A rain garden is a landscaped swale designed to hold and capture storm water until it can percolate into the soil. Rain gardens have become extremely popular, if not required, in cities such as the District as a way of reducing the runoff caused by acres of roofs and paved surfaces.
This runoff harms the environment by washing silt and pollutants into waterways and causing combined storm sewers to discharge untreated sewage into rivers — in our case, the Anacostia and the Potomac.
There is no doubt that rain gardens are a good thing, but from a gardener’s perspective, many of them fall flat. Gardens require maintenance, and too few rain gardens get it, perhaps because they are not thought of as true gardens but something that is engineered, installed and utilitarian. Many start out with far too few plants. Plants that are there die off, and weeds move in.
If you want to see rain gardens that work horticulturally as well as hydrologically, make your way to Bartholdi Park on the southwest side of the Capitol, across Independence Avenue from the U.S. Botanic Garden conservatory. The garden is named for its most famous feature, the luxuriant fountain of sculpted figures, water jets and lamps designed by Frédéric Auguste Bartholdi, better known as the sculptor of the Statue of Liberty.
The fountain was restored several years ago, but the garden was in need of a major renovation itself, especially with the arrival across the street of the American Veterans Disabled for Life Memorial. Because the Botanic Garden has been a leader in a green landscape certification program called the Sustainable SITES Initiative, it was to be expected that the $3.2 million park renovation would adhere to the Botanic Garden’s own environmental principles. This includes the extensive reuse of existing concrete, flagstone and other materials, new permeable paving, habitat for wildlife and, most evidently, the lavish use of rain gardens. There are a dozen or so in and around the garden. About one-third of the planted area consists of these features, although the whole one-acre park is in essence a rain garden because all the storm water is now collected and kept on site.
Completed in the spring of 2017, the plantings became established this spring for the ultimate test — weeks of torrential rain that reversed a drought. Ray Mims, the botanic garden’s conservation specialist on the project, is confident that the rain gardens, which can hold 4,000 cubic feet of water, were able to handle the recent gully washers. “Even with all this rain, we have kept everything on-site,” he said, as he led me through the park.
The rain gardens are equipped with overflow drains. And while the water reached these to feed an ornamental sunken garden, not one of the rain gardens overflowed its perimeter.
Seeing the water accumulate here during the heaviest storms would leave the impression that rain gardens are meant for boggy plants, but that’s a misconception. The key is to use plants that can endure periodic inundation but will live in dry conditions to the point of tolerating drought once root systems are established. This is because a well-designed rain garden has a deep, free-draining soil profile with carefully controlled mixes of aggregate.
At Bartholdi Park, the hollowed-out land and the drains reveal the contours of the rain gardens but not their soil makeup. They are covered in shredded bark mulch, which knits together to prevent washout (pine nuggets would take a voyage). I remember rain gardens I’ve seen dressed with river stones. They look fresh at first but grim after a season or two. “They just get covered up in a short time” with detritus, said Bill McLaughlin, the botanic garden’s curator of plants, who worked with his colleagues and landscape architects at Andropogon Associates in the planting designs.
Each bed has its own combinations. I particularly like one on the east side of the fountain with several sweetbay magnolias, an attractive small tree or large shrub with bluish leaves and fragrant spring blossoms.
You associate some of the lower plantings with moist soil — the royal fern, turtlehead and creeping phlox, for example — but others you’d think would run a mile from flood. These include gaura, heucheras, switch grass and even winecup (Callirhoe involucrata), which I always thought of as a rock garden perennial. The key, McLaughlin said, is to find plants that ecologists call facultative, meaning they are adaptable to temporary extremes.
On the west side of the garden, a rain garden is planted in bold sweeps of hardy mallow, a variety named Robert Fleming that’s about to burst into huge scarlet flowers. Before the mallow stands the rush, juncus, which is so vigorous from the rain that it is beginning to lean. The rush, in turn, is bounded by a tall meadow perennial named hoary skullcap (Scutellaria incana) that is crowned with sprays of purple pea flowers. Why this isn’t used more is a mystery to me. “It really likes the heat,” Mims said. “It doesn’t like wet feet but will take some dampness.”
In a nearby bed, you see it paired with another perennial that deserves greater use, the Stokes aster, a blue flowering daisy represented here with the improved variety Peachie’s Pick.
In another bed, you find a truly handsome native oak, the swamp chestnut oak (Quercus michauxii). Prettier even than the chestnut oak, it’s a species that should be near the top of your list of shade trees, even if you don’t have a rain garden. Here, it anchors a bed of Joe Pye weed erupting into bloom.
You will have a sense that rain gardens can be much more than ditches for storm water. The monsoons of May and June proved the perfect test and in some respects may have been too ideal.
In a drought year, the mallow “may perform unacceptably, we’ll see,” McLaughlin said. And that’s the point. This is one series of rain gardens in town likely to get the continuing care and adjustment that it needs. The main thing I’ve learned about gardening over the years is the work doesn’t end when an area is planted; that’s when it begins.
The sudden collapse of clematis stems is typically caused by a fungal disease named clematis wilt. Prune out all affected stems, and the vine should return. Minimizing physical damage to stems in early spring will reduce the risk of wilt. After winter pruning, make sure stems are staked to reduce cracking from wind whipping.
— Adrian Higgins