Rain Gardens

Rain Gardens - the Natural Solution for Small Urban Spaces
You don’t need to be an engineer to plan and build a rain garden - but you do need to understand storm water management principles.

The following guidelines reflect interviews with Larry S Coffman, Prince George’s County, Maryland, Roger Bannerman, Wisconsin Department of Natural Resources, Madison, Wisconsin, and Chris Cavett, Maplewood, Minnesota.

Site Selection & Rain Garden Design

• Observe the storm water runoff patterns on the site. Does water infiltrate in the ground after a rain or does it pond? Rain gardens should be placed where they will intercept storm water flow - hopefully through diffused, overland flow. Wet areas in the yard that currently collect rainwater are ideal locations.
• If you are using rainwater from the roof, place rain gardens in the vicinity of roof downspouts. Gardens should be 20 feet away from any buildings, including your neighbor’s home. If necessary, create a grassed swale to guide the downspout water to the rain garden.
• Sizing of the rain garden depends on many factors including the size of the drainage area, soil texture, slope, and the goals of the property owner. The Virginia Department of Forestry recommends a garden space that is 5% - 7% the size of the total drainage area.
• Design the garden as a "shallow bowl" to trap the first flush of storm water. The recommended ponding depth with the infiltration capability of the soils - but generally averages 3 to 6 inches. If the site has clay soils, make the depression shallow to reduce the volume of trapped water. If soils are sandy and porous, a deeper rain garden - even exceeding 1 foot in depth - could be considered.
• A grassed buffer (lawn) around the rain garden provides additional water quality and soil erosion control benefits.
• If a constructed rain garden holds standing water for more than 2 days, some modifications in the soils and design will be needed. Rain gardens need to trap storm water temporarily, but should not become a wet pond that breeds mosquitoes.

Soils, Natural Mulches, & Erosion Control

• Build the garden bed with a planting mix of sand (25-35%), compost (50% or more) and native soil (15-25%). For a small rain garden at a home site, variations of these proportions may be workable.
• Stabilize the top of the garden with a natural mulch 2 to 3 inches deep. The mulch acts as a sponge to capture heavy metals, oils, and grease. Bacteria breaks down the pollutants as the mulch decays. The mulch also reduces weeds and maintenance.
• Select a natural mulch such as aged shredded hardwood bark that will gradually decompose, adding compost (humus) to the soil. Apply the mulch to a depth of 2 - 4 inches and replenish, as needed.

How-to-do-it Guidelines for Small Site Rain Gardens
You don’t need to be an engineer to plan and build a rain garden - but you do need to understand storm water management principles. The following guidelines reflect interviews with Larry S Coffman, Prince George’s County, Maryland, Roger Bannerman, Wisconsin Department of Natural Resources, Madison, Wisconsin, and Chris Cavett, Maplewood, Minnesota.

Planting the Rain Garden
In most rain gardens, plants are inundated from time-to-time and left dry at other times. Choose hardy plants that grow in either saturated soils or organic soils. Native wildflowers and shrubs are ideal. Some non-aggressive native grasses may also be useful. Native plant nurseries in Michigan and the Midwest will also have useful information. Consider hiring a consultant familiar with the growth habits of native plants, or requesting help from ecological gardening volunteers.

• Plant selection and placement should reflect various zones of the rain garden. Some sections of the garden will typically have saturated soils, while other areas (along the side of the rain garden) may have organic soils but may not be saturated with water.
• Use plants native to Michigan and the Upper Great Lakes whenever possible. Native plants are adapted to local climate and site conditions. Native plants offer many advantages for water quality and biodiversity. Perhaps most importantly, the deep roots of some native wildflowers and shrubs help absorb storm water, and help to decompose storm water pollutants.
• Wet prairie wildflowers are excellent candidates for sunny rain gardens. Diverse plantings can often be selected to provide continuous bloom and nectar for butterflies throughout the growing season.
• In partial shade locations, consider a rain garden with trees, shrubs, woodland wildflowers, and sedges - recreating a woodland edge. Trees and shrubs have important storm water enhancement benefits and add to habitats for birds and beneficial insects.
• Start your garden with a wide variety of plant species planted in a style to replicate a "natural" look. Plants suited to the site will thrive, while those that aren’t suitable for drainage and site conditions will die out. Avoid monocultures which can be susceptible to disease.
• Involve community groups and children in the planting of the garden whenever possible. Community plantings are ideal opportunities for education outreach and help build community pride in water quality projects. If community residents participate in the planting of the rain garden, they will be more willing to help maintain it.

Maintaining the Rain Garden

• Consider garden maintenance responsibilities before planting.
• Inspect the rain garden after rainstorms. Rain gardens, like other native plant gardens, need attention to maintenance.
• Weed when necessary. Replace mulch and plants when necessary.
• If volunteers are involved, be sure to thank them in an appropriate way.

Native Wildflowers, Groundcovers, & Shrubs for Rain Gardens in Southeast Michigan
A "starter" list of rain garden plants for Southeast Michigan is listed below. Always checks sun/shade conditions before planning your garden. The plants listed are excellent for moist organic gardens that are "dry" within 48 hours of a rain.