When I was on vacation in August, I saw the most inspiring bioswale I have ever seen. Some people may ask, “What on earth is a bioswale?” The answer is simple. A bioswale is a more sophisticated and ecologically sound version of a drainage ditch.
Bioswales are designed to hold, channel and sometimes filter the runoff water that flows from hard surfaces, like parking lots or roads. Located adjacent to the hard surface, the sides of the bioswale slope down gently to a bottom that can be filled with rocks or specifically chosen plants. When runoff water drains into the swale, it slows down and much, if not all, of it is absorbed by the swale. Plants assist in the slowing/absorption process and their roots may also filter out pollutants. Capturing runoff in bioswales is an alternative to traditional storm drain systems, which generally channel untreated runoff into nearby water supplies.
I discovered the magnificent bioswale at Cornell Plantations, the botanical gardens of Cornell University, a place where horticulture, agriculture, ecology and biology come together every day. It came into being as part of an initiative that began several years ago when the University built the beautiful new Nevin Welcome Center for the Plantations. The building is a masterpiece of sustainable design, attaining the U.S. Green Building Council’s LEED–Leadership in Energy and Environmental Design–Gold Certification.
Construction of the Nevin building meant changes to the adjacent parking area. It follows that a sustainable building needs a sustainable setting and the new parking lot, with its abundant small trees and bioswale, is part of that setting. The swale undoubtedly functions well in its ecological role. It also succeeds beautifully as a garden. Most of us don’t need bioswales in our home landscapes, but many of us have low spots that fill with water after rainstorms. Those spots are perfect for some of the same plants and plant combinations used at Cornell.
Choosing bioswale plants means selecting species and cultivars suitable to your climate zone. Native plant species are perfect for this purpose, but hybrids and climate-appropriate non-natives work well too. Swales are subject to flooding and occasional drought conditions, so plants must be able to stand both extremes. The Cornell bioswale functions as part of a botanical garden, so its plants also have to look good year-round. To achieve this, the designers chose species and varieties that feature combinations of interesting foliage, stems, flowers, fruits and even bark. The plant list runs to seven pages and is available at http://www.cornellplantations.org/sites/default/files/bioswale.pdf. Most, if not all, of the hardworking plants on the list are available to home gardeners.
The Cornell swale features enough plant variety to sustain four seasons of interest, but the predominance of grasses and prairie plants means it is at its best in late summer. When I was there, tall pinkish purple Joe Pye-weed—Eupatorium species–vied with several types of sneezeweed, also known as Helen’s flower or Helenium, as centers of butterfly activity. The many coneflowers were almost finished, but a few large daisy-like flowers remained to tempt those same pollinators.
I would love to see the swale in late spring when the various moisture-loving iris varieties are in bloom. Many gardeners are familiar with the graceful Japanese iris or Iris ensata, but may not be acquainted with colorful Louisiana iris, hybrids that seem to be equally at home on the Gulf Coast and in Ithaca, New York.
Many of the plants die back in winter, but winterberry holly—Ilex verticillata—adds color with bright red berries on leafless stems. The tan and cream exfoliating bark of river birch also shows up well during the colder months. Red chokeberry or Aronia arbutifolia features persistent red berry-like fruits as well. These are an edible treat for the birds and a visual treat for garden visitors.
Tough, hardy ornamental grasses tie the landscape together. The predominant grass species is switch grass—Panicum virgatum. A native prairie plant, switch grass grows three to six feet tall, with long, arching foliage. In the late summer the plants sport a pinkish inflorescence that appears from a distance as a rosy haze. Various varieties feature differences in height and coloration, creating subtle contrasts. Like many ornamental grasses, switch grass creates a sense of motion as it sways gracefully in the breeze.
Bioswales, like the one at Cornell, look effortless and natural, but like all artificially created environments, they require maintenance. Weeds and invasive plants must be kept in check, especially when the desirable plants are young. Sometimes new plants must be installed to replace specimens that die. Ornamental grasses, which are often left intact through the winter to add dramatic interest, need to be cut back in the spring so that new growth can emerge unhindered. Some grasses also form large clumps that must be dug and divided every few years. Anyone who has ever done this chore knows that it takes a sharp spade, a garden knife or small axe and a strong back.
If you have low spots where runoff accumulates, try combining some of Cornell’s swale plants to create a mini-swale or rain garden. For small areas, choose disciplined, short ornamental grasses, like fescues. Add one or more of the many available coneflower varieties, along with asters or goldenrod for end-of-season interest. Evaluate after one or two seasons and add or substitute plants. Your planting scheme may not be a swale, but with clever plant choices it can certainly be swell.