How to 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 used to collect, filter, and infiltrate stormwater. It is densely planted with native plants that tolerate both wet and dry conditions. The purpose is to keep stormwater on site and improve its quality as it percolates through the soil to recharge the water table. Plants are imperative because they encourage water to infiltrate the soil as they create pore spaces with their roots. Additionally, they use water and, through transpiration, release it back into the atmosphere.

Rain Garden Benefits

Rain gardens provide many benefits to the earth, wildlife, and people. The earth benefits by improved water quality. Rain gardens filter pollutants and sediment through the plant roots and soil. In addition, rain gardens recharge the ground water and reduce surface water runoff, which prevents pollutants from entering waterways. Finally, holding the water reduces the volume and velocity of runoff thereby reducing soil erosion.

Monarch on Asclepias in Rain Garden

Wildlife benefits from the habitat rain gardens provide. The native plants 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.

People benefit from reduced flooding. Stormwater stays out of streams when it is kept on site. This benefits us both with increased security and increased savings as property owners and taxpayers. And, of course, rain gardens are beautiful too!

Site Selection

We install stormwater projects on properties of all shapes, sizes and types such as residential and commercial properties. Additionally, grant funding is becoming increasingly available for municipal storm water projects. Optimal sites are neither too steep nor too flat and have good drainage.

Location of Rain Garden

Choose your location carefully. Determine the source of the storm water, such as 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.
  • Keep it at least 10 feet away from buildings
  • Do not place it in an area that already holds water- added water will not infiltrate into the soil

Rain Garden Design

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

  • Connect it to the existing landscape
  • Mimic natural ecosystems through plant combinations
  • Use a mix of woody and herbaceous plants, colors, and textures
  • Mass plants for effect
  • Repeat those masses
  • Allow for overflow

Rain Garden Construction

Depending on the existing site conditions and the size of the garden, you can likely build a small rain garden in a weekend. However, quite a bit of physical labor is required to loosen soil and extend downspouts.

Extend downspouts to the rain garden using buried 4″ pipe. This should be at a slope no greater than 33% to reduce the force of the water entering the garden.

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 (2-4 feet). If soil texture is high in clay, add compost improve in 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. You may reinforce the area with filter fabric and large river rock to prevent mulch from washing out of the garden.

Once the garden is formed, plant it densly enough to fill in the spaces quickly. Add either non-dyed shredded hardwood mulch or leaf compost.

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
  • Cut back perennials in the spring
  • Mulch once/year for the first 2-3 years
  • Clear gutters and downspouts as needed
  • Clean out inlets, outlets, and overflows as needed and address any erosion

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 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 sides of the garden. It is inundated after storm events and dry soil between storm events. For the interior “walls” try plants like:

  • Spiderwort/ Tradescantia spp.
  • Blazing Star/ Liatris spicata
  • Lady in Red Fern/ Anthyrium felix-femina
  • Bee Balm/ Monarda didyma
  • Little Joe Pye Weed/ Eupatorium dubium
  • Virginia Sweetspire/ Itea virginica
  • Inkberry Holly/ Ilex glabra
  • Red-twig Dogwood/ Cornus sericea ‘Baileyi’
  • Red Chokeberry/ Aronia arbutifolia
  • Smooth Witherrod/ Viburnum nudum
  • Summersweet Clethra/ Clethra alnifolia ‘Ruby Spice’
  • Serviceberry/ Amelanchier ‘Autumn Brilliance’

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:

  • Husker Red Beardtounge/ Penstemon digitalis ‘Husker Red’
  • Foamflower/ Tiarella cordifolia
  • Coreopsis/ Coreopsis verticillata ‘Zagreb’
  • New England Aster/ Aster novae-angliae
  • Wood Geranium/ Geranium maculatum
  • Black-eyed Susan/ Rudbeckia fulgida
  • Northern Sea Oats/ Chasmanthium latifolium
  • Sweetbay Magnolia/ Magnolia virginiana
  • Red Maple/ Acer rubrum

Other Considerations

If for any reason a rain garden is not appropriate for your site, there are other ways that you can manage storm water.  Here are some alternative ideas:

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

So remember, the environmentally friendly approach to managing rainwater is to keep it on site. Rain gardens are a very effective and beautiful solution.