What are ‘rain gardens’ and why do we need them?

In developed urban areas, low spots that would ordinarily fill with storm water and drain naturally, are filled in, usually with pavement. Because of this, any storm water is typically directed into storm drains that can overflow and mix with sewer systems. Even if the storm water does not mix with the sewage water, it has already washed across parking lots, roofs, sidewalks, or driveways, and is often filled with many common pollutants like oil, chemicals, and even pet waste. This contaminated water eventually makes its way into our streams, inlets and oceans.

A rain garden can help mitigate the damage caused by this routine by mimicking a forest’s natural drainage system. It does this by using the roots of plants (usually a selection of wetland edge vegetation such as wildflowers, sedges, grasses, ferns, shrubs, and small trees) and layers of gravel, soil and mulch, to filter the water as it soaks into the ground.

The runoff water has time to cool in the rain garden, which helps out the aquatic ecosystems that often suffer an upset when warmer storm water directly enters a stream. A sudden influx of warmer water reduces the availability of dissolved oxygen in water.

Rain gardens can also help to minimize flooding of nearby properties or roads, since runoff is slowed and directed for ideal water runoff management in specific locations.

Another great thing about rain gardens is they can be easily integrated into your yard, worked into business sites, or tucked along roadsides. It’s a nice way to tie buildings into their surrounding environments. The garden is simply planted in a shallow depression that allows rainwater runoff to drain into them from hard surfaces like roofs, driveways, or parking lots. The end of your downspout, for example, is where a rain garden can flourish.

What a rain garden is not

A “bioswale” is sometimes confused for a rain garden. Swales serve the purpose of sloping and redirecting runoff water to a particular destination. Most rain gardens are designed to be an endpoint of drainage with the purpose of filtering out toxins. Sometimes bioswales may direct water into a rain garden.

Gardens that nearly always have standing water in them are considered to be water gardens, ponds or wetlands. Rain gardens, however, are designed to slow down the flow of runoff water but do not hold the water for long. Ideally they hold standing water for no longer than 48 hours after the end of a rain.

Rain gardens are not “retention basins” because the water that collects in them is expected to slowly seep into the ground within a day or two and they fill up no more than 2 or 3 inches above the soil bed. With rain gardens, it’s a bonus that the water does not stay around long enough to become a breeding ground for mosquitoes.

It's catching on

Campaigns across the country are sprouting up to urge people and their municipalities to create these helpful gardens. Puget Sound in western Washington has a goal of building 12,000 rain gardens in the area by 2016 in hopes of significantly improving Puget Sound’s water quality.

Kansas City, Missouri has a 10,000 rain garden initiative in which property owners are encouraged to create rain gardens.

New Jersey, Seattle, Portland, Maplewood Minnesota, Delaware, and western Michigan are also jumping on board with similar programs.

How to establish a rain garden and what to consider

To create a rain garden, the idea is to have a simple inflow where rainwater enters the garden and gradually dissipates into the soil while being filtered by the roots of vegetation. It helps to have an above-ground overflow where excess water can exit. The design doesn’t need to be complex. Often adjusting a downspout to drain into a low area may be all that is needed, but remember most plants won’t tolerate their roots being saturated for too long so make sure you also have proper drainage.

Here are some points to consider:

• The rain garden should be at least 10 feet away from your home in order to reduce the possibility of water migrating towards your house foundation, or interfering with any utilities adjacent to the house.
• Do not locate a rain garden over a septic field.
• Try to use a naturally occurring low spot. Adjust downspouts or a sump pump outlet that can be used to direct the water right into your garden. A location in the sun is best.
• If you are capturing water from a hard surface such as a roof, you will need to measure the specific drainage area of that surface and multiply it by the number associated with the type of soil you have in order to get your drainage capabilities.

Measurements go as follows:
Sandy soil- multiply by 20%
Loam- multiply by 30-35%
Clay- multiply by 45- 60%

*For more specific information on the calculation process, go to www.raingardennetwork.com/build.htm.

• If you have a rain barrel water collection system, the overflow can also be directed to a low spot suitable for a rain garden.
• Use native plants in your rain garden to encourage native butterflies, birds, and beneficial insects. Try flowers and grasses that grow in both wet and dry areas.
• If you use trees, use small native ones that preferably grow multiple trunks.
• Consider both tall and short plants. Tall plants such as shrubs like azalea and rhododendrons or winterberry, holly, and chokeberries, are all good options. Some shorter plants like swamp milkweed thrive in wet soil. Black-eyed Susan and Liatris are both highly adaptable plants. Check what is native to your area.
• Once you have chosen your rain garden site you should contact your local organization that locates underground utilities. You’ve heard the slogan a hundred times, “Call before you dig.”
• Remove turf and grass and dig down 4-8 inches.
• The best mixture to start the bottom layer is made up of 60% sand, 20% compost, and 20% topsoil. Recent trends replace compost with biochar. Sometimes gravel can be used in particularly low wet spots to help with percolation.
• Transplant your starters just as you would in the vegetable garden or shrub bed. Your plants should be approximately 1 foot apart.
• Finish off with a layer of mulch. Use coarse, fibrous woodchips that won’t float or blow away. Apply the mulch 2-3 inches deep in order to keep weeds at bay and to hold moisture in.

For a more in-depth description of building a rain garden you might want to consult the site of the Rain Garden Network at the link mentioned above.

Once established, a rain garden requires little maintenance but you do need to keep an eye out to see if the drainage is getting clogged. The best time to look for this is right after a rainfall. If you notice the soil is eroding or that it takes the water more than a couple of days to leave the garden, you may have to make an adjustment by improving drainage. If the climate is very dry when you plant out, you may have to water your rain garden every couple of days, just until you notice the garden looks like it’s growing on it’s own. Once it has, you are well on your way to detoxifying your environment using the simple, natural technique of rain gardens.