How to Build a Rain Garden

close-up of a snail on a leaf

For an eco-friendly lawn alternative that will brighten your yard, rain gardens are all the rage.

Not only are they gorgeous, but they’ll prevent water damage, improve drainage, and protect the environment from stormwater runoff. They’re like glamorous sponges for your lawn.

Ready to wave goodbye to soggy puddles and say hello to beautiful blooms and butterflies? We’ll walk you through the steps to build your own rain garden.

What is a rain garden?

A rain garden (formally known as a bioretention cell) is a basin filled with native plants and permeable soil designed to capture stormwater flowing away from the house.

During storms, rain gardens stop and hold fast-flowing runoff to protect both your home and local ecosystems. Compost-heavy soil and plants with deep roots absorb the water, clean out contaminants, and let the water slowly filter into the surrounding soil. 

Planning your rain garden

Building your own rain garden is an exciting DIY project that you can accomplish in a few days. But to make your spongy garden shine, you’ll need a master plan. 

  • Difficulty level: Easy to medium
  • Time: 2-3 days
  • Cost: $500-$1,600

Steps to plan your rain garden

We will talk about the basic planning stages below, but if you want more details on how to plan your rain garden before building it, watch this video from Washington State University.

1. Choose your location

It’s best to build a rain garden on a slight slope (about a 2% grade). If your slope is more steep, install landscape fabric to prevent erosion and landslides. 

2. Test your soil

Your soil may need texture, pH, and nutrient amendments for optimal plant growth. Contact your local extension office for information about where you can get your soil tested. 

3. Decide on ponding depth 

Based on your soil type and landscape preferences, choose a maximum ponding depth of 6 to 12 inches. Maximum ponding depth is measured at the deepest part of your rain garden: It’s the amount of water that your garden can hold before the water level is above the surface of your surrounding lawn. Most homeowners opt for a 12-inch ponding depth to hold more water.

4. Test percolation rate

Perform a percolation test (AKA infiltration test) to make sure your rain garden can absorb water within one to three days after a heavy storm. If it can’t, you may need to choose a different location.

5. Calculate your garden’s size

Measure impermeable surfaces like driveways, patios, and sidewalks to determine how much water will flow into your rain garden. The area of your rain garden should be 10% to 20% of the area of the impermeable surfaces that contribute runoff. 

6. Choose your garden’s shape

Let your creative juices flow! Ovals, kidney beans, and teardrops are frequent favorites, but you can choose any shape that fits your dream lawn design. Use a rope or garden hose to visualize your design, and then spray paint your final choice to follow as you dig. 

7. Decide when to build

You can build a rain garden any time from spring to fall, but many homeowners prefer spring because it gives flowers time to blossom. 

8. Call 811

Before digging, check that you won’t accidentally hit an underground utility line and knock out your neighbor’s internet. Call at least three days before you dig so companies can send out locators to mark where utilities are buried. 

Building your rain garden

Congratulations, now you’re on to the fun part! It’s time to get your hands dirty and create the lawn sponge of your dreams.

Tools you’ll need for building

  • Measuring tape
  • Shovel
  • Rake
  • Planting knife
  • Mini excavator (optional)
  • Rain garden soil mix or soil amendments
  • Level (Carpenter’s level and a board, laser, water, or other)
  • Topsoil
  • Mulch
  • Plastic piping (smooth with rigid edges) if needed
  • Rocks or gravel for inflow and overflow
  • Native plants
  • Wheelbarrow 
  • Small hand tools for gardening (trowels, rakes, and cultivators)
  • Boulders, decorative stones and rocks, or edging
  • Sod cutter (optional)

Step 1: Remove grass

Using a sharp spade or a sod cutter, remove the grass where your rain garden will grow. You may want to remove an additional foot or more of grass around the perimeter to make room for sloping.

Don’t throw away your excess sod. Instead, transform it into compost material. Lay it out under mulch and cardboard and wait six to 12 months for it to decompose. Then, you can use it to give your rain garden a nutrient lift. 

Step 2: Excavate the basin

It’s the big moment to dig. Enlist your friends and dig with shovels, or use a mini-excavator to dig your rain garden 18 to 36 inches deep based on your desired ponding depth and your soil drainage. 

If you want a deep, 12-inch ponding depth and do not have well-draining soil, excavate 2 to 3 feet of soil. This will give you enough room to make soil amendments. If you’re opting for a shallow, 6-inch ponding depth and your soil drains well, 18 inches is enough. 

If you don’t know whether your soil has good drainage or not, your local extension service is always a good resource.

After you’ve dug to the proper depth, rake over the bottom of the basin to make sure it’s level. This will ensure that water is distributed evenly throughout your rain garden. 

To determine if the bottom is flat, use a straight board with a carpenter’s level on top. Place the board in different areas and in different directions, and then either dig or fill in spots according to the level. Alternatively, you can use a laser level.

Step 3: Install inflow 

Once you’ve dug and leveled your basin, install piping or build rock-lined swales (low-lying channels lined with rocks or grass), depending on your desired look. Piping and swales are considered your inflow because they conduct water into your rain garden, while your overflow will be an extended rocky area that leads excess water away from the house.

Follow these steps to install inflow mechanisms:

  • Dig a trench (at a slope of about 2%) to carry water from a downspout to your rain garden. Your trench should be 1 to 1 ½ feet deep.
  • If using pipes:
    • Line your trench with at least 2 to 3 inches of rocks. Use smooth rocks that are at least 2 inches in diameter. If working near your house, install a pond liner before placing the rocks to prevent water from seeping into the foundation.
    • Choose a pipe that is long enough to extend 1 foot into your rain garden. This way, water fully enters the basin.
    • Lay your pipe in the rock-covered trench.
    • Attach a 90-degree elbow to the downspout. Then, attach the elbow to your pipe.
    • Cover the sides and top of the pipe with 4 to 8 inches of gravel and small stones to prevent erosion. Add sod on top of your pipe if you don’t want it visible in your yard. 
  • If building swales: 
    • Dig your swale 18 to 24 inches wide.
    • Mound soil on either side of your swale to create a berm, following the 2:1 slope ratio rule (mounds should be twice as wide as they are high).
    • Add 4 to 8 inches of gravel and rocks to prevent erosion. 
    • If the slope is steeper than 2%, slow the water by building small rock dams across the swale every 5 to 10 feet.

Step 4: Build an overflow 

To prevent flooding your home or your neighbor’s yard, build a rocky overflow area (also known as an outlet) at the lowest edge of your rain garden. The overflow will cut through your berm and extend away from your home, directing water to a safe place like a storm drain, dry creek, another rain garden, or out toward your neighborhood’s rainwater system (ex. culverts).

Here are some tips for building an effective overflow:

  • Build your overflow using rocks, gravel, and river stones. Your overflow should be at least 4 feet long and slope downward. 
  • Your overflow area must be placed lower than the inflow point but at least 6 inches above the deepest part of your garden. This placement ensures that your water neither runs back towards your home nor flows out of the rain garden too easily.
  • If your rain garden isn’t near a natural drainage system, consider building a second rain garden or bioswale (channel filled with vegetation) to filter the overflow, or attach a pipe to the outlet to carry overflow water to a safe location. 
  • With an overflow, you won’t have to worry about rain running back to your house or damaging your neighbor’s property. 

Step 5: Build a berm

Wondering what to do with the extra soil you excavated? Build a berm! Use some of your excess soil to build a 6- to 8-inch wall of compacted soil around the perimeter or three sides of your rain garden. Watertight berms prevent water from spilling out of your rain garden during major storms. 

If your garden is not on a slope, you can build a berm around the whole perimeter of your garden. 

If your garden is on a slope, build a berm around three sides of your garden. The berm on the lowest side of your garden should come up to the same elevation as the entry to the rain garden at the uphill end.

Berms should have a sturdy base and be twice as wide as they are high (so if your berm is 8 inches high, it should be 16 inches wide). Make sure that your berm is well compacted by stepping firmly on it each time you add a few inches of soil. 

If you plan to plant on your berm, mix compost into the soil to give plants a nutrition boost. Then, you can landscape your berm with ground covers, shrubs, and perennials to give the area some lovely, hardy color. Avoid trying to grow grass on your berm, as grass tends to invade rain gardens.

If you prefer not to have a berm, you can create more gradual side slopes at a ratio of approximately 3:1 (for every 3 feet of distance, land slopes downward by 1 foot). Dig outward from the perimeter of the rain garden, shaping the edges to create a bowl.

Step 6: Replace your soil

Your rain garden needs healthy, well-draining soil to function and support plants. If your soil gets waterlogged easily or has other issues, you’ll need to either replace or amend it before adding soil back into your rain garden.

You have two options for what to do with the soil that you’ve excavated

  • Replace your soil. If your soil has a high clay content, it isn’t the best option for draining stormwater. You may want to replace most or all of it with 12 to 24 inches of bioretention soil mix (which contains 50%-60% screened sand and about 40% compost or a mixture of compost and topsoil). This will give you completely new, well-draining soil.
  • Amend your soil. If your type of soil has moderately good drainage and doesn’t contain too much clay, you can keep most of it. Add compost into your excavated soil for a mixture of 65% excavated soil and 35% compost. For very sandy soils with fast drainage, you may want to add additional compost.

By replacing or amending your soil, you’ll give your rain garden the perfect soil to support plant growth and allow water to drain as it should. 

Step 7: Calculate how much soil and compost you need 

The compost you’ll find at the garden supply store is measured in cubic yards, not square feet. So, you’ll need to know how many cubic yards of soil your rain garden needs to figure out how much compost to purchase.

To calculate how many cubic yards of soil you’ll need, begin by converting your desired depth of soil from inches to feet.

Example: Let’s say your rain garden needs 18 inches of soil. 

1. Convert desired depth in inches to desired depth in feet.

Desired depth in inches x 1 foot12 inches= Desired depth in feet

18 inches x 1 foot12 inches= 1.5 feet deep

In this example, you want your rain garden to have a soil depth of 1.5 feet. 

2. Calculate cubic feet of soil by multiplying your desired depth by the square footage of your rain garden. 

For this example, we’ll use the square footage of 151.5 that we calculated earlier. 

Desired depth in feet x Rain garden square footage = Cubic feet of soil needed

1.5 feet deep x 151.5 square feet =227.25 cubic feet of soil needed

You would need 227.25 cubic feet of soil to fill a rain garden of this size to the desired depth.

3. There are 27 cubic feet in a cubic yard, so to convert, divide by 27. 

Cubic feet of soil needed 27 cubic feet per cubic yard=Cubic yards of soil needed

227.25 cubic feet of soil 27 cubic feet per cubic yard=8.42 cubic yards

For this example rain garden, you would need to add 8.42 cubic yards of soil to the basin.

4. To amend your soil, you need 65% excavated soil and 35% compost. Multiply the total cubic yards by those percentages to find how many yards of excavated soil and compost you need. 

Total cubic yards of soil needed x 65% (0.65)=Amount of excavated soil in your mixture

Total cubic yards of soil needed x 35% (0.35)=Amount of compost in your mixture

Example:

8.42 cubic yards x 0.65=5.47 cubic yards of excavated soil

8.42 cubic yards x 0.35 =2.95 cubic yards of compost

So, for a rain garden of this size, you would need to purchase 2.95 (round up to 3) cubic yards of compost to add to your existing soil.

Step 8: Fill the basin with soil

Now, it’s time to partially refill your rain garden to your ideal ponding depth. Use a wheelbarrow to pour fresh soil into your basin, and spread it evenly with a shovel (or shovels, if you’ve enlisted some good gardening friends). 

After adding 6 inches of soil, stop and walk over the soil to tamp it down. Repeat this process until you get to your desired ponding depth. If you want a ponding depth of 12 inches, leave 12 inches of space at the top of your rain garden. 

Once you’re set on soil, rake thoroughly to even out the area.

Pro Tip: If you’re amending your soil, you can either mix the compost and soil in a wheelbarrow and transport the mixed soil to your basin or dump your original soil and the compost directly into your basin and mix them there. 

Step 9: Fill your garden with plants and decorations

Here comes the fun part! Now that your basin is filled with fresh, healthy soil, it’s time to add plants and decorative features like boulders and edging to your rain garden.

Plants

Think of your rain garden as having three zones: the bottom (the lowest part), the sides, and the top (the shallowest part). Choose native plants with a variety of heights and textures (shrubs, grasses, flowers, small trees, and emergents like sedges and rushes) that will thrive in each zone. 

  • Zone 1: The bottom, in the very center of your rain garden, is the deepest zone. Choose plants that are happy with a “drought and drench” lifestyle. These plants grow well for months in standing water, but they also can survive in dry conditions. Many native grasses, sedges, and shrubs are perfect for this area. 
    • Irises, coneflowers, juncus (common rush), and dogwoods are top picks for Zone 1.
  • Zone 2: The sides of your rain garden need plants that can tolerate occasional ponding and have strong root systems to stabilize soil on slopes and prevent erosion. 
    • Asters, ferns, and snowberries are favorites for Zone 2.
  • Zone 3: The top of your rain garden is rarely wet for prolonged periods, so choose plants like low-growing shrubs and perennials that thrive in drier conditions. Ornamental grasses and wildflowers are also great options.
    • Black-eyed susans, spirea, and lupine are popular Zone 3 picks.

With native rain garden plants, you can look forward to butterflies, birds, and bees making your lawn a cheerful, bustling ecosystem with little maintenance on your part. Check a local native plant list to find the best plants for your rain garden.

Just make sure that when you plant, you’re considering the size of your plants at maturity. Space trees, shrubs, and plants according to their expected final dimensions.

Once your plants are in the ground, add a light layer of topsoil around their roots and then cover the bottom of the rain garden with 2 to 3 inches of compost. Mulch the sides and top of your garden with 3 to 4 inches of wood chips. Mulch will prevent weeds and erosion, and it won’t float when your rain garden fills with water.

Hardscape decorations

For a natural look, add stones and boulders to your edges. You can take a trip to your local quarry to check out their selection of native rocks.

Popular rocks for rain gardens include: 

  • River stones
  • Gravel
  • Lava rock
  • Limestone
  • Granite

You also may want to edge your rain garden with pavers, flagstone, or metal fixtures to give your garden a lovely design, prevent erosion, and stop grass from encroaching. 

Maintaining your rain garden

Rain gardens require very little maintenance once they’re fully established, but establishment can take a while. 

For the first three weeks after planting, water the plants in your rain garden once a week. After that, you’ll need to water deeply and infrequently (as needed) during the dry season for the first two to three years. After the first few years, rain gardens require little to no watering. 

Because rain gardens are filled with native plants, they do not need to be fertilized or treated with harsh chemicals. They may need occasional pruning, and yearly mulching is a good idea. Applying mulch will prevent erosion, inhibit weed growth, and keep the soil moist. 

To prevent the flow of sediment from your rain garden into the storm drain, make sure you have a healthy, clean cover of plants and rocks in place.

What does a rain garden do?

Rain gardens channel water away from the foundation of your home and reduce the level of contaminated stormwater runoff that flows into sewers and local aquatic ecosystems. 

Some of the benefits of rain gardens are that they:

  • Relieve some of the strain on your town’s storm sewer system
  • Prevent home water damage
  • Protect the environment from pollutants in stormwater runoff
  • Protect aquatic life from sudden water temperature changes
  • Recharge groundwater
  • Have plenty of visual appeal

For sloped yards prone to flooding and excess runoff, rain gardens are a great lawn addition. Puddles and pooling won’t stand a chance, and native wildlife such as butterflies and bees will be grateful for a new habitat.

How rain gardens work

When stormwater runoff is moving at full speed, it can pack quite an environmental punch. Rain gardens force runoff to slow down, cool off, and get filtered before it enters waterways.

Here’s a breakdown of what rain gardens do with stormwater runoff:

  • Rainwater rushes from gutters and impervious surfaces (like patios, driveways, and walkways) across your lawn. The water is heated by asphalt and concrete and picks up pollutants like fertilizer, herbicide, and harsh chemicals.
  • Your rain garden diverts the water away from the storm drain. Rain gardens are built at the bottom of a gentle slope so they catch the water as it flows downhill. They have swales (low-lying channels that mimic streams, often lined with rocks) or plastic piping to channel the water from your home into the basin.
  • The rain garden’s basin acts like a bathtub, temporarily stopping and holding the water.
  • Native plants and permeable soil act as sponges. The basin holds the water while native plants with deep roots absorb it. Permeable soil strains out nutrients, chemicals, and sediment from the water.
  • The water drains out of the rain garden into the surrounding soil within 1 to 3 days. By the time the water drains, it’s much cleaner, and its temperature won’t shock the natural ecosystems. 

FAQ about rain gardens

1. Mosquitoes love standing water. Should I be worried about my rain garden becoming a mosquito haven? 

No, you don’t have to worry about mosquitoes as long as your rain garden drains properly. Rain gardens drain water in 24 hours (or at the very most, 72 hours). Mosquitoes take four to seven days to complete their water lifecycle, so they’ll never get a chance to mature. 

2. How much does renting a mini excavator cost? 

You can rent a mini excavator for four hours for about $200 to $250 or rent for a day for $290 to $350. You can find a mini excavator at most garden centers and home improvement stores. 

3. Is there a way to soak up more water during heavy storms? 

Yes, if you’re concerned about major storms, you can install a dry well or even multiple dry wells beneath your rain garden. Dry wells are vertical, underground filtering systems that receive runoff from pipes and allow the water to slowly seep into the surrounding soil. They add water capacity to your rain garden and handle overflow during heavy storms.

4. How do I build a rain garden on a slope? 

If your rain garden is on a slope, you can build a sturdy berm (at least 2 feet wide) on the downhill side of your basin to retain water. Alternatively, you can simply dig deeper on the uphill side of your basin to even out water distribution.

Rain gardens: Building your own or getting help from a pro

Rain gardens are gorgeous and easy to maintain once established, but they certainly require detailed planning, measuring, and a whole lot of digging. 

If you’re short on time or you want a larger garden, call a local lawn care pro to deal with the equations and excavation for you. You can enjoy your lovely garden sponge without sponging sweat off your forehead. 

Main Photo Credit: krzysztofniewolny | Pixabay

Jordan Ardoin

Jordan Ardoin is a writer and indoor plant enthusiast hailing from Florida. In her spare time, she enjoys chasing her two cats around the house and trying to keep her houseplants alive.