Rain Gardens

Rain Gardens are gardens containing flowering plants and grasses (preferably native species of both) that can survive in soil soaked with water from rain storms. However they are not gardens that have standing water. Rain Gardens collect and slow stormwater run off and increase its infiltration into the soil.

These attractive gardens help reduce the rapid flow of stormwater from homes and businesses to storm drains and thus protect streams and lakes from pollutants that are washed from house roofs and paved areas.

Applications

1. Provide a low maintenance, attractively landscaped property with minimum cost.
2. Reduce the amount of household or business stormwater and associated pollutants to area streams and lakes-improve local water quality.
3. Reduce potential of basement flooding.
4. Improve or eliminate wet spots in yard.
5. Increase habitat for beneficial insects, butterflies and birds.“Habitat” means the area with all the resources that critters need to survive.

Examples

Lakeside Stormwater Reduction
Project

Duluth, MN

Project to demonstrate the effectiveness of homeowner BMPs to reduce residential stormwater flows in an older neighborhood.

Lake County Courthouse

Two Harbors, MN

East Ridge Community Church
Duluth, MN

Barkers Island, Lake Superior NERR/UWS
Superior, WI

Materials and Installation

1. Locating the garden:
   
   • 
     From: Rain Gardens: A How to Manual for Home Owners;
     provided by Wisconsin Department of Natural Resources (WDNR) [3.2 MB pdf file]
   • Build rain gardens at least 10 feet from a house or building.
   • Choose a low point in the yard to which stormwater from building downspouts and/or sump pumps outlets can be directed.
   • Study the natural drainage of yard. Place garden to take advantage of natural drainage patterns that will direct any garden overflows away from building.
   • Locate the garden to receive full or partial sunlight.
2. How big should the garden be?
   
   • Garden size depends on the amount of water shed by the roof of a building. A rule of thumb for garden size:
     
     • Determining area covered by building (length of building x width of building).
     • Determine which downspouts will be draining to the rain garden, and which will drain elsewhere.
     • To provide an approximate square footage of garden needed (for a 1 inch rainstorm and a garden that is 6-inches deep, less than 30 feet from building downspout(s) and in clay soil): Multiply the area of the building by the ratio of downspouts feeding the garden, then multiply by 0.32 Example --
       - your house is 40 feet long and 25 feet wide
       - it has 4 downspouts, each one collecting from a similar area of roof,
       - 2 downspouts will be feeding the rain garden
       - your calculation will be: 40 x 25 x (2/4) x 0.32 = 160 sq. ft.
     • Note that the soil type where the garden is to be located influences the size of garden needed.
   • More detailed calculations on garden size determination are found on pages 7-10 of Rain Gardens: A How to Manual for Home Owners- A step by step manual for building a home rain garden (download 3.2 MB pdf).
   • Once optimum garden size has been established, a decision based on land availability, financial resources and available labor should dictate actual garden size built. A garden 2/3 of size of the calculated optimal garden can be almost as effective.
3. Planning and designing the garden.
   
   • Draw your vision of the garden to scale on graph paper. Keep the design simple and limit plant selections to a few species rather than the multitude of available species.
     
     • Select plant species to provide flowers throughout the seasons.
     • For greatest impact plant similar plant species in groupings of three or more.
     • To estimate the total number of plants required, divide the garden’s square footage by .75 and plan to plant at a density of approximately one plant every one to two feet. Labels on nursery stock provide information on suggested distance between plants.
   • Layout garden boundaries or shape by using rope or spray paint to transfer paper design to actual dimensions of yard surface.
   • The preferred time to build a rain garden is in the spring when the soil is easy to dig and selected plants are in a rapid growth phase.
4. Digging the garden
   
   • Prior to digging check with appropriate authorities to verify location of any underground utilities. (Minnesota; Wisconsin)
   • Remove the turf
   • Rain garden surfaces need to be level and a berm approximately the height of the depth of the garden should constructed around bottom 2/3 – 3/4 of the garden. The berm holds water in the garden during rainstorms.
     From: Rain Gardens: A How to Manual for Home Owners;
     provided by Wisconsin Department of Natural Resources (WDNR) [3.2 MB pdf file]
   • Determine the slope of the land where the garden is being built and use the rules of thumb below to guide a decision to what depth the garden should be dug:
     • Less than 4% slope; dig garden three to five inches deep.
     • Between 5 -7% slope; dig garden six to seven inches deep.
     • Between 8 -12% slope; dig garden 8 inches deep.
   • Use the dug soil to fill in and level those parts of the garden that are deeper than the required depth. Also for building the berm.
   • Excellent direction on lay-out and digging a rain garden can be found on pages 10-13 Rain Gardens: A How to Manual for Home Owners- A step by step manual for building a home rain garden. [3.2 MB pdf file]
5. Planting
   • Locate and purchase plants. Select robust, healthy plants.
   • Refer to design plan to locate potted plants in their approximate positions in the prepared garden. Step back and check the effect created by the design. Adjust plant positioning until satisfied.
6. Maintenance
   • Cover garden with shredded mulch to help reduce weeds and retain moisture in the garden.
   • Weeding over the first few years will be necessary. When weeding be certain to remove the entire weed- root and leaves. After two years the native plants will be able to out-compete most weed species and the need for intensive weeding will be greatly reduced.
   • Dead foliage can be left untouched throughout the winter months and adds additional interest to the landscape.
   • In spring dead foliage should be cut out as the new spring growth begins.

Tips and Wisdom

1. Rain Gardens do not have to be elaborate or overly large. Refer to: The Smaller Rain Garden on the Wisconsin DNR website for ideas on developing a small rain garden.
2. Native plants are preferred because they are best adapted to soil and temperature conditions. In regions with heavy clay soil, deep rooted native plants break-up the soil better than typical varieties of lawn grass, thus improving clay soil’s permeability. However, busting through layers of clay takes time so expect clay based rain gardens to take several years to fully develop.
3. Unless there is purist concern for use of actual native plant species, rain garden landscapes can be enhanced by the use of varietals of native species. These ornamental forms of native plants usually possess different flower or leaf color, shape or size; however, most varietals continue to be adaptable to rain garden conditions. Many plant nurseries will have a supply of these types of plants.
4. Make certain that chosen plants are compatible with the climate where the garden is to be developed. Most plant nursery stock will include a label with USDA plant hardiness zone information for that particular species. Another good reference for determining the hardiness zone of a particular plant species is ‘The American Horticultural Society A-Z Encyclopedia of Garden Plants’. 2004. By H. Marc Cathy and Christopher Brickell; D K Publishing Inc, NY, New York. Details determining the hardiness zone of the area where the rain garden is to be installed can be found at: USDA Plant Hardiness Zone Map or GrowIt.com (clearer state maps provided). Be wary of using plant catalogs as a source for information on hardiness zones of plants to be purchased.
5. Sunlight requirements of individual plant species is an important consideration. Nursery plant labels contain this information. Other references include:

   Plants for Stormwater Design: Species Selection for the Upper Midwest. [available online at MPCA]

   The American Horticultural Society A-Z Encyclopedia of Garden Plants. 2004. By H. Marc Cathy and Christopher Brickell; D K Publishing Inc, NY, New York.

   The South St. Louis Soil & Water Conservation District developed a list of plants used for their Northshore Community School Demonstration Project.

   The University of Minnesota-Duluth’s College Street Parking Lot Rain Garden project also developed a comprehensive list of plants.

   The Minnesota Stormwater Manual (November 2005) includes a chapter with recommended plants for a variety of vegetated stormwater BMPs.

6. Use of a rain barrel located under other gutter downspouts and connected with a soaker hose to the rain garden(s) can help spread rainfall over longer periods of time, thereby further slowing the flow of stormwater and increasing its infiltration. See the rain barrel page for more information on installing rain barrels.
7. If personal land ownership provides an available supply of native plants listed for the rain garden, be cautious in removing these wild plants.
   Often wild plants do not transplant successfully or are rare in particular habitats and should not be removed. Plants may also be available from some development sites prior to beginnings of clearing and construction. The cautions associated with the sometimes low success of transplanting and the rarity of the plants to be harvested must be addressed. Advice should be sought from local nurseries or a professional horticulturist. Propagation from seed is a much more desirable approach. However, the germination of seeds of many native plants requires adoption of special techniques. A little time researching propagation requirements for native plants will result in a more successful outcome. To find general information on native plant seed germination techniques see: Wild Ones® Landscaping With Native Plants. The following books discuss useful plants for stormwater design, including the seed germination techniques required of specific species: Plants for Stormwater Design: Species Selection for the Upper Midwest. 2003. By Daniel Shaw and Rusty Schmidt; published by Minnesota Pollution Control Agency and is reproduced online (PDF Files) at MPCA. Restoring the Tallgrass Prairie: An illustrated manual for Iowa and the Upper Midwest. 1994. By Shirley Shirley; published by University of Iowa Press (numerous prairie plants range far north beyond traditional prairie boundaries).

Limitations

1. Rain gardens will not remove permanent stands of water (pool or pond) in a yard. However, water gardens can be designed to incorporate such a feature.
2. Do not locate rain gardens over septic systems or near wells.