How to Build a Rain Garden

Rain gardens are becoming increasingly popular as more homeowners become aware of their impact on the overall environment. Simply put, rain gardens are planned gardens that capture and filter rain water that would otherwise run off or through your property.

The benefits of rain gardens are:

• Decreased pollution of streams and water supplies, because rainwater runoff picks up chemicals from roads, lawns, farms, and parking lots
• Reduced erosion of both surface soil and subsoil, which can result in sinkholes and structural damage
• Improvement of the natural replenishment of groundwater supplies

Creating a Rain Garden is Easy

It’s easy to build a small rain garden that will benefit the local environment and improve the beauty of your immediate surroundings.

Typically, large or medium sized rain gardens should be located at least 10 feet from your house to preclude increasing the amount of water around your foundation.

Rain gardens should not be located immediately over a septic system, as these areas are already very moist.

The surface of your rain garden should be 4 to 8 inches lower than the surrounding surface of the lawn, so that it will trap rainwater for a short period of time, allowing it to seep into the soil, feeding the plants and filtering any potential pollutants that would otherwise run off your property.

How to Build Your Rain Garden

Determine how big your rain garden should be. The size of your rain garden is determined by two factors: 1) the total area of impervious surfaces on your property (roof, driveway, patio, etc) and, 2) your soil type. Here’s a simple rule of thumb:

• For very sandy soil, make a rain garden that is about 20% of the total area of impervious surface.
• If you have good loamy soil, figure on 35%.
• For heavy clay soil, your garden size will need to be the equivalent to 50% of the impenetrable area.

Decide the shape that will fit your existing landscape and increase the aesthetic value of your property. Most rain gardens should be a variation of an oval design, as a long, skinny garden won’t really capture much runoff.

Decide what you’ll plant in your rain garden. The experts recommend “native plants” that is, plants that are native to your area. These plants have demonstrated that they can thrive in your climate. Pick plants that can tolerate short periods of wet conditions, but also a couple of weeks of no rain at all. After all, you don’t want to have to water your rain garden, except in periods of extreme drought, once it is established.

The Process

Just like traditional flower and vegetable gardening, the success of your rain garden will be a function of how well you’ve prepared the soil.

First, remove the sod from the area. This step will make future maintenance of your rain garden easier. Also, removing the sod gives you a head start on building a rain garden that is slightly lower than the surrounding landscape. Rent a sod cutter for any rain garden over 50 square feet. They’re really easy to handle, and, like the Mantis tiller, they’re actually fun to use. Two of the best sod cutters on the market are manufactured by Classen and Ryan. The money you spend to rent a sod cutter will save you a lot of time and also will help you make a better rain garden. If you’re removing good sod, use it to improve another part of your lawn, or share it with friends.

Second, add some compost to the dug out area. Four or five inches of compost will significantly improve both the fertility and tilth of the soil. If you don’t have your own compost, buy some. (Don’t add large quantities of peat moss, as this can often adversely affect the pH of the soil.) With a good rototiller, this part of the job can be done in less than an hour. The tines on the Mantis tiller are especially good at mixing compost and other soil amendments into the soil. If you have very clay soil, and you’re building your rain garden where you sometimes have standing water for several days, adding some sand to the soil at this point will greatly improve drainage.

The old saying “dig a $10 hole for a $5 plant” is true here, too. Soil preparation is the key. If you have, or create, good soil, you’ll have a good rain garden.

Third, plant “native plants” in your rain garden. A simple search will show you that you’re not limited to flowers… rain gardens can include trees and shrubs, too. Many trees and shrubs are ideal for rain gardens, because they develop large root systems that can absorb a lot of water, and they can tolerate a couple of rain-free weeks, as well.

Fourth, mulch the garden. Use a mulch that will allow water to penetrate and one that will help prevent weed growth. Adding a good mulch will help create a low-maintenance garden. This may be the one time that wood chip mulch can be useful.

Fifth, water the garden until it is well established. This might seem odd for a rain garden; however, you do need to nurture newly transplanted trees, shrubs, and flowers for a little while. Once established, however, the plants in your rain garden should be almost maintenance free. In fact, they’ll be working for you improving your landscape and your impact on the environment.

All images courtesy of NRCS, Des Moines, IA