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Chapter 7: A Stormwater Marsh

by Craig Tufts

Did you ever stop to think about how much water rolls off your roof each year? If you live in Tucson, the quantity might be small. Where I live, though, in suburban northern Virginia, summer thunderstorms and winter snows drop 40 inches of precipitation annually. This stormwater rolls down driveways and over roads, collecting motor oil, cigarettes, pesticides and pet wastes from street gutters as it flows towards small streams where, over a period of years, it causes extensive environmental damage.

According to some minor calculations, I figured that just half of the runoff from my roof could supply me with 14,000 gallons of water a year. I reasoned that rather than letting the rain continue to wash unused across lawn and into a nearby stream, I could use that water in a more productive way. Such a large quantity could fill a big fish tank or moisten a nifty little marsh, for example. Since childhood I've liked the moisture-loving plants that grow in marshes—cardinal flower, common rush, great blue lobelia. Many of these would thrive in regular soil as well, but I wanted to establish a plant community, an ecosystem, rather than simply design a formal landscape.

What's a Marsh?

A naturally occurring marsh is a wetland characterized by predominantly herbaceous sun-loving plants, some shrubs and, for at least part of the year, standing water. In time, as shrubs and sedges are replaced by trees, a marsh eventually becomes a swamp (a wetland dominated by trees). However, by controlling the water and with judicious weeding, a marsh can be made to retain its character and will continue to host a variety of herbaceous plants.

Building a Marsh

Plants like summersweet, Clethra alnifolia, were selected not just for their flowers, fragrance and seasonal interest but also for their attraction to butterflies and other wildlife.

I chose the site for my marsh based on access to water from the roof of my house, which fortunately was close to a site flat enough to accommodate a marsh. Although you could use a driveway or other surface to supply stormwater runoff, a roof is ideal both because gravity works in your favor to bring water to your marsh and because the water quality tends to be good. The only potentially polluting nutrients my roof may contribute are some bird droppings.

To grow the types of plants I wanted I needed a sunny location, so I chose the south side of my house. A third consideration was soil quality. Very sandy soil is too permeable to adequately hold water. Luckily, I had a good deal of absorbant clay in my soil.

As with any project involving significant landscape change, I needed to consult my local homeowner's association and municipal government to make sure the intended marsh was permissible before I began excavating.

My marsh project began to take shape eight years ago in early winter. Step one was to outline the perimeter of the proposed site. My marsh, about 20 by 30 feet, is irregular in outline, due more to encounters with bedrock than to my sense of design. Using a small rototiller, spading fork and a digging bar to wedge the rocks out of the ground, I stripped the sod from my outlined area, then excavated the subsoil and rock to about 14 inches deep, allowing for an emergency spillway, an area of turfgrass at the lower end of the marsh; during heavy rains, the water can collect here without causing erosion. As I excavated, I used levels supported on two-by-fours to make sure that the ground was even.

Next, I connected a length of flexible plastic pipe to a downspout from my roof, angling it away from the house and burying it beneath the ground and into the marsh. However, during the next rainstorm, when the water backed up inside the pipe, I discovered I needed to increase the overall angle by 4 inches. What's more, because I had used perforated pipe—simply because it had been lying around—I found that too much water was leaking out en route to the marsh and flowing back towards the foundation of the house. I tried using clay to seal the soil surface around the pipe, but that proved ineffective. So, in April, I replaced the perforated pipe with a solid piece and re-excavated the ditch at a greater angle, thus stopping what had become an overactive sump pump in my basement.

Marsh Plants

Because my soil had the same consistency as construction debris—clay and rock fragment—I worked sand, a dozen buckets of compost, a good quantity of the original subsoil and leaf mold into the top few inches of soil. Using my tough little rototiller, I mixed the marsh soil in place. I mimicked a vegetation zone pattern by simply varying the original basin depth, then backfilling uniformly with my marsh soil.

Plants like summersweet, Clethra alnifolia, were selected not just for their flowers, fragrance and seasonal interest but also for their attraction to butterflies and other wildlife.

The last and most enjoyable task was plant selection. My choices weren't only based on plant size, fragrance, hardiness, color and seasonal interest. Because I was determined that my marsh attract butterflies and hummingbirds, I factored in their needs for nectar and chose other plants that provide food for their caterpillars.

I planted Joe-pye weed to attract butterflies, cardinal flowers for hummingbirds and great blue lobelia for bumblebees. Some of the woody plants, especially buttonbush and sweet pepperbush, attract butterflies as well as other pollinators. Some of these insects are excellent biological pest-control agents, too.

Eventually, this mass of herbaceous plants and shrubs provided nesting areas for birds, including cardinals, song sparrows, catbirds and common yellowthroats. Although I wasn't intending to attract mammals—deer, raccoons and squirrels hardly need an incentive—small species do use the marsh as a place to hide and forage for the plants' fruit.

Plant placement was largely determined by my desire to create a screen from my neighbor's house on the upslope of the marsh and by the plants' water requirements. Because the roots of spicebush have to remain fairly wet, it went into a relatively deep area of the marsh, near the water's entry point. Sweet pepperbush needs less water, so I placed it in a drier section.

I look forward to the day when spicebush, sweet pepperbush, buttonbush, silky dogwood and wafer ash will tower over the cardinal flower, monkey flower, swamp milkweed, turtlehead and pickerel weed. Joe-pye weed now looms over the downstream end of the marsh, providing a transition between the marsh, which supports many pollinator species, and the garden specifically designed as a nectar feast for butterflies. Eight years after its creation, my marsh supports about forty-five plant species.

Marsh Maintenance

For the most part, I've let nature take its course and done little in the way of maintaining my marsh. A two- or three-inch rainfall may result in standing water for a day before it's absorbed into the ground. And once, during a severe summer drought lasting more than three weeks, I did add graywater pumped from my washing machine. After concluding that I didn't like the look of that and wasn't too comfortable with the types of nutrients I was potentially adding to the soil, I did draw on some city water for a couple of days as the drought continued.

Although marsh gardens are largely restricted to regions with fairly good rainfall such as the Northeast, Southeast and Northwest, if you live in a drier area your yard could potentially support a marsh as well, provided you line the excavation with a plastic liner. A number of municipal and county governments around the country are promoting "rain" gardens as a way to lessen sudden surges into local streams. Certain areas, including Prince George's County, Maryland, and the Portland, Oregon, metropolitan area, are educating construction engineers to establish areas in new developments where stormwater can collect, sit and then slowly percolate into the ground.

Some day, I'll live next to a natural marsh. Meanwhile, I'm enjoying the added dimension that my homemade mini-marsh has brought—the butterflies, birds, amphibians and moisture-loving plants.