden, but it can be used provided the ponding area is level.

Soil Science for Gardeners book by Robert Pavlis
Rain garden ponding area
Rain garden ponding area, source: Marika Li

Decide on Size and Shape

Rain gardens come in many different shapes and sizes, so this part is largely up to your personal taste. The size should be whatever is appropriate for the site, though bigger is better if you live in a high rainfall area.

If you want a natural looking garden, then it should have a curving, organic shape. More defined, geometric shapes will give the garden a formal or modern look.

Map out the shape of the garden before you start digging, to confirm the final design. Ground paint, a watering hose or stakes can be used for this. If it’s winter time, use the snow.

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How Well Does Your Soil Drain?

The ponding area of a rain garden needs to be deep enough to hold water, but it is not designed to hold that water for more than 24 to 36 hours. Both the ideal depth and construction method depends on how well your soil drains. Sand drains quickly and clay drains more slowly. To find out how quickly your soil drains, do a perc test as described in this video (you can ignore the calculations in the video).

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If the water drained out in less than 24 hours, your soil is ideal for a rain garden and you won’t have to amend the soil for drainage. If it takes more than 24 hours, consider amending the soil with sand, as described below.

How Deep Should Rain Gardens Be?

Ideally, the rain garden is designed so that water from a heavy rain storm does not overfill the garden. Many tutorials will tell you to dig according to a calculation made using the square footage of your roof and/or driveway, as well as the expected rainfall in your area. For the purposes of this tutorial, we’ll keep things simple. After all, gardening should be fun and you don’t need to have a perfectly calculated rain garden to help your community with stormwater and pollution.

The center of the final ponding area should be about 6 inches deep.

Amending the Soil

There are two ways to amend the soil in the rain garden. If you soil drains well and grows plants well, you don’t have to do any amending.

If your soil does not drain within 24 hours using the above perc test, then it is a good idea to dig the hole a bit deeper, add sand to the excavated soil to make a 50/50 mixture and use this better draining soil to raise the depth to 6″.

If you want to improve the health of your soil you can amend with compost. Dig the hole a bit deeper, mix soil and compost together and replace the soil to a depth of 6″.

A good bioretention soil mix can be made from 50% sand, 25% compost and 25% soil. This will drain well and grow great plants.

Digging Sloped Sides

Dig the sides of the garden so they have a gradual slope that runs from the edge, to the bottom of the hole. This allows water to flow into the garden gradually without soil erosion, makes planting easier and it decreases the risk of trips or falls.

The Inlet and Outlet for a Rain Garden

Concrete swale directing water from the road to the rain garden
Concrete swale directing water from the road to the rain garden, source: ensia

A proper rain garden should include an outlet and may benefit from an inlet, which is just a fancy way of saying that there should be a couple of little dips at two ends of the garden that allows water to flow in when it rains and out when it gets too full.

A simple way to create this is to slope the lawn from the house to the rain garden so that water simply runs into it. To make this work better, create a swale (low spot) which directs water to a specific point where you want it to enter the garden. You can also extend the bottom of a downspout to empty directly into the garden, but aesthetically that does not look as good, unless it is buried.

An outlet should always be added. Think of the outlet as a safety value. If you get an extremely heavy rain storm, and your rain garden can’t hold all of the water, where will the excess go? It should flow away from the house and probably to the street. This outlet is just a low spot along the edge of the rain garden.

White rocks used for the inlet and outlet
White rocks used for the inlet and outlet, source: Rain Dog Designs

Add Your Rain Garden Plants

Planting in a rain garden is done just like any other garden. All the usual design principles apply, such as using varying heights, colors, and textures for maximum interest. For some plant suggestions, click here.

Keep the new garden well watered for a few weeks, but don’t overwater. The common advice to water daily is wrong.

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Mulch the Rain Garden

After putting in the plants, it’s a good idea to put a generous 2-3 inch layer of mulch on top. Mulch keeps the soil moist when it’s not raining and prevents weeds. You can use any type of mulch you like, although wood chips are best. Stay away from bark since it tends to float.

A heavy stream of water will move most mulch and in such conditions larger stones or rocks should be used. A think layer is better at keeping weeds in control.

Enjoy Your New Rain Garden

A rain garden is maintained in the same way as any other garden. Take care of the plants, replace mulch as needed, and keep it weeded.

You are now ready to enjoy your brand new rain garden!

Written by: Marika Li

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

2 thoughts on “Building a Rain Garden – A Step-by-Step Guide”

  1. When you mention digging, is it about the surface elevations rather than the depth of aerating the soil for a bed?

    Using the Rain Garden Ponding Area image as a visual example, does digging 6″ mean that the ponding area needs to be 6″ lower in elevation than the upland area?

    Does a rain garden require a visual depression, or could a deeply aerated bed without a visual change in surface elevation serve the same purpose in water collection, as long as there is an inlet and outlet and assuming there will be no traffic on the bed creating soil compaction?


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