Avant Gardening

How-To Build A Rain Garden – Ask Avant Gardening

Gardening | Landscaping

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What is a Rain Garden?

A rain garden is a shallow depression in the landscape designed to temporarily hold runoff water in order to allow it time to infiltrate through the soil instead of rushing into storm drains and ultimately surface water, e.g., lakes, rivers, and streams. When designed and installed correctly, rain gardens can help solve an urban problem of water pollution, provide food and habitat for important birds and insects, and provide aesthetic pleasure with (in the long run) minimal maintenance.

I heard Rain Gardens look weedy… Who wants that in their yard?!

Many poorly designed rain gardens have given the concept a bad name, arguably, due to poor plant choices and improper installation.  But they really can look beautiful and blend well with the rest of your landscape.

In this article we will highlight some important features to consider when building a rain garden.  Admittedly, there are many more components to a rain garden to consider than these highlighted few – Please refer to this WI DNR Rain Garden Manual for more information, it is a great resource.  For further assistance, please do not hesitate to Contact Avant Gardening in help designing, installing, and/or maintaining your rain garden.

1. Location

  • At least 10 feet away from any foundation
  • In an area that allows runoff water to flow towards it
  • Often in an area that water naturally flows towards
  • Or route water towards it with drainage lines
  • Not in the area where existing tree roots will be disturbed
  • Not on overly steep slopes (unless you want help building a retaining wall)
  • Not in an area subject to erosion

rain garden 2 rain garden 1

2. Soil Test

It is important to know your percolation rate to determine your rain garden specifics. Soil texture will provide information on suitable plants for your area.

  • Soil samples can be tested at UW Madison for $15
  • A Ribbon Test can approximate your percentage clay
  • Perform a Utility Locate
  • You can perform your own percolation test by digging two 1.5’ deep holes in the center of the proposed site on the uphill and downhill sides. Fill the holes with 12” of water and observe the drainage rate. After 12-24 hours the holes should drain. Repeat the process after a day. If the holes drain in 12 hours your percolation is rate 1”/hour, if in 24 hours .5”/hour. If it takes longer than that you should move locations or consider amending the location. If the rain garden does not drain properly the soil and plants will be damaged and you are providing mosquito habitat.

3. Sizing

  • Calculate the surface area of runoff water you will be managing (e.g., roof, patio, and/or driveway)
  • Use your percolation rate:
    • At 1”/hr multiply area by 10% for a 6” deep raingarden or 7% for 9” deep
    • At .5”/hr multiply by 20% for a 6” deep raingarden or 14% for 9” deep
  • Be sure to provide an overflow away from your foundation, rain gardens will fill up in torrential rains
  • Consider keeping your rain garden narrow and thin so that maintenance can be performed from outside of it minimizing soil compaction
  • If you cannot fit the calculated size rain garden in your property know that any properly functioning rain garden is beautiful and still helping
  • Design can match your existing landscape or provide a unique accent. Have fun!

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4. Installation

  • Kill existing lawn to ease in maintenance later
  • Layout your shape with marking devices (paint, hose, flags, etc.)
  • Site grade stakes on the uphill and downhill sides, the raingarden will be level from side to side and front to back
  • Dig to your calculated depth making sure your bottom is level
  • If through your soil analysis amendments were suggested add them now. Often the addition of organic matter and/or sand is suggested. If 3” OM and sand is suggested than dig your rain garden 3” deeper and rototill the amendments into your rain garden.
  • Use excavated soil to build a berm on the downhill side compacting in 2” lifts
  • Place the overflow area along the compacted berm flowing away from your house
  • Route runoff water towards rain garden either with surface or subsurface drainage

IMG_20150729_1016380645. Plants

  • Generally plants that can withstand long periods of drought but still survive temporary inundation of waters work the best. i.e., native species and cultivars thereof (aka nativars)
  • Herbaceous perennials are most common (a combination of grasses and flowers) but many trees and shrubs will work in larger raingardens
  • Know your soil texture and light requirements
  • Consult the DNR Information for ideas or contact our office for further assistance
  • Know your plant spacing (most perennials will be planted about 1’-2’ on center)
  • Mulch the area in shredded hardwood mulch for water conservation at 2”-3” deep

IMG_20140924_114048_0326. Maintenance

  • Water as needed due to seasonal requirements for the first year or two while roots are established
  • Vigorously weed your garden to keep competition down and not allow weeds to get established
  • Make sure your raingarden is draining properly
  • Note any erosion from your overflow spot and reestablish vegetation

If you carefully select an area and plan accordingly a rain garden will not only add beauty to your property but also help your larger community by providing many ecological services.  Once established, rain gardens are minimal maintenance.  With thriving plants, weeds have a harder time taking hold.  You will enjoy watching from year to year as the look of your rain garden moves around to match each year’s differences in precipitation and temperature. Enjoy!