How to Landscape a Drainage Swale

Vegetative drainage swales are a landscape element on commercial properties, particularly used in parking lot stormwater run-off management. Drainage swales also play a role in residential rainwater and snow melt management, directing water from downspouts or driveway run-off to a yard drain or street storm drain. Like agricultural ditches, drainage swales not only safely control water flows, but carry with them fertilizers and other pollutants from your roof, lawn and gardens. Plantings along and in the drainage swale will improve its appearance and mitigate its potential negative environmental impacts.

Things You'll Need

• Backhoe
• Sand
• Gravel
• Topsoil
• 6 inch perforated drain tile
• Choice of flood-hardy ground covers: water cress, marsh marigold, grass or sedge
• Choice of flood-hardy perennials: day lilies, cat tails, irises, rushes
• Choice of flood-hardy shrubs: pussy willows, red osier, yellow-twig dogwood
• Choice of flood-hardy trees: Sycamore, ash, swamp white oak, red maple

Instructions

1. Dig a ditch 4 feet deep by 6 feet wide for the desired course of the drainage swale, using a backhoe, for residential landscaping drainage; engineering and design standards for commercial property and parking lot stormwater drainage may require a larger drain profile. Place a 1 foot thick layer of gravel in the bottom of the ditch with the backhoe. Lay 6 inch drain tile on top of the gravel. Cover with an additional 1 foot thick layer of gravel.

2. Mix topsoil and sand in equal volumes. Place a 1 foot thick layer of this mixture on top of the gravel in the ditch. Fill the ditch to the surface with topsoil. Grade the swale to a gentle dip; the University of Illinois Extension Local Community Resources project recommends no more than a 10 degree to 15 degree slope to the surface of the swale.

3. Plant your choice of flood-tolerant ground covers across the entire surface of the drainage swale, following the supply nursery directions as to planting spacing and depths.

4. Plant groupings of flood-tolerant perennials in the center of the drainage swale at any locations where standing water is likely, such as at flatter spots or on curves. The Washington State University Clark County Extension office recommends wetland plants such as rushes, cattails, and sedges for absorption of standing water.

5. Plant flood-hardy shrubs and trees along the upper edges of the drainage swale to provide landscape interest, shade, wildlife habitat, and to soak in water through their massive root structures.

Tips & Warnings

• Observe local stream environments and attempt to mimic the patterns of local stream-side shrub and tree growth to maximize both positive environmental impact and landscape design appeal.

• Although drainage swale design is a landscaping project, the quantity of fill required is likely to trigger the requirement of a development permit in many jurisidictions. Check with your local land planning office to obtain any necessary permits before beginning your drainage swale project.

References

• University of Illinois Extension: Multifunctional Landscaping--Putting Your Parking Lot Design Requirements to Work for Water Quality
• Washington State University Clark County Extension: Improving Drainage
• Ohio Department of Natural Resources: Ohio Stream Management Guide--Trees for Ditches