How to Make a Rain Garden

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[JODI SULPIZIO] Rain gardens are shallow landscape features designed to capture storm water from downspouts, driveways, and other surfaces.

By installing a rain garden, you can reduce runoff by capturing storm water and allowing it to absorb back into the ground.

Let's find out how to make one.

A successful rain garden installation starts with a good plan.

Careful site selection, design, and installation are needed to create an attractive garden that blends with the existing landscape and avoids creating new problems.

As you begin planning a rain garden, check with your municipality about storm water rules and plant height ordinances that may impact your design options.

The rain garden should be designed to capture runoff from a roof, driveway, sidewalk, road, parking lot, or a compacted lawn.

The garden here captures runoff from a downspout that otherwise would have flowed onto a driveway.

To determine possible rain garden locations, walk the property in the rain to observe storm water flow.

Look for low, flat areas that collect storm water but are not permanently waterlogged.

Knowing your soil type is critically important.

Rain will soak into a sandy soil at a quick rate of 2.5 inches per hour, while rain soaks into a silty soil at a rate of one-half inch per hour.

And in clay soil, the rate is only a third of an inch per hour.

Good drainage is required.

A percolation test can be done to ensure adequate drainage.

Dig several one-foot deep by one-foot wide holes in the spot where you would like to install your rain garden.

Fill the holes with water and monitor the time it takes for the water to drain.

If it takes more than 24 hours for the water to drain, you should consider another site.

For example, the fresh storm water in this newly installed rain garden will drain within 24 hours.

Rain gardens should be at least 10 feet from a building's foundation to avoid creating basement water issues.

Do not construct a rain garden near a septic tank, leach field, drinking water well, tree roots, waterlogged areas, or utilities.

If the area is not flat, measure the slope.

Steep slopes greater than 10% are not ideal sites for a rain garden.

Parts of the rain garden include an inlet, often lined with rock to slow the flow of incoming storm water; a border created by a low berm with gradual slopes to create a shallow basin bottom to hold water; and an erosion-resistant outlet for overflow, often lined with rock.

Rain garden size depends upon the estimated volume of water to be captured.

The rain garden's size should be 10 to 20% of the impervious surface area draining into the garden, but this area can vary greatly based on the infiltration rate of the soil and the depth of the garden.

Let's figure out how big our rain garden needs to be.

First, we calculate the roof's surface area.

The roof is 50 feet long and 12.5 feet wide.

Multiplying those together gives us 625 square feet.

Now, how much rain water will that roof collect during a one-inch storm?

We multiply 625 square feet by 0.083, since one inch of rain equals 0.083 feet, and get 52 cubic feet of storm water.

Next, let's size the rain garden.

If the garden has a six-inch ponding depth, which is 0.5 feet, each square foot of the garden can hold 0.5 feet of water.

So we take 52 cubic feet of storm water and divide by 0.5 feet, which gives us 104 square feet.

That's the size our garden needs to be.

Before finalizing, check that the size works for your site.

If not, you can adjust the ponding depth, garden shape, or location to make it fit.

Before you dig, always call Pennsylvania 811, PA One Call.

The service sends someone out to mark all underground utilities, preventing costly and potentially fatal accidents.

This service is free to homeowners.

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After you dig, observe the new area after a rain.

Note the areas that hold water the longest.

Moisture-loving plants go there.

Choose native plants that match the site condition, soil type, and sun exposure.

Native plants provide important food and habitat for wildlife, especially insects and birds.

Consider installing a rain garden on your property or in your community today.

Contact your local extension office to speak with a water resources team member.

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