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Designing Tattnall Square Park’s Rain Gardens

By Andrew Silver, Friends of Tattnall Square Park

RG1I’d never have imagined that people would love our park’s new rain gardens as much as they have, and I wouldn’t have imagined that they’d have looked so good after so little time. Still, when Friends of Tattnall Square Park first teamed with Mercer engineering students to design a rain garden, we had no idea that the road to success would take months of planning, changes, revisions, and tweaking. All we knew is that we had an oversized 60-plus car parking lot, a tiny inlet into the park, and lots of erosion and storm water eventually heading down the sewer at the low end of our park, sweeping sediment and pollutants along with it. The rain gardens have become some of the most popular sites in the park, but in order to spare you some of our steep learning curve, here are some rain garden tips that I wish would have been emphasized more in the sources we consulted.

  1. “Any drop of water saved is a good thing.” Helen Kraus, co-author of Rain Gardening in the South, a great rain garden source, gave us this great bit of advice. The engineering students who advised us originally proposed a massive 2,000 square foot garden that they designed to hold every drop of water in two year, two hour storms. But when our volunteer gardeners set eyes on the proposed gardens, they said, “there’s no way we can pay for the plants and no way to maintain the gardens if we planted them.” So I asked Dr. Kraus if we could cut the design by a third and still do some good. “As long as your garden isn’t too small—a 200 foot garden for your parking lot, for instance,” she told me, “any drop of water saved is a good thing.” We cut one of three gardens, cut our budget, and created a sustainable beautiful garden.
  2. Shore up your inlets and outlets. Storm water submits rain gardens to massive amounts of erosive pressure, particularly at the inlets and outlets of the gardens. Make sure that these are properly bulked up with river rock and reinforced before the first storm sweeps them away. Go wider in the application of the rocks than you think necessary.
  3. Protect your rain garden from storm water. . . for a month or two. I know this sounds counter-intuitive, but given the erosion potential of a serious storm, minimizing storm water flow into your garden for its first season will keep the sod and plants from getting blown out in storms. We didn’t know this and paid the price for our eagerness when a storm hit before we planted and blew out a side of our berm. If it’s possible to redirect the storm water or temporarily stop its flow, do it.
  4. Get your berms right before the flooding comes. First off, you’ve absolutely got to have a 3:1 ratio on your berm slopes or you will find yourself, as we did, watching a storm blow out an entire chunk of a poorly sloped berm. Second, be sure to lay sod down on your berms as soon as possible, or you’ll watch them, as we did a week after the berm blow out, melt away in a hard rain.
  5. Don’t bother with automatic irrigation. We had the choice of spending thousands of dollars on automatic irrigation for our rain gardens, and we chose not to for budget reasons. And that was a good thing. Because, as it turns out, our two rain gardens have vastly different drain times and, more importantly, the rain gardens themselves have vastly different “zones.” The center of the rain garden is the “wet zone,” and it has fundamentally different water requirements than the “xeric zone,” the high and dry outer ring that covers the berms. In between these two is a kind of moderately wet zone that has its own special needs. Unless you’ve got the money to purchase a multi-zone automatic irrigation, hand watering with sprinklers gives the greatest control over properly watering plants. Just have a hose bib nearby and get your hoses ready.
  6. Know your nursery lists before you begin designing. We made the mistake of theoretically designing before we had the chance to know the holdings of our local nurseries. So when we heavily designed with cardinal flower and then found no economical source for cardinal flowers, we had to go back to the drawing board. It was far more helpful to find a great nursery or two and then begin choosing, especially since delivery costs can add a great deal to your budget. Also: if possible, observe your rain garden once built before purchasing plants. One of our rain gardens drains in a matter of a few hours, while the other nearby takes at least 48 hours to drain. We had no idea of the disparity, and so we had to redesign completely for different ecosystems.
  7. A deeper soil sample is a better soil sample. The textbook way to test for soil percolation rates is to use a dual ring infiltrometer test. But that test only gauges the relative porosity of the soil in the first few inches of your bed. In the case of one of our gardens, about a foot and a half of porous, sandy soil sat on top of a shelf of hard-packed impassable Georgia clay. Our rain garden works, but it’s a slower draining garden than our tests predicted. Had we taken a deeper core sample of more than 24 inches, we’d have seen the clay and planned accordingly.
  8. Have volunteers do the planting. We saved thousands of dollars by getting about thirty neighbors and supporters to plant 1,600 square feet of rain gardens. They did a great job, and they saved us money, but most importantly, each volunteer now has a strong feeling of ownership over the gardens they helped plant. And you can’t beat photographs of children planting flowers.

We were all novices at rain garden design and preparation, but we managed to create two beautiful, functional gardens that have harvested storm water, attracted wildlife, and wowed a great many park-goers. In short order the gardens have become the most photographed part of the park. But it would have made our jobs a whole lot easier had we known some of the tips above before we got started on our work.

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  1. […] Designing Tattnall Square Park’s Rain Gardens | Andrew Silver | City Parks Blog […]

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