How to Design and Build a Rain Garden

We love rain gardens. We may be partial because over the years we’ve designed and built some beautiful and successful rain gardens for our clients. Also, there’s really nothing better than a beautiful landscape that’s highly functional. Now, if you’re like many people, you’re probably thinking:

What is a Rain Garden?

A rain garden is a shallow depression in the land that is used to collect, filter, and infiltrate stormwater. It is densely planted with native plants that tolerate both wet and dry conditions. Rain gardens keep stormwater on site and improve water quality as it percolates through the soil to recharge the water table. Native plants are a key element to a successful rain garden, absorbing the water and through transpiration, releasing it back into the atmosphere. Additionally, they encourage water to soak in by creating space in the soil with their roots.

Rain Garden Benefits

Rain gardens provide many benefits to people and wildlife including improved water quality, reduced flooding and erosion, and increased habitat and beauty.

When rain hits hard surfaces like roofs, driveways and roads, it picks up pollutants and runs quickly into our waterways causing localized flooding. This increased speed also causes erosion, furthering the problem of water pollution. For instance, in Pennsylvania, the sediment from erosion is the largest surface water pollutant by volume. Furthermore, water temperatures increase when running across these surfaces which is harmful to aquatic life.

Improved Water Quality

Rain gardens filter pollutants and sediment through the soil and plant’s roots while recharging the ground water. This reduced runoff prevents pollutants from entering our waterways. Moreover, temporarily holding the water reduces the velocity of runoff thereby reducing soil erosion. And rain gardens keep water cool, benefiting fish and other wildlife.

Reduced Flooding

As storms get more powerful, localized flooding increases, stressing roads, bridges, and other infrastructure. When stormwater is kept on site, it recharges our ground water. Then slowly and steadily, cool water releases into our streams and waterways through seeps and springs. This benefits us both with increased safety and increased savings as property owners and taxpayers. And, of course, rain gardens are beautiful too!

Better Habitat

The native plants in rain gardens provide nectar and pollen for butterflies, moths, and bees. These plants also provide seeds, berries, and attract insects that birds eat. And the temporary marsh-like environment provides habitat for frogs, toads, turtles and other amphibians and reptiles.

Site Selection

You can install rain gardens on properties of all shapes, sizes, but make sure your site is neither too steep nor too flat. Optimally, you should have between a 4% and 12% slope. Your site should also have good drainage, so it is important to evaluate your soil. It should not be compacted or have too much clay, and you will use this information to select appropriate plants. Make sure the water drains within 72 hours to eliminate the possibility of mosquitos breeding.

Location of Rain Garden

Choose your location carefully. Determine the source of the stormwater that you are trying to capture. This may be a rooftop, driveway, or neighboring property. Then use these guidelines:

  • Start at the top. It is usually best to create a rain garden above wet areas. Locate your rain garden at the highest point possible that still captures the water.
  • Determine the size. Ideally, this is about 1/3 the size of the impervious surface are of the source of water. If you don’t have the space, smaller is fine.
  • Keep it at least 10 feet away from buildings and 25 feet away from septic systems.
  • Do not place it in an area that already holds water- added water will not infiltrate into the soil.
  • Install multiple rain gardens along a slope to capture more water.

Rain Garden Design

Rain garden designed to blend into existing landscape

Use these design strategies to build a rain garden that is beautiful as well as functional:

  • Connect it to the existing landscape. Rain gardens do not look their best when they are in the middle of a grassy area.
  • Mimic natural ecosystems through plant combinations.
  • Use a mix of woody and herbaceous plants, colors, and textures.
  • Cover the ground with native plants.
  • Allow for and plan the direction of the overflow.

Rain Garden Construction

Volunteers planting a rain garden

Call before you dig. In Pennsylvania you must call 811 to have utilities marked before you can start your project.

Determine the garden depth. If you have clay soils, the maximum recommended depth is 6”. Rain gardens with sandy soils can be 8” to 12” deep.

Excavate the rain garden and place the soil on the downhill side to build the berm and retain water. Deeply loosen the soil in the bottom of the garden. If soil texture is high in clay, add compost to improve drainage. Then level out the base so that water spreads evenly across the bottom.

It is important to design an area for overflow during larger storm events. Direct water away from buildings and neighboring properties. Overflows should be lower than the top of the berm, but higher than the inlet.

Once the garden is formed, plant it densely enough to cover the ground quickly and then apply mulch. Use either non-dyed shredded hardwood mulch or leaf compost.

Attaching Downspouts

Some rain gardens can be built without the need for pipes, such as ones capturing water from roads or driveways. However, rain gardens that are designed to capture water from roofs are frequently connected via underground pipes. To attach your downspout to your rain garden via pipes, take the following steps:

  • Determine what supplies you need. You will need downspout connectors, 4” pipe, connectors, glue, a drain cap, a level, and a saw. Measure the size of your downspouts, the distance between your downspout and the rain garden, and decide if there will be any bends in your run. Avoid corrugated drain pipes, because they tend to trap dirt and they do not drain as well as smooth pipes. Solid pipes come in 10’ lengths, and connections come in both 45-degree and 60-degree options.
  • Gather material from your local hardware store.
  • Dig your trench. You will need a pitch between ¼”-3” per foot. A 10” deep trench allows for 6” of soil over a 4” pipe which is usually enough to grow grass over top.
  • Lay your pipe. Ensure the proper pitch, make your connections, and then glue them together. To allow for cleanouts, do not glue the downspout connector.
  • Backfill the trench. Cover the pipe with soil and tamp lightly.
  • Finish with a cap.

Rain Garden Maintenance

The maintenance of a rain garden is as simple as regular garden maintenance. There are two key elements to the rain garden: the garden and the downspouts (if applicable). If both elements are maintained regularly, a rain garden is a long-lasting, functional element to the landscape. Maintenance tasks include:

  • Manage weeds regularly
  • Perform an ecological spring cleanup and cut back perennials in the spring.
  • Mulch once/year for the first 2-3 years. After plants fill in, mulch may no longer be necessary.
  • Clear gutters and downspouts regularly.
  • Clean out inlets, outlets, and overflows as needed and address any erosion.

For more information on garden maintenance, read our Simple Gardening Tips From the Pros.

Plant Selection

There are many, great, native plants to use in a your design. Native plants are adapted to our cold winters, wet springs, and very dry summers.  For ideas, look at native plants that grow near wetlands, streams, rivers, and ponds and mimic those combinations.

There are 3 basic zones to a rain garden:

Zone 1– This is the bottom of the rain garden. It is inundated with water after storm events, and has wet/mesic soil between storm events. Select plants that grow in wetlands, ephemeral streams, or along the water’s edge. Try plants like:

  • Swamp Milkweed/ Asclepias incarnata
  • Creek Sedge/ Carex amphibola
  • Rose Mallow/ Hibiscus moscheutus
  • Blue Flag Iris/ Iris versicolor
  • Winterberry Holly/ Ilex verticillata
  • Soft Rush/ Juncus effusus

Zone 2– This is the slanted area of the depression. It is inundated after storm events and dry soil between storm events. For the interior “walls” try plants like:

  • Serviceberry/ Amelanchier sp.
  • Red Chokeberry/ Aronia arbutifolia
  • Lady in Red Fern/ Athyrium felix-femina
  • Summersweet Clethra/ Clethra alnifolia
  • Red-twig Dogwood/ Cornus sericea
  • Joe Pye Weed/ Eupatorium sp.
  • Inkberry Holly/ Ilex glabra
  • Virginia Sweetspire/ Itea virginica
  • Blazing Star/ Liatris spicata
  • Bee Balm/ Monarda sp.
  • Spiderwort/ Tradescantia sp.
  • Smooth Witherrod/ Viburnum nudum

Zone 3– This is the top edge of the rain garden. It is inundated after extreme storm events and can be very dry between storm events. For the uppermost perimeter of the rain garden, try plants like:

  • Red Maple/ Acer rubrum
  • New England Aster/ Aster novae-angliae
  • Beardtounge/ Penstemon digitalis
  • Coreopsis/ Coreopsis verticillata
  • Wood Geranium/ Geranium maculatum
  • Sweetbay Magnolia/ Magnolia virginiana
  • Black-eyed Susan/ Rudbeckia fulgida
  • Foamflower/ Tiarella cordifolia

Other Considerations

If for any reason a rain garden is not appropriate for your site, there are other ways that you can solve drainage and erosion issues, and manage stormwater Here are some alternative ideas:

  • Rain barrels
  • Green roofs
  • Bioswales
  • Porous surfaces
  • Deep-rooting meadows

So remember, the environmentally friendly approach to managing rainwater is to keep it on site. And rain gardens are a very effective and beautiful solution. Do it yourself, or contact us for help!