Thursday, Sep 27, 2012

Suck it up
A rain garden absorbs water runoff on your property
By MARK SALERNO, CMHC, Special to QMI Agency

Reducing stormwater runoff from your roof, driveway and other hard surfaces in your yard can ease the strain on municipal sewer systems and prevent harmful pollution from entering lakes and streams.

An attractive and effective way to reduce runoff is to install a rain garden – planted or stone-covered beds specifically designed to receive stormwater and allow it to be absorbed into the soil.

Rain gardens are relatively easy and inexpensive to design and build, but there are a few things you need to consider. The two most critical technical considerations: the water must drain into the soil and not stand in the bed for more than two days; and the garden should not create drainage problems on your property or your neighbour’s.

Placing your rain garden at a low point on your property or somewhere along an existing drainage path is ideal. It’s also best to locate it in a relatively flat area, with slopes of less than 12%. Be sure to place it at least four metres (13 ft.) away from your house foundation, septic beds or neighbouring homes to ensure that you direct stormwater away from vulnerable areas.

Rain gardens do not work well on clay soil because drainage in clay is slow. If your soil has a higher clay percentage, you will need to add sand, fine gravel and/or organic matter to improve permeability.

Often, rain gardens are designed to capture roof runoff via a downspout extension, a swale or an underground pipe. Your downspout extension should be inserted right into the rain garden, but, to prevent erosion, place a small bed of pea gravel, decorative pebbles or a concrete splash pad under the extension. If possible, you can also have the downspout discharge onto a lawn area that drains into the rain garden.

Deciding on the size and shape of your rain garden will be your next step in the process. It will function best if you make a reasonably accurate estimate of the amount of water that will flow into it from your roof and lawn, and how quickly the water will be absorbed by the soil in your rain garden.

Canada Mortgage and Housing Corporation’s (CMHC) fact sheet "Rain Gardens: Improve Stormwater Management in Your Yard" provides a method for calculating the best size for your individual property as well as instructions on how to form and install your rain garden. Essentially, to capture as much stormwater as possible, a rain garden should be at least one-and-a-half times longer than it is wide.

Best plants

The best plants for rain gardens include perennials, shrubs, grasses or ferns that can tolerate both wet and dry conditions and are adapted to your climate and your specific soil and sunlight conditions. You might also choose to cover the rain garden with loose, hard materials, such as pea gravel or river stone.

A rain garden can provide an eco-friendly habitat for birds, butterflies and other fauna and create a pleasant oasis in your yard. CMHC has plenty of information to help you with your landscaping projects. Visit us online or call our toll-free number: 1-800-668-2642.

Mark Salerno is district manager for the Greater Toronto Area at the Canada Mortgage and Housing Corporation. You can reach him at 416-218-3479 or

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