Slecting the Best Site for the Rain Garden: Slope and Soil

Before you get started on building your rain garden, you want to know the answer to two basic questions: "What's my slope?" and "What kind of soil do I have?".  This article will help you get to the bottom of these two questions.

What’s My Slope?

 To calculate the slope of the intended rain garden, pound one stake uphill of the site and the other at the downhill end approximately 5 m (15 feet) apart. Tie the string to the stakes and make sure it is horizontal with a carpenter’s levels. Once the string is properly secured and level, measure the length of the string between the two stakes and then the height of the string from the ground at the downhill stake.   The percent slope will be equal to:

Percent Slope = Height x 100

                         Length

Slope <4%, easiest to build 8-13 cm (3-5 inch) deep rain garden.

Slope 5-7%, try to build it 15-18 cm (6-7 inches) deep.

Slope 8-12%, build it 20 cm (8 inches) deep.

If the slope is greater than 12%, it would be easier to install a garden elsewhere.

How Do I Know What Kind of Soil I have?

Before you even think of putting a shovel in the ground, you also need to figure out what type of soil your rain garden site has. Sandy soils have the fastest infiltration; clay the slowest. Sandy soils are described as gritty and coarse, silty as smooth but not sticky, and Clay as sticky and clumpy. Clay garden must be bigger to handle infiltration and should have sandy pits installed in them to help infiltration. Here is an easy test to help you see if your rain garden will work well at your selected spot:

Soil test: Dig a hole 15 cm (6 inches) deep and fill with water and allow it to filter away. If it takes more than 24 hours to drain, the soil is more clay-based and is not good for a rain garden. You can mitigate the lack of drainage by adding sandy soil pockets during installation, but that takes time and money.