By Barbara Perry Lawton
In the early 1990s, stormwater specialists in Prince George’s County, Maryland were the first to develop raingardens, constructions that alleviate watershed erosion problems. They recognized that America’s early and original ecosystems allowed rain to filter slowly through the plant foliage and soils of forests, meadows, and wetlands, replenishing subsoils and aquifers. Most streams and rivers were, as a result, unpolluted, clear and clean.
Raingardens, comparative newcomers to the garden scene, are designed to catch and clean runoff water in an attractive way, replacing eroding watersheds in private gardens and also arboreta, housing developments, parking lots and similar areas. They eliminate the need for more expensive solutions to runoff and standing water, such as regarding, and treat rain as a valuable asset, not a waste product.
Raingardens are making such a difference in erosion-prone areas that the Environmental Protection Agency (EPA) is giving grants to groups willing to heal eroded land and degraded streams. EPA estimates that suburban properties are responsible for approximately half of all manmade pollutants that end up in our waterways. Small wonder that EPA is encouraging the development of raingardens.
Site raingardens in sun to partial shade where there is a lot of water runoff. Look for low places where water naturally gathers. Swales and areas at the base of slopes or hills are likely sites. Parking lots typically offer raingarden sites at their lower ends where water runoff is greatest.
Transport rainwater from downspouts or driveways by gutter pipe or French drains to the foundation of the raingarden, which is a shallow basin a foot or so deep. The entrance area into the basin should have a patch of gravel or pebbles to baffle and slow the water. Fill the center of the basin with mixes of soil, organic matter, sand and/or gravel to encourage the water to percolate into subsoil. Soils with a pH of 5.5 to 6.5 are best for the biochemical reactions that convert pollutants to harmless substances. The plants in a raingarden slow down the water and thus increase the infiltration of the water into the soil, just as forests and prairies do in the natural landscape. Mulching with organic matter will reduce weed problems.
Spring is a good time to create a raingarden. That is usually a rainy season and it will be easier to dig as well as easier to study the water flow. But you can start planning your raingarden now, taking the time to study what happens when there is a downpour. Where does the water come from? Where does it go? What area is involved? How long does the water stand after the rain stops?
The size of the raingarden will depend upon the amount of water coming from the source. The larger the volume of water, the larger the proposed raingarden should be. Make the raingarden approximately twice as long as it is wide, with width measured from uphill to down. Position it roughly perpendicular to the source of the water if possible.
Define the raingarden with garden hose or rope, and then kill the existing grass and other plants with glyphosate herbicide or black plastic. Create a berm by digging on the uphill side of the raingarden, using that soil to build a gently sloping berm at least a foot wide along the low side. Make the area between the upper edge of the raingarden and the berm level. Use a carpenter’s level to check periodically. A curved berm will be more attractive and look more natural than a straight one.
Native plants are often used in raingardens, thus continuing the natural theme. Raingardens have different moisture zones. The upper zone nearest the source of the runoff – often gutters or eaves – is the driest and can be planted with most native shrubs, trees, and perennials. A middle zone where the soil is usually moist but may have droughty periods, is a good site for such plants as hardy hibiscus, cardinal flower, black-eyed Susan, sweetbay magnolia, and river birch.
The lowest part of the raingarden is where the soil is most moist even though it, too, may have dry periods. Plants for this area of the raingarden include swamp milkweed, sedges, and New England aster. For areas in the raingarden where the soil stays moist or even wet for long periods at a time, rushes and reeds are among the good choices. If the area has standing water for lengthy periods, rushes, sweet flag, pickerelweed, hardy canna and arrowhead will thrive.
Barbara Perry Lawton is a writer, author, speaker and photographer. She has served as manager of publications for Missouri Botanical Garden and as weekly garden columnist for the St Louis Post-Dispatch. The author of a number of gardening and natural history books, and contributor to many periodicals, she has earned regional and national honors for her writing and photography. Barbara is also a Master Gardener and volunteers at the Missouri Botanical Garden in St Louis MO.