A rain garden is a form of specialty gardening which involves excavating an area to create a 4- to 12-inch deep depression and then planting native perennials, grasses, and shrubs into this depression. This shallow basin garden is positioned so that it collects rainwater runoff from roofs, sidewalks, driveways, and streets. Then the rainwater soaks into the soil and is absorbed by plant roots, evaporates into the atmosphere, and slowly drains from the garden in about 48 hours. Garden plants filter out pollutants and also can provide food and shelter to birds and butterflies. As if that's not enough, a rain garden also adds a beautiful point of interest to a home's landscape. As a conservation measure, this type of rainwater management is preferred to expensive drainage systems, and is much better for the environment than allowing rainwater to rush down the street, collecting pollutants from the road's surface, and emptying into streams or lakes.
Rain Garden Location and Size
The pathway to creating a successful rain garden begins with the gardening mantra: right plants, right place. Proper planning at the beginning will simplify garden maintenance over the years.
Once you know where underground utilities lie, consider the following guidelines for locating and sizing your water garden:
- Allow for 10 feet or more distance from any buildings.
- Allow at least 35 feet distance from septic system drain fields and 50 feet from drinking water wells.
- Rain gardens located in the front yard should be constructed 10 feet from the home but close enough to catch water coming from a downspout.
- Rain gardens located in the backyard should be constructed 30 feet or more from the back of the home to collect rainwater from the downspout and lawn.
- Choose a full or partially sunny, flat location.
- Complement the entire landscape design and position the rain garden to allow for viewing from indoors.
- Avoid play areas or traffic areas to avoid the risk of people tripping.
You must also determine whether the soil drains adequately. Dig a hole 6 inches deep and fill it with water. It should dry out within 24 hours. If it does not, the soil likely has a high clay content and is not suitable for a rain garden. You can try amending the soil by removing about 12" of topsoil from the garden and replacing it with a mixture of sand and compost.
There are a number of calculations that you can make to determine the size of your rain garden, but the goal is to manage 100 percent of the rainwater runoff while allowing none to flow into the streets or storm sewers. A typical rain garden occupies an area 100 to 300 square feet in size, and is 4 to 8 inches lower than the rest of the yard. The actual size also depends upon the size of your yard, your climate, how much money you plan to spend on the project (including excavation, soil amendment, plant material, and mulch), and how large a garden you're willing to maintain. Bigger isn't always better, since even a small rain garden can manage up to 90 percent of water runoff.
The best time of the year to construct a rain garden is the spring because the ground is softer to dig and the new plants will have an entire growing season to establish themselves. Prepare the ground for digging by removing the established turf. An easy way to do this is by covering the area with a sheet of black plastic until the grass dies. Then, shovel soil out of the garden area to construct a berm around the three sides of the garden facing away from the rainwater collection side. The goal is to funnel water into the garden and prevent it from flowing out.
The overall height of the berm should be at the same height as the uphill side of the garden, and it should be about 12 inches wide. Compact the berm by stomping on it. The sides of the berm should slope gently to naturally integrate it into the surrounding yard. Plant grass or another ground cover on the top and outside of the berm. The base of the garden at the bottom of the depressional must be level so water doesn't pool.
Rain Garden Plants
For best results, choose plants that are native to your area. As you select a variety, consider the following characteristics and create a mixture of plants so that your garden will always look interesting.
- Bloom time
- Plant height
- Overall texture
Shop for mature plants rather than young seedlings, because the mature plants will have a bigger root system, which is necessary for a rain garden. Set out the potted plants in the garden area, grouping three to seven of the same species together. This will produce pockets of color throughout the garden. Be sure to include sedges, rushes, or grasses to form the necessary mass of roots throughout the garden. You can use a rough one-foot grid as you set out all the plants, then step back and evaluate whether plants need to be moved. Once you're satisfied with the arrangement, dig a hole for each plant twice the width of the plant's rootball and equal in depth. Place the plant in the hole so that the top of the rootball is flush with the level of the soil in the base of the garden. Add soil to fill the remaining space around the plant and tamp it into place. Repeat the process for each plant.
Add a thick layer of wood chip mulch, about two or three inches deep, to retain moisture and prevent weeds from growing. Note that bark mulch is not well suited for a rain garden because it generally repels water and is lower in nutrients than wood chip mulch. Water the entire garden thoroughly with a sprinkler, soaker hose, or watering wand attached to a garden hose.
Rain Garden Maintenance
Water the garden regularly during the first two years while plants establish their root systems. Keep the top 6 to 12 inches soil moist at all time, using supplemental watering whenever natural rainfall is not enough. . Generally, water gardens require 1 inch of water each week. To test soil moisture, poke your finger into the soil next to the plant about 2 inches deep. If it feels dry, water the garden.
Remove weeds by hand, with a trowel, or with a pointed shovel, as necessary. Weeding is especially important in the spring when the weeds are small, before they spread seeds or runners. The rain garden will still fulfill its purpose if it contains weeds, but the garden plants will become less healthy as they compete for nutrients, water, and sunlight. Also, weed-free gardens are more attractive.
Redistribute the existing mulch, if necessary, and add more mulch every two or three years as it breaks down. Do not use chemical fertilizers or pesticides because they will contaminate the rainwater.
If some perennial plants go dormant in the fall, you can either cut them off or leave stems and seed heads in place to feed birds through the winter and then trim them off in the early spring as new growth starts. Prune shrubs in the early fall. You can decide whether you want to remove dead ornamental grasses or leave them as nesting material for birds.
It may be necessary to use a garden spade to maintain the height of garden inflow and outflow as soil and rocks shift and settle. Keep water moving freely through the garden. And remember, your original plan can change as plants grow.