Rain Gardens
First published October 11, 2007
Alpine gardens, English country gardens, Japanese gardens, xeriscape gardens, native gardens…the variety of themed gardens out there is diverse, to say the least. And although many of them may not suit your property or personal taste, there is one that I think worthy of trialing in most yards: The Rain Garden.
Now, if you’ve never even heard the term rain garden, you’re not alone. It’s a relatively new concept that transcends the aesthetic or edible world of plants, and enters the domain of gardens grown to mitigate the effects of pollutants. Yes, I know; that sounds rather complicated, but in actuality, a rain garden is as simple as can be—a garden strategically placed to intercept and collect water that runs off roofs, driveways and yards, in essence, allowing it to infiltrate the soil rather than to run off into storm sewers. That’s not where the good news stops. Because this type of garden acts as both a natural biofiltration system and as a temporary water reservoir, it’s able to significantly reduce the amount of contaminated storm water that has direct access to our rivers, streams and lakes. On the flipside, what rain gardens are not, are ponds. Although both are designed to collect and hold water, a rain garden is designed to collect and hold water for only about 6 hours after a storm.
Location, location, location
Not surprisingly, a rain garden is typically placed where it will collect the most amount of runoff. For example, it can be located on the downhill side of a paved surface to reduce the amount of water that enters the sewer system or positioned near the discharge end of your downspout. But the most important thing to remember is not to locate a rain garden closer than three metres to your home. Basements make lousy bio filters, and I can guarantee that you won’t give a damn about the environment when you are pumping water out of your house. On that note, rain gardens built in spots too low to drain will also become poorly designed water ponds.
One size fits all
When planning the garden’s construction, the first principle to remember is that any sized rain garden is better than no rain garden at all. However, if you want to take a scientific approach to minimize rooftop runoff, start by calculating the size of your roof. For example, let’s say that your roof is 186 square metres and that rainwater runs to each corner equally. That means that each of the four down spouts drains about 46 square metres of roof. Next (stay with me now!), divide the 46 square metres by 6 (because someone somewhere discovered that was the magic number) to determine the optimum size of garden you need, which in this case is about 8 square metres.
The construction is relatively simple. Choose a location that’s not on too steep of a slope (this will eliminate erosion), dig the garden to a depth of 15 cm and ensure that you have well-drained soil—the sandier, the better. A good way to test your soil for drainage is to dig a 15-cm deep trial ‘pit’ and to fill it with water. If it drains within 24 hours, you’re set. If water remains after 24 hours, either replace the soil or amend it with a mixture of coarse sand and loam. The only step that’s left after that is to plant the entire rain garden (not just the periphery) with suitable plants. The good thing is that there are plenty of excellent choices. For example, astilbe, iris and ferns thrive in rain gardens, as do dogwoods and nannyberries. In fact, the list of suitable plants is much larger than the list of unsuitable ones.
It really is a have-your-cake-and-eat-it-too scenario. Not only are rain gardens beautiful, they also help to keep our lakes, rivers and creeks cleaner. Hmm…less polluted water that enters our water treatment plants…perhaps there could be a tax savings for those who have properly designed rain gardens. Just a thought during an election year.
