March is a month of change – from winter to spring, from cool temperatures to warmer, from frozen soil to early mud season. So far, winter 2024 has brought strong contrasts and variability, warmth and abrupt temperature changes, as well as precipitation falling as rain, snow, sleet, or wintry mix. This month we are working outdoors as conditions permit. Garden planning and plant orders are still in progress here and for gardeners in the community who are sharing their plans and questions.

Rain gardens are an excellent way to incorporate native plants in your landscape and realize the benefits they provide. When heavy and frequent rains fall, a rain garden captures the water on site. During the growing season, water infiltrates into the soil because of the deep root systems of perennial native plants. When the plants are growing actively (spring through late summer) they also take up the collected water. As a result, water does not stand in the garden for more than a day or two. Ideally, a rain garden is planted with and managed to support a diversity of native plants. Those provide habitat and season-long resources for pollinators, songbirds, mammals, and others. Even during severe drought rain garden plants survive, though they often don’t grow as tall as usual. Some species may be suppressed during dry years and more drought-tolerant species can expand into the open spaces, and the garden still provides habitat.

What is a rain garden? It is a basin or low-lying area where water collects after rain. Direct downspouts away from foundations and toward these areas, which can be excavated and graded to create a desired garden space. Because even a shallow basin has a range of wetter and drier areas, plants with different moisture requirements can be used. For example, in both of our rain gardens here at the Arboretum, blue joint grass (Calamagrostis canadensis) grows in the lowest and intermediate zones. Other species thriving in the lowest areas of the basin include wild iris (Iris virginica), red milkweed (Asclepias incarnata), turtlehead (Chelone glabra), cardinal flower (Lobelia cardinalis), and great blue lobelia (Lobelia siphilitica).

In the mid- to upper zones of the basin, in full sun, use plants suited to mesic soil conditions. Examples include Joe-pye weed (Eutrochium maculatum), New England aster (Symphyiotrichum novae-angliae), bottle gentian (Gentiana andrewsii), Culver’s root (Veronicastrum virginicum), and mountain mint (Pycnanthemum virginianum).

Our large rain garden is 30 meters in diameter. It receives rainwater from about half of the north side of the Visitor Center roof and holds about 100 species. In both rain gardens, we leave the previous year’s plants in place over winter. Usually in late March or early April our professional crew uses prescribed fire to remove standing plant material and stimulate spring growth. The blackened soil has more available nutrients and absorbs heat more than in unburned areas.

The Friends of the Arboretum online sale offers a rain garden mix for sunny sites as well as other suitable plants for wetter areas. Buttonbush (Cephalanthus occidentalis) is a beautiful shrub for low-lying areas. It can reach 12 feet in height and width, blooms in July, and attracts many bees and butterflies.

Planting a rain garden will provide flowering resources, habitat, and infiltration. If rain gardens are installed throughout the community, they can help protect downstream waters from sediment and excess nutrients.

—Susan Carpenter, native plant garden curator