How to Build a Rain Garden

A rain garden collects water runoff and allows it to soak into the ground. Filled with moisture-tolerant plants, it can also be a beautiful addition to a landscape.

Why Build a Rain Garden

There's a new sort of garden in town. It's easy to install, looks good year-round and has a positive impact on the environment. It's a rain garden.

A rain garden is a special type of garden, designed to collect stormwater runoff from a roof, driveway or other impervious surfaces. Rather than rushing off into a storm sewer or a local waterway, the rainwater collects in a shallow depression in your yard. This area is planted with native grasses and wildflowers that are specially selected for their ability to gradually absorb and filter stormwater.

Rain gardens can have a significant impact on the water quality in our communities. Studies have shown that as much as 70% of the pollution in streams, rivers and lakes has been carried there by stormwater. By taking responsibility for the rainwater that falls on your own roof and driveway, you'll be helping to protect our rivers, streams and lakes from stormwater pollution. Adding a rain garden to your yard will also provide food and shelter for wildlife, and give you a whole new garden that's hardy, low maintenance and naturally beautiful!

Choose the Site

Your rain garden should be located at least 10 feet from the house. A natural site is a low spot in your yard that often collects water after a heavy rain. Ideally this area receives full sun, but at a minimum it should receive a half day of sunlight. There should be a natural slope (at least 1 percent grade) leading from the water collection area (your roof or driveway) down to the rain garden. Choosing a relatively level spot for the garden will keep digging to a minimum.

This hydrangea (Hydrangea arborescent 'Annabelle') is a native species well-suited to a rain garden planting.

Prepare the Soil

Once you've identified the new garden's location, remove the sod and dig a shallow depression approximately 6" deep. Slope the sides gradually from the outside edge to the deepest area. Use the soil that you remove to build up a slightly raised area on the lowest side of the garden. This berm will help contain the stormwater and allow it to percolate slowly through the rain garden.

If your rain garden is no more than about 6" deep, stormwater will usually be absorbed within a one- to seven-day period. Because mosquitoes require seven to 10 days to lay and hatch their eggs, this will help you avoid mosquito problems. If you want to create an area with standing water for fish and amphibians, you can make one part of your rain garden deeper, perhaps as much as 18 inches in the deepest spot. Depending on the type of soil you have (sand, clay, loam), you may need to line that area of the garden with plastic to help retain a small pool of water.

A typical residential rain garden is 100 to 300 square feet, but any size rain garden is fine. Most people just size the garden to suit their available space. You can calculate the ideal size for a rain garden, based on the surface area of your roof, soil type and the garden's distance from your house. (For more detail, see the links at the end of this article.)

The downspout from your roof or sump pump outlet from your basement should be directed toward your rain garden depression. This can be accomplished by a natural slope, by digging a shallow swale, or by piping the runoff directly to the garden through a buried 4" diameter plastic drain tile.

A relative of mint, bee balm is a native plant that thrives (and will spread) in moist soil.

Choose Native Plants

Native plants are the best choice for rain gardens. They withstand difficult growing conditions and require little care. When choosing the plants, consider height, bloom time and color. Clumps of three to seven plants of the same variety will look better than a patchwork of singles. Be sure to mix native ornamental grasses and sedges in with your perennial wildflowers to ensure the garden has a strong root mass that will resist erosion and inhibit weed growth.

The following plants are ideal for rain gardens; they are native to North America and can withstand fluctuating water levels in full sun to part shade:

Perennials

• Blue flag iris (Iris versicolor)
• Cardinal flower (Lobelia cardinalis)
• Joe Pye weed (Eupatorium perfoliatum)
• Marsh marigold (Caltha palustris)
• Swamp milkweed (Ascepias incarnata)
• White turtlehead (Chelone glabra)

Shrubs

• Buttonbush (Cephalanthus occidentalis)
• Inkberry (Ilex glabra)
• Smooth alder (Alnus serrulata)
• Red-osier dogwood (Cornus sericea)
• Sweet pepperbush (Clethra alnifolia)
• Ninebark (Physocarpus spp.)

New plants should be watered every other day for the first two weeks or so. Once they are well established, your garden should thrive without additional watering. Fertilizers will not be necessary, and only minimal weeding will be needed after the first summer of growth.

Joe Pye weed (Eupatorium purpureum) is a good perennial plant for a rain garden, and the flowers will attract pollinators, too. Photo: Amanda Everse

Winter Maintenance

Most rain gardeners wait until early spring to cut back the prior year's growth. Leaving seed heads and spent foliage in place through the winter provides visual interest as well as cover and food for many kinds of wildlife. Once spring comes, burning off the dead material is the best way to knock back weeds and stimulate new growth. If burning is not an option, mow the dead plants or cut them back with a scythe or pruning shears.