Good for the Earth: How to Make a Rain Garden

Updated on May 13, 2019
Maren Morgan M-T profile image

Maren gardens in Pennsylvania, specializing in earth-friendly, unconventional, creative, joyful artistry. She is committed to eco health.

What Is a Rain Garden?

A rain garden is a plant-filled low level area of land to which rain waters flow and are absorbed by the ground within 48 hours.

What It Is Not

A rain garden is:

  • Not constantly wet, therefore it's not a pond, swamp, marsh, or wetland
  • Not a "minimum" size
  • Not a plant-void pile of rocks
  • Not necessarily connected to downspouts, pipes, or underground plumbing
  • Not a mosquito breeding place

What Great Thing Do Rain Gardens Do?

Rain gardens divert rainwater from directly entering a municipal stormwater system or from directly flowing across surfaces into streams and rivers and temporarily holds it and cleans it.

You may ask: so what? Is rain going into sewers and rivers bad?

Answer: Sometimes yes. Hard to believe, but it can be bad.

Two Ways Rainwater Can Travel

Note the storm sewer grate at the bottom left of diagram. It sends dirty water directly into a bay or river.
Note the storm sewer grate at the bottom left of diagram. It sends dirty water directly into a bay or river. | Source

How Rain Can Be Bad?

In urban, suburban, and built-up communities, developers and citizens never imagined how much land would be covered with concrete.

They also never imagined how many people would crowd into these living spaces.

Furthermore, our forebears could not imagine a number of things, such as:

  • How many fossil-fuel powered vehicles there would be
  • How much air pollution and water pollution could occur
  • How much soil erosion near streams and rivers would exist

But, now we are where we are.

Rain falling on these areas washes across dirty rooftops, driveways, patios, parking lots, roads, and other impervious surfaces picking up asphalt bits, motor oil and all sorts of filth. It rushes into storm drains and, from there, directly into streams and rivers, carrying pollutants picked up along the route. Another "baddy" is torrential rain rushing across fertilized farm fields directly into rivers, carrying fertilizer (bad for the waterway, as the Chesapeake Bay has found) or carrying good topsoil and sediment away from the fields (bad for the farm.)

How Does a Rain Garden Clean Water?

Two things happen:

  1. Like a game of tag, the low-lying rain garden catches the dirty rainwater. It holds it long enough that the polluted water is absorbed into the garden's ground.
  2. The roots of the plants in the garden are virtual sponges, and the mulch layer is a filter. Rain gardens trap up to 90% of nutrients and non-nutritional chemicals in the rainwater and up to 80% of sediments from rainwater.

Rain Gardens Have Natural Cleaners: Plant Roots

A top layer of mulch, then a 12-20 inch layer of compost mixed with soil holds all those fantastic plant roots - Mother Nature's filtering system. Below that is a layer of coarse sand or gravel.
A top layer of mulch, then a 12-20 inch layer of compost mixed with soil holds all those fantastic plant roots - Mother Nature's filtering system. Below that is a layer of coarse sand or gravel. | Source

How Does a Rain Garden Reduce Floods?

Again, two wonderful things happen:

  1. The low-lying rain garden catches the rainwater. The water can't rush into the streets, low areas under train trestles that always flood, or the streams that rise above flood stage. The rain garden holds the water long enough that it is absorbed into the garden's ground.
  2. The water seeps through the special soil and the sand or gravel layer into the subsoil. From there, the rain slowly recharges aquifers, groundwater, and wells just as it used to do hundreds of years ago before the invasion of concrete and macadam.

Hey Rain, Gotcha!

Even a modest sized rain garden is a container for rain.
Even a modest sized rain garden is a container for rain. | Source

Let's Build One!

Absolute first step is ....

Call 811 Before Choosing Rain Garden Location!

In the United States, the phone number 811 connects you to a center which alerts all your local utilities to visit your property and spray paint (it washes away) the ground wherever their underground lines are.

You absolutely, positively do not want to interfere with underground electricity, communications cables, or natural gas lines. Otherwise, you will have (past tense) worked on your Eternal Rest Garden, instead of a rain garden.

In other countries, please do what is appropriate to learn where the safe, soil only spots are on your property.

Where to Place a Rain Garden

Generally agreed guidelines:

  • Ten or more feet away from buildings (you don't want deterioration of a foundation)
  • Not on top of utility lines, a septic tank, or a well
  • Not where it's always soggy and never dries up (this may be due to bedrock or clay which will not allow water to "percolate" [be absorbed] into the ground)
  • Not where it hits tree roots

Ten Feet Away

A rain garden must be at least ten feet away from buildings.
A rain garden must be at least ten feet away from buildings. | Source

How Deep?

18 to 30 inches.

Dig to a depth of 18 to 30 inches but don't have the perimeter drop off like the deep end of a swimming pool. The edges should gently slope up to the natural ground level.

Connect or Don't Connect to a Downspout

Either way works if the location requirements are met. There are a few people with a strong preference for having pipes bringing roof drainage from a downspout to the garden. However, this is a matter for each homeowner to decide. The layout of the land, walkways, and structures will weigh on what is practical and aesthetically pleasing.

See the diagrams below for both designs.

With Downspout Extension to the Rain Garden

Source

A Swale Guides Water to Rain Garden

A narrow low channel in the yard guides rainwater to the rain garden.
A narrow low channel in the yard guides rainwater to the rain garden. | Source

Supplies and Steps

  1. Dig at your chosen location and save a little less than half of that soil for step 3
  2. Line the bottom of the depression with a layer of small gravel or coarse sand 4 to 8 inches deep.
  3. Make a mixture of the saved garden soil and compost.
  4. On top of the sand or gravel, fill the depression with this mixture to a level about 5 inches below the natural ground level and plant your vegetation in it.
  5. Spread mulch over all to a height ending about 2 inches below the natural ground level.
  6. Optional: line the edge of the rain garden with large decorative stones.
  7. Optional: guide the pipe (if you are using one) to permanent placement past the edge and into the rain garden so that water flows inside.

Water Flows Easily to Rain Garden at a Natural Low Point

No pipes or swales are needed to guide rainwater if it already follows a path to a lower area of a property.
No pipes or swales are needed to guide rainwater if it already follows a path to a lower area of a property. | Source

Plant Choices and Placement

You are encouraged to use native plants that can handle wetness and drought. This certainly reduces the amount of care and maintenance you'll have later. In addition, native plants give food and habitat to your native bugs, butterflies, birds, and critters.

Also, you should use potted plants or bare-root plants, not seeds. Seeds could wash away before getting established as plants.

Follow garden design principles. Putting taller plants and grasses in the center of the garden and encircle with smaller and shorter plants so that you can see and enjoy every plant.

Some designers feel that you should not put a tree or bush in a rain garden. This is so flexible, depending on your region, the overall area of the garden, and more. Please consult local experts when you are seriously selecting your rain garden plants.

Plants Must Handle Wet and Dry Conditions

Rain garden plants must be able to "tolerate wet feet," but not depend on constant wetness.
Rain garden plants must be able to "tolerate wet feet," but not depend on constant wetness. | Source

A Great Shovel

Jackson Eagle Long- Handle Round Point Shovel, No. 2 Blade, 46" Handle, Steel
Jackson Eagle Long- Handle Round Point Shovel, No. 2 Blade, 46" Handle, Steel

A round point garden shovel is my go-to tool for digging the rain garden depression hole. I love using my muscles, digging without motor humming, and doing my part to avoid adding fossil exhaust fumes into the environment.

 

Regional Variations

It is amazing what differences exist around the world in expectations for water absorption and what plants will succeed. As you explore creating your own rain garden, you will find many resources online and perhaps in your government.

In the United States, you can consult land grant colleges, agricultural extension services, and county, state, and federal conservation departments, to name a few. In other areas, finding government entities and charities which support water conservation may be your best bet.

Rain Soaking on Plants

A rain garden encourages the rain to tarry.
A rain garden encourages the rain to tarry. | Source

Hooray For You!

By putting a rain garden on your land, you have:

  1. filtered pollutants
  2. recharged groundwater
  3. conserved water
  4. protected waterways
  5. removed standing water from your yard
  6. reduced mosquito breeding
  7. reduced potential of home flooding
  8. increased beneficial insects that eliminate pests
  9. given habitat to birds and butterflies
  10. survived drought season
  11. reduced garden maintenance
  12. enhanced property value

(All these benefits are from a publication of the USDA, Natural Resources Conservation Service, Caribbean Area.)

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

    © 2019 Maren Elizabeth Morgan

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