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Gravity assures that where water falls, it runs – down roofs, down streets, downhill all the way to the sea. Draining water efficiently from homes, streets and businesses has long been relegated to the world of engineers and community planners. Roofs are pitched, landscapes graded and sidewalks sloped to quickly carry rainwater away. Home gutters, drain pipes and driveways flush and rush stormwater toward curbs and into storm sewers that pipe the water directly into streams, lakes or treatment plants. All that engineering was deemed necessary because paved surfaces prevented rain from doing what it was meant to do naturally: filter through the ground into the water table. But we're slowly appreciating that there's another way to get stormwater out of gutters and away from foundations while keeping this liquid asset near our front and backyards – rain gardens.
Rain gardens are shallow depressions with loose soil designed to collect rain from impervious surfaces such as roofs and parking lots, slow it down and give plants, bacteria and soils time to clean the water as it seeps into the ground. The concept is simple enough for anyone with some green space to put into practice. More important, according to Roger Bannerman, a DNR stormwater specialist, these gardens can be sustained like any other garden without using manmade energy. Rain gardens can help recharge local aquifers, protect communities from flooding and slow the flow of city street pollutants like fertilizer, road salt, oils and pesticides.
We're slowly getting past the mindset of viewing stormwater as a waste product that needs to be piped and ponded, says rain garden advocate Larry Coffman, associate director for the Department of Environmental Resources in Prince George's County, Maryland. Rainfall and snowmelt are typically piped to storm sewers or collected and directed to detention ponds that give roiling stormwater and sediments time to settle out before the water flows offsite.
By keeping stormwater more widespread close to where it falls, rain gardens provide less opportunity for stormwater to dissolve and scour contaminated oils, metals and other chemicals from the roads and curbside. Rain gardens actually filter pollutants from the water as it percolates through the soil on its way to becoming groundwater.
Declining groundwater levels due to human use not only drop the water table but significantly affect aquatic ecosystems. Groundwater flowing into streams is "vital to their well-being," says Ken Potter, UW water resources engineer. During periods of little rain, groundwater discharge forms the main flow in streams, and this cooler water "can be critical since cold water holds more oxygen." So, rain gardens make for better surface water quality, groundwater quality and overall hydrological health.
The concept started in 1988 while Coffman was thinking "out of the box." He had heard about the practice of growing plants over septic system drain fields as a way to break down pollutants. Why not use this kind of onsite treatment and infiltration (known as bioretention) in an urban setting? Coffman and others felt that the name bioretention was too technical and cold. So they coined the term "rain garden," which they agreed was perfect: simple and catchy.
More catchy than the name is the idea itself. Individual homeowners, neighborhood developers and commercial businesses are all experimenting with designs to incorporate rain gardens on their properties. Since rain gardens are designed to handle average rainfalls, not big blowout storms, traditional curbs and gutters are still needed. But these gardens can provide an important buffer in both new developments and landscape renovations.
Coffman foresees a world in which rain gardens are popping up as tree boxes, peeking over ditches and peering off rooftops, not to mention blooming in yards, campuses and parks. If that seems utopian, just take a look around. Such gardens are now commonplace in new subdivisions, in parking lot renovations, and in rehabs at churches, schools, strip malls, gas stations and residential neighborhoods. They don't involve a lot of centralized planning. They don't require much space. They can fit into oddball shapes, and are readily added to existing buildings. Rain gardens look nice, and you don't need to be an engineer to build one. Anyone can make a rain garden – including you!
A good guide to get going
To plan a rain garden step-by-step, we highly recommend getting a copy of the free publication Rain Gardens – A how-to manual for homeowners, available from DNR Service Centers as DNR Publication WT-776-2003, from UW-Extension offices, which charge shipping and handling to mail out the publication (UWEX Publication GWQ037), or ordered by phone at 1-877-947-7827. It can also be viewed and printed for free in PDF format at on the web at UW Extension-Publications.
A lot of your planning can be done outside of the gardening season. First decide what you want to accomplish. Rain gardens can be placed fairly near your house (at least 10 to 20 feet from the downspout and foundation) to catch roof runoff or farther out if you want to collect water from your lawn and roof.
Sketch It Out – Draw a rough sketch of your property and house. Note the locations of property boundaries, septic fields, buried utility lines (including sewer laterals, water lines, power lines, phone lines and gas lines). Don't plan a rain garden under big tree roots that would be tough to excavate and where additional water might damage trees. Also don't be tempted to place a rain garden in places in your yard where water already ponds. Your goal is to encourage infiltration, and wet patches already indicate where soil is compacted and drainage is slow. You also don't want to place a rain garden in high traffic areas where children play or where foot traffic will only further compact or muddy up wet soils.
Gauge the Slope – Once you've picked a relatively flat area of your yard, you'll want to gauge the slope, test the soil type and determine the plot size. Typically rain gardens only need to be about four to eight inches deep. You can determine the slope with a few wooden stakes, a piece of string and a carpenter's level, a simple line level or a laser level, all of which are available at hardware stores.
Measuring slope is a simple calculation. Stretch the string horizontally between stakes placed about 15 feet apart. Check that the string is level. Measure the distance (width) in inches between the two stakes, then measure the height on the downhill stake between the string and the ground. Divide the height by the width and multiply by 100 to find the percent slope in that part of your lawn. If the slope is less than four percent, plan on a rain garden about three to five inches deep. If the slope is between five and seven percent, figure you will need a rain garden that is a bit deeper, say six to seven inches. If the slope is between eight and 12 percent, you will need a rain garden about eight inches deep. For greater than 12 percent slope, consider working with a professional landscaper because you'll have to dig a bit deeper and will likely need some equipment.
Look at Soil Types and Estimate Garden Size – The rain garden manual offers similar practical tips for evaluating if the soil is generally sandy, silty or clayey. Sandy and silty (also called loam) soils drain much faster than clayey soils. The size of the rain garden will have to be larger if it is draining clayey soils that allow water to seep in more slowly. Typically soils are mixtures of sand, silt and clay. Once you have an idea of the soil types and drainage pattern, you can decide to either work with what you've got or increase your soil's permeability. "Mixing compost or mulch into your soil really increases infiltration later," Bannerman says. "Incorporating dead leaves or dried grass clippings boosts permeability as well."
The manual also provides tips on estimating the square footage of the area that will drain toward the rain garden(s) from roofs and yards. Both are important factors in setting the size, shape and configuration. A rain garden in sandy soils needs to be about 20 percent of the size of the drainage area, 30 percent for silty or loamy soils and 60 percent in clay soils. Typical rain gardens for residential homes range from 150 to 400 square feet and are wider than they are long. The how-to manual gives directions for shaping the garden and creating a lip or berm on the left, right and downhill side of the garden to contain water.
Give your rain garden an outlet, too. Just like an overflowing tub, there are times when it may overflow the berm. Rain gardens are meant to handle average storms and you don't want to drown or wash out your plants when a big rain comes along. When it overflows, you want to give the water a path to follow the natural drainage pattern and keep flowing away from your house or lot.
Plant Selection – In most rain gardens plants are alternately deluged then left high and dry. So, the trick is choosing hardy species that can handle both situations. Some of the best vegetation for rain gardens in the Upper Midwest comes from the native prairies that adapted to these same conditions. Prairie flowers have deep roots that may go down ten feet. When the weather is dry, they can access sources of water unavailable to other vegetation, and when it's wet, their root structures provide deep conduits where rain can flow.
If a prairie garden isn't what you envisioned, there are more traditional trees, shrubs and flowers that "don't mind getting their feet wet," says UW-Extension Home Horticulturist Helen Harrison. She recommends starting with smaller quantities of a variety of plants to "see what works with your site and what doesn't, then smooth out any kinks."
The manual offers tips for designing attractive gardens and shares eight patterns including ample plant selection. If the scientist and engineer in you designed the garden specifications, then free the artist within to select a visual palette for the floral display. Think about plantings that will provide a variety of height, color and texture in each season. If you want the garden to act as a screen to provide a bit of privacy, block the wind or provide shade, consider planting taller species along that border and cascade plants that grow in descending heights toward the side that you see most frequently from your house, patio or windows.
Setting plants that will bloom at different times of the gardening season will extend your flowering period. Plants with flowers, fronds, curly leaves, variegated foliage, fruits and colors all add texture and interest to the plantings. In laying out your garden plan, think about clumping three to seven or more plants of the same color to establish splashes of color in the garden patch, then repeat these groupings if you want a more cohesive, traditional look. Also consider mixes of flowering and fruiting plants with some grasses, sedges and rushes. These will develop dense underground networks of roots that can outgrow and crowd out weed species. Consider planting low groundcover that can spread and fill in over time to shade out weedier species.
You may have other aims for the rain garden like attracting butterflies or creating backyard habitat and food for birds. Build those into your plan as well. Or you may want some taller species in part of the garden to provide a shady patch that's perfect for a garden bench or trail border. Take the time to visit local arboretums, nature centers and garden centers to get ideas about hardy perennial species that won't require much maintenance and will develop a nice mat of roots to absorb and hold moisture after a rainstorm.
When it's time to plant, follow the old tree planter's maxim and dig a $2 hole for every dollar's worth of planting. Since rain gardens are subject to sudden flushes of forceful water, you are better off starting with one- to two-year- old plant sets that already have well established roots. Dig each hole twice as wide as the plant plug. Loosen the soil and plant them just deep enough to keep the crown at grade level. As each area is planted, cover the spaces between plantings with double-shredded mulch to minimize moisture loss, act as a weed barrier and allow stormwater to infiltrate. Water the plants gently with about an inch of water a week unless the weather does that work for you. Once the plantings are in place for a few years, their roots should be deep enough that they can fend for themselves as long as you top dress them with a little fresh mulch during their first few years.
Mulch keeps the weeds down, acts as a sponge to hold in moisture and works as a filter to remove oils and other grit near the surface before the rain seeps down toward groundwater. Avoid cypress mulch. It's effective, but it's made by chopping down rare, old growth cypress in Southeastern wetlands. Shredded hardwood mulch works particularly well in rain gardens because it doesn't float or blow away.
Maintenance – No matter what kind of garden, expect that you'll need to weed until the plants you want become well established. don't rely on pesticides and fertilizers to keep a rain garden growing. You don't want those compounds soaking downward or running off onto paved areas. As the plantings mature, dense roots will squeeze out weeds. When you put your garden to bed for the season, leave the seed heads and stalks in place to provide winter cover and then cut them back in the early spring when four inches or so of new growth appear. A string trimmer or gentle wiggling of the old stems usually is adequate to loosen up the dried, dead stalks from the previous year without disturbing the root or new growth.
Enjoy! – The last step in this whole process, emphasizes Bannerman, is to enjoy the results! Your rain garden will add more birds, butterflies, colors, sounds and smells to your life. And start feeling good that this garden offers a path and a means to get rainwater back in the ground and back into the local water cycle where it belongs.
David L. Sperling edits Wisconsin Natural Resources magazine. This article is based on "Rain Gardens" by Karen Cozzette.
Blooms to watch
Here are just a few of the possible plantings to consider in a rain garden: