Planting a rain garden

Gardening with CharlieBy Kathy Bond-Borie, Guest Columnist

Storm water runoff can be a big problem during heavy thunderstorms. As the water rushes across roofs and driveways, it picks up oil and other pollutants. 

Municipal storm-water treatment plants often can’t handle the deluge, and untreated water ends up in natural waterways in many areas. The U.S. Environmental Protection Agency estimates as much as 70% of the pollution in our streams, rivers and lakes is carried there by storm water. 

To reduce excess water runoff, many towns are encouraging businesses and homeowners to install rain gardens in their yards: specially constructed gardens located in low areas of a yard where storm water can collect. The idea is to have the water funnel naturally to this garden, which collects runoff and stores and filters it until it can be slowly absorbed by soil.

Sizing Up a Rain Garden

 A rain garden’s size and location depends on your yard. 

If you’re the type of person who likes precise measurements, there are guidelines you can follow for estimating the ideal size for your particular situation. 

For example, you’d want to measure the area of your roof that will be draining into the gutter leading to the rain garden, as well as the size of any paved areas that will be contributing to runoff into the garden. 

If your soil is sandy (which drains quickly), you’d want your rain garden to be about 20% to 30% of the area that will be draining into it (roof plus driveway, etc.). If you have clay soil, your ideal rain garden would be 60% of the drainage area. 

But don’t let these numbers intimidate you. Any size rain garden is better than none at all. 

The ideal spot for a rain garden is in a natural depression. You can also funnel water from downspouts or gutters into the garden. The soil should be well drained so the water doesn’t sit in the garden for more than 2 days. 

A special rain garden soil mix of 50% to 60% sand, 20% to 30% topsoil, and 20% to 30% compost is recommended. You can dig this mixture into the soil to a depth of 2 feet before planting.

Black-eyed susansChoosing Plants 

The most difficult part of building a rain garden can be plant selection. 

The plants in a rain garden must be able to tolerate sitting in water now and then, so native plants and wildflowers are good choices because they’re so adaptable. You probably already grow many of them: ferns, ornamental grasses, sedges, iris, milkweed, asters and black-eyed Susans (right), to name a few. 

The idea is to create a naturalistic planting that’s easy to maintain (no fertilizer needed) and welcoming to butterflies, bees and other creatures. 

A former floral designer and interior plantscaper, Kathy Bond-Borie has spent 20 years as a garden writer/editor, including her current role as horticultural editor for the National Gardening Association. She loves designing with plants and spends more time playing in the garden—planting and trying new combinations—than sitting and appreciating it. 

Photos: Center for Neighborhood Technology, National Gardening Association/Fotalia