#63 Designing a Rain Garden PDF  | Print |  E-mail
Written by Mitzi VanSant   
Monday, 10 May 2010 19:13
In my last column I went into detail about planning and constructing a rainwater harvesting system. They can be costly, especially those with larger collection vessels. Another les expensive method is to capture and collect it in a rainwater garden, allowing it to soak into the ground and replenish groundwater and aquifers below. Some of the stored water will also be used by the plants within the garden, and returned to water vapor in the air. This is much more advantageous than allowing it to run off into storm drains, then storm sewers, and eventually creeks and rivers.

Preventing water runoff is important in many ways. Water rushing off your property and into the street will pick up pollutants from autos that eventually end up in our waterways. Rainwater running off your cultivated lawns and gardens may carry fertilizers and pesticides into the same bodies of water. A much better option is to slow runoff, allowing it to percolate through the soil where soil bacteria can break it down before it enters water stores below ground.

You will first need to select a site for the garden. Note the existing runoff patterns to see where water collects naturally. Anywhere down slope and at least 10 feet away from the house foundation will work. Having it somewhat near the downspouts is also helpful. It cannot be over a septic drain field, and you should avoid areas where utilities are buried underground as they may be disturbed by digging. You’ll also want to make sure that any existing trees in the area can adjust to the extra water, and be careful when excavating that you don’t damage their roots.

Once you have chosen the site, you need to evaluate the existing soil for its ability to drain. You can dig a hole and fill with water; if it drains at less than one inch per hour you will need to break up the soil and add compost. For aesthetic/design reasons, try to find a way to connect the area to an existing flower bed, rather than having it as an unrelated “island” bed in the landscape.

Shape the low area so that its deepest point is about 6-8 inches, and then contour it out to the edges. If the general slope of the land is downhill, you may need to create a small berm at the lower end to prevent water from escaping. Should the soil need improvement to drain, dig a few inches deeper, add compost to fill to that previously determined grade, and rototill or shovel it into the existing soil.

The connecting grade from the downspout to the rain garden can be managed in several ways. It may be as simple as a small swale in the lawn, with the water from the downspout being slowed by a rain chain, diverter or splash guard. Another method would be to create a small dry creek bed that could carry the water from the downspout to the garden area.

If the rain garden is large enough, mixing trees, shrubs, grasses, and perennials will give the best effect. If it is small, grasses and perennials can also be beautiful and effective. Some of our native grasses have very deep roots and assist water in soaking into the soil. Once completed, the idea is to have the area hold water for perhaps a few days, but drain before standing water can produce mosquito larvae (3-7 days).

Now we come to the really fun part, choosing plants for the garden. Native plants work best, as they are adapted to extremes of drought and heavy rainfall. They also don’t require fertilizer and pesticides, which would negatively affect water quality as a result of application. In my next column I will provide suggestions for planting a Central Texas rain garden, with an emphasis on plants from the Blackland Prairie and Post Oak Savannah ecosystems.
 

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