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How to Build a Rain Garden

Rain Gardens are like a sponge –they soak up water, clean pollutants, and slowly release it back into the ground.

What is a Rain Garden and What Does it Do?

After rainfall hits the ground, it becomes “runoff” and begins its journey back to a waterway, picking up any harmful chemicals or pollutants on its way. In urban areas, runoff can be more intense because buildings, pavement, and homes leave little area for water to sink back into the ground. This runoff overpowers our streams leading to erosion, flooding, and habitat loss. Rain Gardens are a natural and beautiful way to reduce and clean stormwater. These shallow, depressed gardens are designed to collect rainwater and allow it time to filter into the ground. The result is cleaner water and less dirty runoff overwhelming our storm systems and waterways.

Now, how do I build one?

Before you start to dig, let’s figure out where to put your Rain Garden. Each site is different, but use the following general guidelines to help your rain garden truly bloom.

Location, Location, Location

  • Build your rain garden at least 10 feet downhill from you or your neighbor’s house to avoid water getting into your foundation.
  • Ensure you aren’t building your rain garden above a septic system or shallow underground utilities.
  • Disconnect downspouts that are routed into pipes or the ground and try to locate your garden to catch and treat the water.
  • Make sure your rain garden isn’t within a streams floodway to avoid plants washing away.
  • Ensure the soil is a least 24 inches deep in your garden for proper drainage.
  • Avoid areas with existing trees because it can damage roots.
  • Make sure the slope of your site is less than 12%. A site too steep will drain too quickly and needs increased excavation work.
  • Your rain garden shouldn’t be located in an area of your yard where water pools because the water can’t drain quickly enough.
You’ve got the perfect spot? Let’s test it!

Water Infiltration Test:

Once you have picked your potential location, you will need to test the soil to determine if it will drain properly. First, dig a hole 12 inches deep, fill it with water, and allow the water to saturate the surrounding soil. Next, refill the hole, and time how long it takes to drain.

If it drains in:

  • Less than 24 hours, your infiltration rate is good.
  • Between 24 and 48 hours, your soil will infiltrate but should be amended with a mixture of 20-30% of the existing soil, 20%-30% compost, & 40-60% sand to a depth of 6 inches.
  • Greater than 48 hours is not the best spot for a rain garden, but if no other options, you can replace the soil to a depth of 2 feet with a mixture of 20-30% imported topsoil, 20-30% compost, & 40-60% sand OR install an underdrain system, gravel, or both. Contact Metro Water Services Stormwater Department for details at 615-880-2420.
We’ve got the spot and it tested great! Now, how big?

Sizing your Garden:

Rain Gardens typically range from 100 – 300 square feet to catch most of the runoff from your yard, but no matter the size, catching, and infiltrating runoff will improve our water quality.

We got the basics- Now, let’s get started!

Tools you’ll Need:

  • Tape measure
  • Shovels
  • Rake
  • Trowels
  • Wheelbarrow
  • Carpenter’s level
  • Marking Paint
  • String
  • Protection for eye, hand, and feet
  • Hardhats if using machinery such as a bobcat or backhoe

Site Preparation:

  • Outline the rain garden on the ground with loose chalk, spray paint, stakes, flags, or a garden hose.
  • Install appropriate erosion controls such as silt fence or fiber logs if you are creating runoff sediment or mud that will enter storm drains or water bodies.

Excavations:

  • Dig your garden the size you have determined in the planning phase and remember to take into account the soil amendment depth and the final 3-inch mulch layer. Your final rain garden should be around 6 inches deep.
  • Do not compact the soil during excavation and make the bottom flat and level so the water will infiltrate evenly and not pool.
  • If your rain garden is on a slope, place excavated soil on the downhill side to be used later to form the berm.

Amending the Soil:

  • If your infiltration rate calculation indicates your soil needs amending, mix small portions at a time by hand or with machinery. Allow it to settle overnight and add additional soil if needed. Keep the soil level.

The Berm:

  • If the garden is located on a slope, use the remaining excavated soil to construct a berm on the downhill side of the rain garden.
  • The berm should be rounded and gradually taper on the sides until it meets the existing lawn. Once the berm is shaped, compact it with your feet or a tamping bar. The berm will act as a dam to hold more water in the garden.
  • To prevent erosion, the berm will need to be planted with grass or incorporated into the planting design.

Planting:

  • Carefully choose native plants that are quality, established nursery stock.
  • Check out our Planting Guide for plant lists that prefer sun or shade!
  • Store plants in a protected shady area until ready to plant to prevent them from drying out during storage or installation.
  • Dig holes twice as wide as the root ball and plant the crown of the plant level with the existing soil.
  • Tamp the soil around the roots gently and do not step on or compact the rooters.
  • Water immediately after installation and keep tags during the warranty period.

Edging & Mulching:

  • Create a strong edge for your rain garden with trenches, metal, plastic, stone, brick, or even a thick border of native grass to create a strong visual line and prevent weeds. Ensure the edge is buried low enough for runoff to flow over it into the garden.
  • Use mulch to retain moisture, prevent erosion, control weeds, and nourish the soil.
  • Spread 3 inches of pine straw or shredded wood mulch over the rain garden taking care not to damage plants.
  • Include an overflow channel and pack with stone.

And just like that – you’ve successfully installed a Rain Garden! Check out our Resource Guide for more in-depth tips for planning, designing, and maintaining your Rain Garden.
Live in Davidson County, TN? Request a FREE Rain Garden consultation from the Cumberland River Compact to see if you are eligible to receive 25 gallons of plants for your rain garden.
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