Step 1: Sizing and Siting the Rain Garden

Site Selection

Home rain gardens can be in one of two places – near the house to catch only roof runoff or farther out on the lawn to collect water from the lawn and roof. To help decide where to put a rain garden, consider these points:

• The rain garden should be at least 10 feet from the house so infiltrating water doesn’t seep into the foundation.
• Do not place the rain garden directly over a septic system.
• It may be tempting to put the rain garden in a part of the yard where water already ponds. Don’t! The goal of a rain garden is to encourage infiltration, and your yard’s wet patches show where infiltration is slow.
• It is better to build the rain garden in full or partial sun, not directly under a big tree.
• Putting the rain garden in a flatter part of the yard will make digging much easier. For example, a rain garden 10 feet wide on a 10% slope must be 12 inches deep to be level, unless you import topsoil or use cut and fill.

What size should the rain garden be?

The surface area of the rain garden can be almost any size, but time and cost will always be important considerations in sizing decisions. Any reasonably sized rain garden will provide some stormwater runoff control. A typical residential rain garden ranges from 100 to 300 square feet. Rain gardens can be smaller than 100 square feet, but very small gardens have little plant variety. If a rain garden is larger than 300 square feet it takes a lot more time to dig, is more difficult to make level, and could be hard on your budget.

The size of the rain garden will depend on..

oWhat size looks best on your landscape

oWhat type of soils the garden will be planted in

After choosing a rain garden depth, identify the lawn’s soil type as sandy, silty, or clayey. Sandy soils have the fastest infiltration; clayey soils have the slowest. Since clayey soils take longer to absorb water, rain gardens in clayey soil must be bigger than rain gardens in sandy or silty soil. If the soil feels very gritty and coarse, you probably have sandy soil. If your soil is smooth but not sticky, you have silty soil. If it is very sticky and clumpy, you probably have clayey soil.

oHow much roof and/or lawn will drain to the garden

If you want to build a garden that can handle all the runoff it is likely to receive, the next step in choosing your rain garden size is to find the area that will drain to the rain garden. As the size of the drainage area increases so should the size of the rain garden. There is some estimating in determining the size of a drainage area, especially if a large part of the lawn is up-slope from the proposed garden site. Use the suggestions below to estimate the drainage area without spending a lot of time, and then turn to pages 8 and 9 of Wisconsin’s Technical Rain Garden Manual to calculate the actual size of your garden.

Rain gardens less than 30 feet from the downspout

  • In this case, where the rain garden is close to the house, almost all water will come from the roof downspout. Walk around the house and estimate what percent of the roof feeds to that downspout. Many houses have four downspouts, each taking about 25% of the roof’s runoff.
  • Next find your home’s footprint, the area of the first floor. If you don’t already know it, use a tape measure to find your house’s length and width. Multiply the two together to find the approximate area of your roof.
  • Finally, multiply the roof area by the percent of the roof that feeds to the rain garden downspout. This is the roof drainage area.

Rain gardens more than 30 feet from the downspout

  • If there is a significant area of lawn uphill that will also drain to the rain garden, add this lawn area to the roof drainage area. First find the roof drainage area using the steps above for a rain garden less than 30’ from the downspout.
  • Next find the area of the lawn that will drain to the rain garden. Stand where your rain garden will be and look up toward the house. Identify the part of the lawn sloping into the rain garden.
  • Measure the length and width of the uphill lawn, and multiply them to find the lawn area.
  • Add the lawn area to the roof drainage area to find the total drainage area.

oHow deep the garden will be

A typical rain garden is between four and eight inches deep. A rain garden more than eight inches deep might pond water too long, look like a hole in the ground, and present a tripping hazard for somebody stepping into it. A rain garden much less than four inches deep will need more surface area to provide enough water storage to infiltrate all the storm water from larger storms.