Though the purpose and the function of a rain garden are simple, there are a number of requirements that need to be fulfilled in order to ensure that it functions well. Time spent in good planning and design will result in a building process that goes smoothly and successfully.

Basic Site Requirements

• A rain garden should be reasonably close to and slightly downhill of the runoff that it will receive, so that water can freely flow towards it. Note the location of your roof downspouts and the approximate slope of your lawn, driveway, or garden beds.

• It should be located at least 10 feet away from your home, so that even in a "100-year" storm, it cannot saturate the surrounding soil enough to reach your foundation. It will need to have an overflow outlet that is directed away from your home and neighboring properties.

• A rain garden should not be built near a well, septic tank, or steep slope, over underground utilities, or in an area where standing water accumulates during typical rains.

Soil Drainage Requirements

All types of soil - whether it contains sand, silt, or clay- can be utilized for rain gardens, but it's very important to know how well the incoming water will percolate (drain) from the site before doing any major excavation. Here's how to test your soil's infiltration rate:

1. Dig a hole 12 inches deep. A post-hole digger or narrow shovel works well for this.

2. Use a permanent marker to make inch-marks on a small garden stake and then place it firmly in the hole, buried up to the lowest mark.

3. Use a garden hose to slowly saturate the ground around the hole, and then fill the hole with water up to the top inch-mark. Make a note of the time.

4. Monitor the hole once every two hours, and time how long it takes for the water to drain. Make a note of how long it takes for complete drainage. Divide the water depth by the hours it takes to drain. That number is your infiltration rate. (IE: 12 inches divided by 8 hrs = 1.5)

5. If it's the rainy season, you will only need to do the test once. If it is the dry season, do the test 3 times and then average the results, in order to approximate winter drainage conditions.

If your infiltration rate is 1.0 or more, you have excellent drainage. If the rate is between 0.5- 0.9, the soil is well-draining enough for a rain garden. If it is less than .5, then the soil is poor-draining and may hold water for extended periods during the rainy season, but the site may still be able to be used. If the rate is less than .01, then consider a different location for the rain garden.

Calculating the Square Footage and Depth Needed

If your soil drains well enough for a rain garden, the next step is to calculate the square footage and depth needed.

1. Estimate the square footage of the impermeable surfaces that you would like to drain into the rain garden. For example, if your home is approx. 30' x 50' and ½ of your downspouts are to be directed into the rain garden, then 30 x 50= 1500, and 1500 divided by 2 = 750 sq. ft.

2. Estimate the space that is available on your property for a rain garden. You can create more than one rain garden if need be, but you will need to do an infiltration test for each site.

3. In soils with an infiltration rate of 1.0 or more, you will only need to excavate 9 inches deep.

4. In soils with an infiltration rate of .5-.9, the rain garden should be 18 inches deep (12 inches of soil mixture and 6 inches of ponding depth). Its square footage should be 10% as large as the impermeable surface area that drains into it. For example, a 75 sq. ft. rain garden for 750 sq ft of impermeable surface area.

5. In soils that are poor draining, an 18-inch deep rain garden must be 20% or more of the square footage of the impermeable surface area. If the rain garden's depth is increased to 30 inches, it can be reduced to 15% of the square footage.

Determining the Slope of Your Site

The degree of slope at the site you have chosen for your rain garden will determine how it is built, its overall shape, and how runoff is routed into it. To calculate the slope, you will need 2 stakes, a length of string, and a line level (under $5.00 at most hardware stores).

1. Drive a stake into the soil at the highest and the lowest points of the site.

2. Tie the string tightly to the base of the stake at the highest point, and loosely to the stake at the lowest point. Attach the line level to the string, raise or lower the string on the low stake until the bubble is at its center, and attach the string firmly to the low stake at that level point.

3. Measure the length of the string from stake to stake (width) and from the string to the ground at the downhill stake (height). Divide the height by the width, using the same measuring increments. Then multiply by 100 to get the slope percentage. For example, a 12-in. height divided by 200-in. length = 0.06 x 100 = 6% slope.

Determining the Shape of the Rain Garden

A rain garden can be shaped to fit the site, and can mimic your existing landscape beds, resemble a natural pond, or reflect a personal preference. If the rain garden is to be located on a slope of more than 5%, it will function best if it is approx. twice as long as it is wide, with the long side parallel to the slope.

It's helpful to experiment with the rain garden design and layout before you do any excavating. Use an old garden hose, stakes and string, or a stout rope to try out different shapes, then use utility-marking spray paint to mark your preferred choice on the soil surface. Before making your final decision, remember to ensure that water will easily flow towards the rain garden from the downspout, driveway, or other impermeable surface that is to be directed into it.