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.
