Constructing a Small Rain Garden

Constructing a Small Rain Garden

By Pat Vance

The Philadelphia Water Department (PWD) has made a strong commitment to controlling erosion and excess run-off in the city, including some imaginative and innovative large-scale projects. I found an inexpensive, simple PWD project I could do in my own yard with just a shovel!

The house I moved into a few years ago had a half-sized basketball court in the backyard. I briefly considered removing it until I calculated the expense and work involved. Then I decided to

make the best of it. However, after a couple of heavy rains, I realized I would have to do something about the rain water run-off.

I consulted a web site from the PWD with guidelines for building a rain garden. 

Here is the url:

http://www.phillywatersheds.org/whats_in_it_for_you/residents/how-build-rain-garden

A rain garden is a swale or depression surrounded by a berm, positioned where rain run-off can be temporarily held until it can infiltrate into the ground.  Rain from impervious surfaces such as roofs, parking lots, or the occasional basketball court can be diverted into the garden rather than causing erosion or ending up in the storm sewer. The water is held only temporarily. A rain garden is not a pond or water feature. The water will drain into the ground over a fairly short time.

There is a link on the PWD site for a PDF with guidelines for building a garden. It lists some guidelines for the selection of a good site:

1. The garden should be at least 10 feet from building foundations. This guideline is the most important to follow, even if it means that rain garden may fall short in some of the other considerations.

2. Position the garden to accommodate most, if not all of the water draining from the surface. In my case, if I built my garden along the entire edge of the court, I would place it too close to the foundation of my house. So I restricted the size accordingly.

3. The garden should be at least 20% of the surface area of the drainage area. The area of the space I had available was just slightly over 20%, but even if I had not been able to build a garden quite so large, I figured that whatever I did would be helpful.

4. Drainage should be adequate to allow infiltration of rain water collected. The PWD web site also includes a simple way to test this. Here's the URL:

http://www.phillywatersheds.org/whats_in_it_for_you/residents/infiltration-test

Remove the top and bottom of a coffee can and then push or hammer the can a couple of inches into the ground. Measure the side of the can above the ground and fill the can with water. Start a timer and one inch per hour, the site is adequate.

calculate the time it takes for the water to drain from the can. Repeat this test a few times to assure accuracy. If the drainage rate is at least

Now some guidelines for shaping the garden.

1. The depth of the swale should be 6-8 inches.

2. The height of the berm should be at least 4-6 inches.

3. The height of the berm above grade, should be no more than one-third the width of the berm. My berm was 3 or 4 inches high, so I made sure it was at least a foot wide.

To construct the garden, I placed a row of stakes along the court, and another row on the opposite side of the garden. I ran string between the rows of stakes and adjusted it to be level with the court. Then I measured the stakes at the outside of the garden for the height of the berm

A note of caution: Be sure it is safe to dig! Contact Pennsylvania One Call system by dialing 811 to be sure you are not disrupting utility lines! Hitting one would be extremely dangerous!

As I dug out the swale, I used a ruler to measure down from the string. As I removed the dirt from the swale, I placed it on the berms.

When I had the garden shaped correctly, I used a hose at full force to watch where the water flowed

from all angles of the court.

The final consideration: Which plants to use? All the usual considerations for planting a garden apply with one notable addition: the soil will be very wet at times, so be sure to choose plants that can tolerate that. The PWD pamphlet has a list of some possible native plants, but I decided to go with what I had available.

A friend was splitting a large patch of Japanese Iris, a plant that can tolerate high moisture. Another friend was splitting Athyrium or Lady Fern and I used those as well.

And the results? A resounding success! This inexpensive, simple project has drastically reduced the run-off from the court, and I have a garden that's lovely to look at.