Though a rain garden may seem like a small environmental contribution toward a mammoth effort to clean up our water supply and preserve aquifers, collectively they can produce significant community benefits. For instance, if homeowners in a subdivision each decide to build a rain garden, the neighborhood could avoid installing an unsightly retention pond to collect stormwater run-off. So you see, the little steps you take at home can make a big difference.
Most of the work of building a rain garden is planning and digging. If you recruit some helpers for the manual labor, you can easily accomplish this project in a weekend. As for the planning, give yourself good time to establish a well-thought-out design that considers the variables mentioned here. And as always, before breaking ground, you should contact your local utility company or digging hotline to be sure your site is safe.
Preparing the Land
Soil is a key factor in the success of your rain garden because it acts as a sponge to soak up water that would otherwise run off and contribute to flooding, or cause puddling in a landscape. Soil is either sandy, silty, or clay-based, so check your yard to determine what category describes your property. Sandy soil is ideal for drainage, while clay soils are sticky and clumpy. Water doesn’t easily penetrate thick, compacted clay soils, so these soils need to be amended to aerate the soil body and give it a porous texture that’s more welcoming to water run-off. Silty soils are smooth but not sticky and absorb water relatively well, though they also require amending. Really, no soil is perfect, so you can plan on boosting its rain garden potential with soil amendments. The ideal soil amendment is comprised of: washed sharp sand (50%); double-shredded hardwood mulch (15%); topsoil (30%); and peat moss (5%). Compost can be substituted for peat moss.
While planning your rain garden, give careful consideration to its position, depth, and shape. Build it at least 10 feet from the house, and not directly over a septic system. Avoid wet patches where in filtration is low. Shoot for areas with full or partial sun that will help dry up the land, and stay away from large trees. The flatter the ground, the better. Ideally, the slope should be less than a 12% grade.
Residential rain gardens can range from 100 to 300 square feet in size, and they can be much smaller, though you will have less of an opportunity to embellish the garden with a variety of plants. Rain gardens function well when shaped like a crescent, kidney, or teardrop. The slope of the area where you’re installing the rain garden will determine how deep you need to dig. Ideally, dig four to eight inches deep. If the garden is too shallow, you’ll need more square footage to capture the water run-off, or risk overflow. If the garden is too deep, water may collect and look like a pond. That’s not the goal.
Finally, as you consider the ideal spot for your rain garden – and you may find that you need more than one – think about areas of your yard that you want to enhance with landscaping. Rain gardens are aesthetically pleasing, and you’ll want to enjoy all the hard work you put in to preparing the land and planting annuals and perennials.
Building a Rain Garden
1. Choose a site, size, and shape for the rain garden, following the design standards outlined on the previous two pages. Use rope or a hose to outline the rain garden excavation area. Avoid trees and be sure to stay at least 10 ft. away from permanent structures. Try to choose one of the recommended shapes: crescent, kidney, or tear drop.
2. Dig around the perimeter of the rain garden and then excavate the central area to a depth of 4 to 8″. Heap excavated soil around the garden edges to create a berm on the three sides that are not at the entry point. This allows the rain garden to hold water in during a storm.
3. Dig and fill sections of the rain garden that are lower, working to create a level foundation. Tamp the top of the berm so it will stand up to water flow. The berm eventually can be planted with grasses or covered with mulch.
4. Level the center of the rain garden and check with a long board with a carpenter’s level on top. Fill in low areas with soil and dig out high areas. Move the board to different places to check the entire garden for level. Note: If the terrain demands, a slope of up to 12% is okay. Then, rake the soil smooth.
5. Plant specimens that are native to your region and have a well-established root system. Contact a local university extension or nursery to learn which plants can survive in a saturated environment (inside the rain garden). Group together bunches of 3 to 7 plants of like variety for visual impact. Mix plants of different heights, shapes, and textures to give the garden dimension. Mix sedges, rushes, and native grasses with flowering varieties. The plants and soil cleanse stormwater that runs into the garden, leaving pure water to soak slowly back into the earth.
6. Apply double-shredded mulch over the bed, avoiding crowns of new transplants. Mulching is not necessary after the second growing season. Complement the design with natural stone, a garden bench with a path leading to it, or an ornamental fence or garden wall. Water a newly established rain garden during drought times – as a general rule, plants need 1 in. of water per week. After plants are established, you should not have to water the garden. Maintenance requirements include minor weeding and cutting back dead or unruly plant material annually.