By Bill Sublette
Piedmont Master Gardener
The latest federal climate data confirms what we’ve all seen in our rain gauges in recent years: extreme precipitation events (days with two inches or more) are happening more frequently in Virginia. As heavy rainfalls trend upward, so does stormwater runoff and the pollution it carries into our local watersheds.
According to Virginia Cooperative Extension, a typical homesite with about 2,300 square feet of roof and pavement will send more than 62,000 gallons of runoff into the surrounding watershed each year. If unchecked, the stormwater flowing off roofs and driveways picks up contaminants such as fertilizers, pesticides, auto fluids and animal waste and takes them down storm drains or directly into nearby streams. Intense runoff also erodes soils, scours out aquatic habitat and increases sediment pollution.
The western Albemarle County watershed feeds the Rivanna River, which in turn feeds the James River and ultimately the Chesapeake Bay. This means polluted runoff from our area contributes to problems afflicting the Bay, including excess nutrients—principally nitrogen and phosphorous—that cause algal blooms and oxygen-starved dead zones. We can all help address this issue by adopting strategies for collecting stormwater runoff or slowing it down so that more of it seeps into the ground (a process known as infiltration) or evaporates before it can leave our yards.
A simple first step is to add trees to the landscape. Researchers have found that urban trees retain on average 20 percent of annual rainfall in their canopy and increase infiltration by up to 3.5 times as compared with treeless spaces. Landscape designers Elisa Meara and Alex Thompson recommend going a step further to create layered landscapes that combine a canopy layer of native trees, an understory layer of native shrubs and a ground layer of native grasses, perennials and groundcovers. In addition to helping to control runoff and erosion, planting in layers mimics natural environments that provide food, shelter and nesting places for pollinators, birds and other wildlife.
Meara and Thompson, who are with the firm Native Plant Landscape Design Corp. in Falls Church, will give an online talk on attractive and sustainable stormwater solutions at 7 p.m. March 30 as part of the Piedmont Master Gardeners’ spring lecture series. In their design work in northern Virginia, where intense development and the trend toward larger houses have increased stormwater problems, they often encounter landscapes where runoff originating from impervious surfaces—a paved driveway or patio, for example—flows virtually unimpeded across turfgrass lawns. To slow the water, promote infiltration and clean any runoff that escapes, they will replace a section of lawn close to the pollution source with a layered arrangement of densely planted native species, leaving as little bare earth as possible.
“By using native plants and by using the layering and what we call ‘green’ or living mulch [i.e. dense planting of groundcovers, such as native sedges], we are increasing percolation and filtering the water so it is clean when it reaches the water table,” said Meara. “We are also providing habitat for wildlife, food for overwintering birds and nectar for pollinators. And we don’t run the risk of introducing plants that will invade our forests.”
“We believe plants should do more than one thing in the landscape,” added Thompson, “and native plants have the edge on that because they are so much more interesting to wildlife,”
The designers also point out that simply allowing leaves to stay on the ground in wooded areas or delaying cleanup of the garden until spring will help cushion the impacts of stormwater runoff. To hear more of their ideas, register for their lecture at piedmontmastergardeners.org.
Although dense plantings of natives can do a lot to control runoff, sometimes a more engineered solution is required. Virginia Cooperative Extension offers a series of helpful factsheets on stormwater management options, available at www.pubs.ext.vt.edu/SPES/SPES-9/SPES-9.html. Here are a few they recommend.
Redirect Water from Your Downspouts
The largest impervious surface at most homesites is the roof of the house. With flexible fittings and extension pipes attached to gutter downspouts, you can direct rainwater toward places where it can infiltrate the soil, such as a flowerbed or a rain garden, described below. If your only option is to direct rainwater to your lawn, avoid sending runoff down steep slopes. Also, aerate your lawn every few years to reduce compaction and promote infiltration. You can attach a popup spreader at the end of the extension pipe to distribute the rainwater more evenly, or just place a few medium-size stones at the end of the pipe to slow the flow.
Install Rain Barrels
When attached to gutter downspouts, rain barrels reduce stormwater runoff while capturing water you can use later to irrigate your lawn and garden. A rain barrel is typically equipped with a spigot at the bottom and an opening near the top to release overflow, which should be directed to places where it will seep into the ground—such as a rain garden. Rain barrels will fill up in a flash, so it’s essential to handle the excess. Meara and Thompson advise against routing it into edible gardens, noting that water coming off a rooftop can pick up pollutants that settle there between rainfalls, such as fine particulates from diesel exhaust.
To learn more about rain barrels, mark your calendar to attend a workshop from 6 to 7 p.m. May 11 at Pen Park, jointly sponsored by the City of Charlottesville, the Piedmont Master Gardeners, the Albemarle County Service Authority and the Rivanna Stormwater Education Partnership. You will be charged a fee, but you will walk away with all you need to install a rain barrel at your house. Details and registration instructions will be posted on the sponsor websites.
Establish a Rain Garden
A rain garden is a shallow depression that captures runoff and holds it for just a few days, allowing it to evaporate, infiltrate the ground or be taken up by plants. A perc test, which measures absorption of water by soil, may be necessary to determine if the infiltration rate is sufficient. If designed correctly so that ponded water is gone within 48 hours, a rain garden won’t breed mosquitoes. At times it will be very wet, and at other times it will be very dry, so it’s important to select plants that tolerate this range of conditions and to place them appropriately.
Site the rain garden along the route of where stormwater naturally flows on your property or where it has been redirected from downspouts and impervious surfaces. It should be at least 10 feet from the foundation of your house and should not be on a steep slope, over a septic field or in a soggy area where the water table is high. The finished depth, after all soil amendments are made, should be six to eight inches below the surrounding area.
Rain gardens are an ideal setting for native plants. Virginia Cooperative Extension offers an extensive list of rain garden plants, many of them native. (Search for VCE Publication 426-043.) You can cross-reference this resource with Albemarle County’s native plant database (http://webapps.albemarle.org/NativePlants/list.asp) to identify native species particularly suited for rain gardens in our area and the conditions they require.
These include trees and shrubs that can thrive in wet and dry places, such as Sweetbay Magnolia (Magnolia virginiana), River Birch (Betula nigra), Red Osier Dogwood (Cornus sericea) and Ink Berry Holly (Ilex glabra). For the moderately wet part of the garden, options include Witch Hazel (Hamamelis virginiana), Joe Pye Weed (Eupatorium maculate), Cardinal Flower (Lobelia cardinali) and White Turtlehead (Chelone glabra). For the lowest, wettest area, you can choose Eastern Rosemallow (Hibiscus moscheutos), Royal Fern (Osmunda regalis), Cinnamon Fern (Osmunda cinnamomea) or Pickerelweed (Pontederia cordata), to name a few.
The Virginia Department of Forestry offers a detailed, well-illustrated technical guide to creating rain gardens and some sample plant lists. Visit dof.virginia.gov and search for Rain Gardens to find a link.
Plant Vegetated Buffers
If your property borders a pond or stream, a vegetated buffer will protect water quality by slowing runoff, filtering out pollutants and stabilizing the shoreline. Comprising a mix of perennials, grasses, shrubs and trees, a buffer is relatively inexpensive to establish and easy to maintain. In addition to water protection, a vegetated buffer provides an array of ecological and aesthetic benefits, including support for wildlife, especially if the border is planted with native species. The online resources cited above will offer a helpful guide for choosing upslope plants that can withstand dry conditions and downslope plants that will succeed in wetter environments.
These stormwater management measures are even more effective when combined with one another. For example, a rooftop redirection system can channel runoff into a rain garden, which in turn empties into a stream buffer. Meara and Thompson call this a “treatment train.” You can mix and match based on your topography and available land area. In all cases where excavation is required, consult va811.com (a.k.a. Miss Utility of Virginia) before you dig to locate underground utilities.