Divert your roof rainwater to make a side yard marsh.

Did you ever stop to think how much water rolls off your roof each year? If you live in Tucson, the quantity might be small. But in Northern Virginia where I live, summer thunderstorms and winter snows drop 40 inches of precipitation annually.

Someday, I'll live next to a marsh. In the meantime, I'm enjoying one that I've built in my yard.

Some minor calculations indicated that just half of my roof could supply me with 14,000 gallons of water a year. Instead of drowning my lawn, I reasoned that quantity could fill a big fish tank or moisten a nifty little marsh. Yesterday, I pronounced the marsh a success: butterflies in droves began "mudpuddling" on the moist marsh soil.

The system directing water off our roofs is simple. Rain runs down the roof, follows the gutter to the down-sprout and is soon streaming across lawn or sidewalk. Much of this rain then surges into our storm sewer system with its accumulated stock of motor oil, cigarette filters, pet waste, fertilizers and pesticides.

My marsh project began to take shape in early winter, interrupted by periods of back strain and seasonal periods of frozen ground. Step one was to outline the perimeter of the proposed marsh.

My marsh is irregular in the outline. This is due more to encounters with bedrock than any sense of design on my part. I stripped sod from a 450 square foot area, then excavated the subsoil and rock to an average depth of 14 inches.

I connected my water source, a downspout from our roof, to a length of solid, flexible plastic pipe. This I angled down and away from the house so that water would enter the top of the marsh. The next rainstorm however, showed that the pipe angle was less than was necessary, and that my fill material was too porous. In April, I dug out the pipe, excavated the ditch with a greater angle and backfilled with clay.

Next, I prepared a soil mixture that contained some sand, a dozen buckets of compost, a good quantity of the original subsoil and a small pickup truck full of sand leaf mold. The marsh soil was mixed in a place with my tough, little rototiller. Areas of differing moisture were created by simply varying the original basin depth, then backfilling uniformly with my marsh soil.

The last and most enjoyable task was one of plant selection. As butterflies and hummingbirds rank high in my plans, I factored in their needs with questions of plant size, fragrance, hardiness, color, and seasonal interest. Spicebush, sweet pepperbush, buttonbush, silky dogwood, wafer ash and prickly ash now tower over cardinal flower, great blue lobelia, swamp milkweed and white turtlehead. Joe pyeweed looms over the downstream end of the marsh and provides a transition between the marsh and butterfly garden.

Nature soon took over as the landscaper of my marsh. As the shrubs grew, songbirds and cardinals, catbirds and thrushes took up residence. My labors now include watching the birds and butterflies visit the marsh, and digging up the progeny of the original planting as gifts to friends.

Steps to Take:

• As with any project involving significant landscape change, before beginning to excavate, check with your homeowners' association and municipal government to make sure your intended marsh project is permissible.

Choosing a Site:

• Choose a site based on access to water and which is flat enough to accommodate a marsh and to hold water. Using a roof to supply water is a good idea, since gravity will work in your favor to bring water to the marsh and because the water quality tends good.

• Base your site selection on the kinds of plants you want to grow. For example, if you want to grow sun-loving plants, choose a sunny location on the south side of your home.

• A third consideration is soil type. Very sandy soil is too permeable to adequately hold water. Soil with a good deal of absorbent clay works well.

Installation:

• Outline the perimeter of the proposed site.

• Strip the sod from the outlined area and excavate the subsoil and rock to about 14 inches deep, allowing for an emergency spillway (an area of turf grass at the lower end of the marsh).

• During excavation, use a level to make sure the ground is even.

• Connect a length of flexible plastic pipe to a downspout from the roof, angling it away from the house and burying it beneath the ground and into the marsh. Make sure the angle of the pipe is great enough to avoid water back-up during a big storm.

• Prepare the soil. If your soil is very clayey, work sand, compost, and other organic material into the top few inches of soil. Mimic a vegetation zone patter by varying the original basin depth and backfilling with your marsh soil.

• Choose plants that provide food and cover for wildlife.