Rain Gardens

112 Larson 10

Rain gardens are landscaped areas designed to capture and treat rainwater that runs off roof and paved surfaces. Runoff is directed toward a depression in the ground, which is planted with flood and drought-resistant plants. As the water nourishes the plants, the garden stores, evaporates, and infiltrates rainwater into the soil. The soil absorbs runoff pollutants, which are broken down over time by microorganisms and plant roots.

Rain gardens are a relatively low-cost, effective, and aesthetically pleasing way to reduce the amount of stormwater that runs off your property and washes pollutants into storm drains, local streams, and the Monterey Bay. While protecting water quality, rain gardens also provide attractive landscaping and habitat for birds, butterflies, and other animals, especially when planted with native plants.

Is a Rain Garden Feasible for My Project?

Rain gardens are appropriate where the following site characteristics are present:

  • Rain gardens should be installed at least 5 feet from building foundations. The ground adjacent to the building should slope away at a 2% minimum slope. A downspout extension or ”swale” (landscaped channel) can be used to convey rain from a roof directly into a rain garden. Rain gardens can also be located downstream from a rain barrel overflow path.
  • Rain gardens should be at least 3 feet from public sidewalks (or have an appropriate impermeable barrier installed), 5 feet from property lines, and in an area where potential overflow will not run onto neighboring properties.
  • The site should have well-drained soil and be relatively flat. Soil amendments can improve infiltration in areas with poor drainage. Add about 3 inches of compost to any soil type and till it in to a depth of about 12 inches.
  • A front or backyard can work well for a rain garden, especially in areas where the slope naturally drains the stormwater to the rain garden location.

How Large Does My Rain Garden Need to Be?

A general recommendation for a garden with a 6-inch ponding depth is to size the rain garden to approximately 4% of the contributing impervious area. Your soil type will affect how the rain garden should be sized because the water infiltration rate depends on the soil type; rain gardens should be larger in areas with slower infiltration. The following table can be used as general guidance.

Contributing Area
(sq. ft.)
Rain Garden Area
(sq. ft.)
500 – 700 24
701 – 900 32
901 – 1,100 40
1,101 – 1,300 48
1,301 – 1,500 56
1,501 – 2000* 70

*Projects adding roof or other impervious areas in excess of 2,000 sq. ft. should add 20 sq. ft. of rain garden

How to Plan and Install a Rain Garden

  1. Before choosing the location of your rain garden, observe how rainwater is distributed across your home and yard. The ideal rain garden location is a flat or gently sloped area and is down slope from a runoff source.
  2. Site your garden at least 5 feet away from any structures (unless an impermeable barrier is used) and 5 feet from property lines.
  3. Avoid siting your garden over underground utilities and septic systems, near large trees, or next to a creek, stream or other water body.
  4. Your rain garden will overflow in large storms. Therefore, all garden designs should include an overflow system. One option is to build the perimeter of the garden so that it is perfectly level and to allow water to gently spill over the top during large storms. Another option is to build in a spillway that connects to another landscaped area, or the storm drain system.

Plan the Size of Your Rain Garden

  1. Once you have determined where your garden will be sited, look at the surrounding area and identify which surfaces will contribute runoff to the garden. Is it all or just a part of the roof, patio, or driveway?
  2. Estimate the roof area by measuring the length and width of the building foundation and adding a few inches for the overhang. Multiply the length times the width to determine the contributing area. Once you have calculated the area of each contributing surface, add them up to obtain the total contributing area.
  3. Refer to the chart on page 1 to identify the size of the rain garden you will need to manage runoff from the contributing area.

If you do not have the space, budget, or interest in building a garden of this size, you may consider capturing some of your roof runoff in rain barrels to reduce the amount of runoff, or discharge the overflow to another landscaped area.

Planting Your Rain Garden

my rain garden layout

You can design your rain garden to be as beautiful as any other type of garden. Select plants that are appropriate for your location and the extremes of living in a rain garden

Site Considerations:

  • How much light will your garden receive?
  • Is your property near the coast or located in an inland area (this affects sun and temperature)?
  • Are there high winds near your home?

Recommended plant characteristics:

  • Native plants adapted to local soil and climate,
  • Drought tolerant,
  • Flood tolerant,
  • Not invasive weedy plants,
  • Non-aggressive root systems to avoid damaging water pipes,
  • Attracts birds and beneficial insects.

Design Checklist

When installing a rain garden, the following design considerations are recommended.

  1. Locate the rain garden at least 5 feet from home foundation, 3 feet from public sidewalks, and 5 feet from private property lines. If rain gardens need to be located closer to buildings and infrastructure, use an impermeable barrier.
  2. Locate the rain garden to intercept and collect runoff from a roof downspout or adjacent impervious area.
  3. Size the rain garden appropriately based on the soil type and drainage area (see Page 1).
  4. Do not locate the rain garden over septic systems or shallow utilities. Locate utilities before digging by calling Underground Service Alert at 811 or (800) 227-2600.
  5. Locate the rain garden on a relatively flat area, away from steep slopes. If you plan on moving a large quantity of soil, you may need a grading permit. Contact your local municipality for further assistance.
  1. Consider installing an underdrain to enhance infiltration in very clayey soils. Contact municipal staff for guidance on how to properly install an underdrain.
  2. An overflow should been incorporated in the rain garden to move water that does not infiltrate to another pervious area and away from the home’s foundation or neighboring property.
  3. Drought and flood resistant native plants are highly recommended and a variety of species should be planted. Avoid invasive plants. visit www.centralcoastlidi.org for a list of plants appropriate for rain gardens from the applicable countywide stormwater guidance.
  4. Avoid invasive plants. A list of invasive species may be found at the California Invasive Plant Council website (www.cal-ipc.org).

Additional Resources to Explore:

-       Slow It Sink It Spread It, A Homeowner’s Guide to Greening Stormwater Run-off www.rcdsantacruz.org/publications

-       Monterey Bay Friendly Landscaping Rain Garden Design Templates www.green-gardener.org

Maintenance Considerations

Once a rain garden is installed, the following steps will help the garden function effectively.

  1. Rain gardens should be irrigated periodically (as needed) during dry months, especially while plants are being established. Plants should be inspected for health and weeds should be removed as often as necessary.
  2. Maintain about 2 inches of mulch in the rain garden and replace as needed. Mulch with a material that will not float away such as compost or finally screened soil amendment made from recycled green waste. Wood chips can be used to mulch plants located above the maximum water line an adjacent to the rain garden.
  3. Areas of erosion should be repaired. Further erosion can be prevented by stabilizing the eroding soil with ground cover or using energy dispersion techniques (e.g., splashblock or cobbles) below downspouts.
  4. Avoid using synthetic fertilizers or herbicides in your rain garden because these chemicals are water pollutants.
  1. Standing water should not remain in a rain garden for more than 3 days. Extended periods of flooding will not only kill vegetation, but may result in the breeding of mosquitos or other vectors.