Rain gardens to capture stormwater runoff
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What is a rain garden?
A rain garden is a planted depression (low area) that allows rainwater runoff from impervious surfaces (like roofs, driveways, walkways and parking lots) to soak in.
Rain gardens are beneficial because they:
- Remove pollutants from water before it enters surface waters.
- Prevent erosion by holding soil in place with their deep roots.
- Attract birds and butterflies.
- Require little watering and maintenance once established.
The benefits of rain gardens
Whether you live in the city or along a lake or river, managing stormwater runoff is important. Rooftops, roads, driveways and sidewalks are hard surfaces that prevent rainwater and melting snow from reaching the soil and soaking into the ground. These hard surfaces also tend to collect nutrient-rich yard and pet debris, oil and radiator fluid from autos and other debris and pollutants.
During a rain event or snow melt, fast-moving runoff:
- Washes debris and pollutants away, often directly into lakes, rivers and wetlands.
- Erodes soil and carrys it into our surface waters.
- Can affect aquatic life if warm runoff enters lakes and rivers directly. In summer, runoff is often warmed as it flows over hard surfaces.
A rain garden is one way to address all of these problems. Rain gardens collect stormwater runoff and prevent it from flowing directly into lakes, rivers and wetlands.
Because rain gardens allow runoff to soak into the soil:
- Pollutants are filtered out before entering the groundwater.
- Sediments settle and plants absorb nutrients.
Design and placement
Rain garden designs can be simple or elaborate, depending on your gardening interest and experience. Before you start digging, it's best to sketch a design. You'll need to consider:
- Placement of the garden.
- Size of you need.
- Shape you want.
- Type of soil you have.
- Plants you’d like to include.
- You may need more than one rain garden to accommodate the runoff.
Step 1: placement of the garden
First, determine areas of your property that are suitable for a rain garden. These will generally be low areas that are the recommended distance away from other features.
Rain gardens should be:
- 10 feet (or more) away from buildings to prevent foundations and basements from being damaged by water.
- 35 feet (or more) from septic system drain fields.
- 50 feet (or more) from drinking water wells and well away from utility lines.
Call the Digger’s Hotline (800) 242-8511 to locate electrical, gas or telephone lines.
Step 2: test the soil
Next, test the soil in areas that are both suitable and near the sources of runoff. The kind of soil where you create your rain garden is very important. The soil needs to be porous enough to soak up water within 48 hours to prevent plants from drowning and mosquitos from breeding. This is also the standard shortest period between two rainstorms.
To simply test a soil’s ability to absorb water:
- Dig a wide hole 10 inches deep and fill it with water.
- If the water disappears within 48 hours, the site is suitable for a rain garden.
- If your first site fails the 48-hour test, test the soil at other potential rain garden sites on your property.
Step 3: shape and size
Rain gardens can be designed in any shape. Crescent or kidney shapes are attractive. A long, narrow rain garden may be better if you're placing it between structures, such as a house and sidewalk.
The size of your rain garden will depend upon the size of the roof, driveway or other hard surface being drained.
- Typical rain gardens range from 100 to 300 square feet in size.
- Gardens will handle the runoff from a hard surface that is about three times their size.
- For larger surfaces, more than one rain garden may be needed to handle the runoff. For example, large roof tops may need a rain garden near each down spout.
Step 4: selecting plants
Choose plants that:
- Are appropriate for the soil type in your rain garden.
- Will also tolerate standing water for up to 48 hours.
- Many native plant species are well suited for rain gardens.
If you are constructing a rain garden near a lakeshore or riverbank, you may be required to use native plants, depending upon local ordinances. Check with your local Soil and Water Conservation District.
Constructing and planting
Construction can begin once garden size, shape, location and plants have been decided.
- Lay out a rope or hose in the desired shape to use as a guide for digging.
- The depth may vary from 4 to 10 inches.
- For best infiltration the bottom of the rain garden should be level.
- If your garden is placed on a slope, use the soil from digging to create a berm on the downhill side of the rain garden.
- Excess soil should be removed from the site.
Reviewed in 2018