April Showers Bring… Rain Gardens?

I love rain gardens. I may be partial because over the last 4 years I’ve designed and built some really cool and successful rain gardens for properties of all shapes and sizes. But to me, there’s really nothing better than a beautiful landscape that’s also highly functional. Now, if you’re like most of the clients, friends, and strangers I talk to about rain gardens, you’re probably still thinking:

“What is a rain garden?”

rain garden diagramA rain garden is a shallow depression in the land used to collect, filter, and infiltrate storm water. It is densely planted with native plants and tolerant of extreme wet and extreme dry conditions. The purpose of a rain garden is to keep storm water on site and improve its quality as it percolates down through the soil to recharge the water table. Plants are imperative to a rain garden because they not only encourage water to infiltrate as they create pore spaces with their roots, but they use that water and, through transpiration, release it back into the atmosphere.

Benefits

Rain gardens provide many benefits to the earth, wildlife, and people. To the earth rain gardens improve the quality of water by filtering out pollutants and sediment through the plant roots and soil. Rain gardens recharge the ground water and also reduce surface water runoff, which prevents pollutants from entering waterways. The act of holding the water also reduces the volume and velocity of runoff which reduces soil erosion.

Monarch on Asclepias in Rain GardenWildlife benefit from the habitat rain gardens provide. The native plants in rain gardens provide nectar and pollen for butterflies, moths, and bees; many provide seeds, berries, and attract insects that birds eat. And the temporary marsh-like environment can provide habitat for frogs, toads, turtles and other amphibians and reptiles. However, mosquitoes should never be a problem as rain gardens are built to drain within 72 hours, preventing the standing water necessary for mosquito breeding.

People see reduced flooding in local communities when storm water is kept on site, which benefits us both with increased comfort and increased savings as taxpayers. The constant changes within a rain garden also encourage people to interact with the landscape, whether it’s monitoring the water levels, searching for caterpillars, or managing the perennials. And, of course, rain gardens are beautiful too!

Site Selection

Rain gardens can be installed on properties of all shapes and sizes. Commonly they’re used on residential and commercial properties, municipal land, homeowners associations, and in parking lot islands. Although rain gardens work on any type of property, a few considerations will ensure a successful garden:

  • Decide from where you want to collect water
    • Do you want to capture water from your roof by extending your downspouts to a rain garden?
    • Do you want to collect surface flow from a parking lot or driveway?
    • Keep rain gardens at least 10 feet away from the foundation of buildings
    • Do not create in an area that already holds water- added water will not infiltrate into the soil
    • For rain gardens larger than 200 square feet, consider building two rain gardens

Design

Just like any other garden, a rain garden should be visually appealing. A few simple design strategies will help build a rain garden that looks like more than just a plant pocket in the middle of a lawn.

  • Designed Rain GardenEstablish location
  • Connect with the existing landscape
  • Mimic natural ecosystems through plant combinations
  • Use a mix of woody and herbaceous plants
  • Mass plants for effect
  • Repeat those masses
  • Allow for overflow

Construction

Depending on the existing site conditions and the size of the garden, rain gardens can often be built with little to no machine help. Quite a bit of physical labor is involved to loosen the base layer of the garden or if the downspouts need to be extended. For a professional landscape company to build a rain garden it may take from 3 to 10 days or more depending on how extensive the garden is. For homeowners, it may take 3 weekends or more, again, depending on how extensive it is.

Once the location is determined, any downspouts should be extended to the rain garden at a slope no greater than 33% to reduce the force of the water entering the garden. If the rain garden is collecting surface water, grading may be done to optimize the surface area being collected as well as reduce erosion.

When the base of the rain garden is excavated, excess soil should be placed on the downhill side of the garden to build the berm or aid in grading. The base of the garden should be turned deeply (2-4 feet) and if soil texture is very clayey, high quality compost can be added to increase pore space in the soil, aiding in drainage. Then level out the base so that water spreads evenly across the rain garden.

rain garden in progress

A rain garden during construction.

It is important to allow an area for overflow during larger storm events. An overflow channel should be built into the berm which directs water away from buildings. It should be lower than the berm, but higher than the inlet. We typically reinforce the area with filter fabric and large river rock to prevent mulch from washing out of the garden.

Once the shape is formed, the rain garden must be planted and mulched with either non-dyed shredded hardwood mulch or leaf compost. Mulch keeps the soil moist, prevents soil from washing away, and helps filter sediment and pollutants entering the rain garden.

Maintenance

The maintenance of a rain garden is about as simple as regular property 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 expectations include:

  • Cut back perennials in the spring and/or fall
  • Remove excessive amounts of fall leaves
  • Mulch once/year for the first 2-3 years
    • Maintain 2-4” layer of mulch
    • Use non-dyed shredded hardwood or leaf compost
    • Keep gutters clear
    • Clean out inlets, outlets, and overflows as needed

Plant selection

There are many great native plants that can be used in a rain garden. Local, native plants are adapted to our cold winters, wet springs, and very dry summers.  Some consideration to soil moisture should be taken to ensure the survival of the plants. Select plants that naturally occur in conditions similar to a rain garden. I like to look at what grows around wetlands, streams, rivers, and ponds and mimic those combinations. There are 3 basic zones to a rain garden:

Zone 1/ FACW- Inundated after storm events, wet/mesic soil between storm events. Select plants that grow in wetlands, ephemeral streams, or grow down along the water’s edge, without growing in the water. For the bottom of the rain garden, 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/FAC- Inundated after storm events, dry soil between storm events. For the interior “walls” of the rain garden, 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/FACU/UPL- Inundated after extreme storm events, 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 still ways, big and small, that you can keep storm water on site.  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, and rain gardens are a very effective and beautiful solution.