For gardeners like me, rainfall is a blessing. Precipitation brings the promise of healthy plants and the ensuing joy of watching buds and blossoms burst forth. On the flip side, too much rain can be harmful or even destructive. A rain garden is an effective, natural method of filtering and controlling waterflow that also adds beauty to the landscape. What follows is an introduction to rain gardens to spark your interest in creating one on your own property.
What is a rain garden?
A rain garden is a shallow depression in the land, usually on a slope, that contains native perennials, flowering plants, grasses, and shrubs. By design, it is a temporary holding spot for rainwater runoff that comes from rooftops, driveways, sidewalks, lawns, and nearby hardscape areas. Basically, a rain garden collects rainwater, absorbs it, then slowly releases that water into the ground below. In so doing, a rain garden replenishes ground water and prevents the sudden gushing of runoff filled with potentially hazardous materials into various bodies of water.
Why create a rain garden?
The main function of a rain garden is conservation of water. A high volume of rain or sustained rainfall over a relatively short period of time produces unwanted stormwater runoff, especially from impervious surfaces such as concrete or asphalt. This stormwater carries just about everything along its way, from animal waste to fertilizer, pesticides, garbage, motor oil, gasoline, chemicals, and bacteria. Laden with these non-point source pollutants, untreated runoff flows into nearby streams, ponds, lakes, rivers, bays, and eventually into the ocean. According to data from the U.S. EPA, rainwater runoff is responsible for 70% of all water pollution, so this is a significant problem. A rain garden removes many of the undesirable contaminants and sediment through percolation, plus it enables 30% more water to soak gradually into the ground around the collection site. In other words, a rain garden improves water quality, supports flood control, and absorbs essential groundwater.
Beyond the functional benefits for water quality and physical terrain, a rain garden contributes to landscape aesthetics and provides sustenance for valuable pollinators such as birds, butterflies, and other insects. A rain garden should include native plants well-adapted to local climatic conditions that can handle high levels of moisture and thrive without fertilizer. These native plants are important sources of nourishment for local pollinators, who in turn help to ensure reproduction of those species. Once these plants are established (after the first year), they generally require very little maintenance.
How do you establish a rain garden?
As you approach the planning phase, keep in mind that a rain garden can be designed to fit your site and accommodate your preferences. Basically, the area should be approximately 20% of the surface area of your roof or the hardscape area near your house that will drain into the rain garden. If that’s not feasible for your setting, a smaller rain garden can still have a positive impact. A rain garden should be at least 10 feet away from your house and placed perpendicular to the natural slope for maximum stormwater capture. A 12% grade works well. Ideally, a rain garden is positioned to catch waterflow from a couple downspouts, and it should not be near a well, drain field, or septic system. Avoid the dripline of trees, since those roots may not be able to manage extended periods of soggy soil.
When determining the desired size and shape of a rain garden, aim for the length to be twice as long of the width. Curved designs, such as an oval or kidney shape, fit nicely into natural surroundings. The recommended depth of a rain garden varies, depending on soil type and texture. For clay soil, common in the Virginia Piedmont, the top 8-10 inches of soil should be removed and replaced with sand, due to slow infiltration. For sandy soil, remove 7-8 inches of existing soil and replace it with a mixture of sand and topsoil. For silty soil, dig up 5-6 inches of existing soil and amend that with sand and topsoil mixture. The rain garden should be a shallow depression just slightly lower than the surrounding earth.
IMPORTANT: Be sure you contact Ms. Utility in advance of excavation, so digging won’t interfere with buried electrical, gas, water, sewer, or telecommunications lines. Ms. Utility will mark those lines, so you know exactly where they are. TIP: Take photos for easy reference in the future.
Following excavation, make sure the bottom of the rain garden is level and scarified (scraped to make a rough surface, rather than smooth), and plan for soil amendments. Enhancing soil texture will improve the capacity of a rain garden to hold water, and enriching soil quality will help plants thrive. Depending on existing soil type (as noted above), a mixture of coarse sand and loamy compost should be added into the planting area. Optimally, a rain garden infiltrates water at about ½ inch per hour. You may want to conduct an infiltration test to determine whether the chosen site with amended soil will function well. By the way, a rain garden should allow water to seep into the ground within 36 hours, so that mosquitos do not have a chance to get established there.
Selecting Plants for a Rain Garden
The next step is the most enjoyable: choosing plants for your rain garden. Fortunately, many excellent online resources can help you make appropriate selections based on your chosen site conditions: size, full/partial sun or shade, soil type, grade of the slope, and possible need for deer-resistant plants. I highly recommend these free resources:
- Montgomery County, MD RainScapes planning document offers user-friendly planting design templates and detailed descriptions of recommended plants.
- Planning your garden from Rutgers University and the New Jersey Native Plant Society and Water Resources Program is also a fantastic guide.One of my favorite sources for choosing rain garden plants is from the University of Wisconsin Extension Program. This how-to manual provides plant lists by Latin and common name, bloom time and color, plus height and spacing requirements, with charts organized by soil type and sun/shade conditions. A terrific resource to make your dreams of a beautiful rain garden come true!
Maintenance of a Rain Garden
Good news! Once you have installed your rain garden, taking care of it is surprisingly easy. With native plants, only occasional watering is needed during dry periods, and you can avoid fertilizer and pesticides. Weeding is the main activity in the first year, but after plants are well-established, weeds are less likely to be problematic. You will also want to remove debris, such as leaves, from the rain garden to facilitate drainage and prevent that area from retaining standing water.
Does it work? My Experiment
As testimony to the effectiveness of a rain garden, I share my own story. This past year we built a new solar home on an elevated ridge in Earlysville. The house sits on a natural slope, so we were aware of potential issues with stormwater runoff. A rain garden seemed like a good solution. I designed one to: 1) capture rainwater from gutters and downspouts, 2) carry that rainwater in underground pipes, and 3) release that runoff into a kidney-shaped rain garden. This area is strategically located about 25 feet away from the house at a lower elevation. After observing waterflow on rainy days during the construction process, I figured out the natural pathway for that stormwater runoff.
We hired an excavator to dig a shallow hole at the identified location where the water-laden pipe from downspouts would empty on rainy days. We placed a layer of stones at the bottom because of clay soil here and topped it off with sand and then loamy topsoil. For aesthetic reasons, we added large boulders (from another part of our property) all around the rain garden. I researched, selected, and planted native species recommended for rain gardens, with shrubs, young plants, and seeds in the mix. I crossed my fingers that our set-up would work. Sure enough, just three weeks after our move-in, a torrential downpour arrived. Grass was not yet established, so I watched with trepidation as rivers of water went rushing downhill. Aha! The rain garden filled with water, serving as a temporary holding pond. Slowly, over the next few hours, that water seeped into the ground below. Without the rain garden, water bursting from downspouts would have gushed down into the pond and streams below our house, carrying good topsoil and whatever else the stormwater picked up enroute. So far, so good.
As you may recall, our region experienced a drought this past spring and summer. I was concerned about those new plantings and watered the rain garden every three weeks. I weeded the area a couple times, so that undesirable plants would not steal precious water. To my astonishment, the native plants survived and thrived. By September we enjoyed views of colorful flowers, tall grasses, and small shrubs with berries, which numerous pollinators visited on a regular basis. In November, I trimmed the longer stalks, leaving stems at a height of two feet for insects to use as homes during the winter months. As December began, the rain garden continued to be a source of beauty in our side yard, with friendly birds using the boulders and plants as landing spots while searching for food nearby. I would say my experiment was a success!
Conclusion
With this introduction to rain gardens, I hope you consider establishing one of your own. The resources below will give you a wonderful head start. Special thanks to Dorothy Tompkins for her expert input on this topic. A visit to Dorothy’s property during my PMG training was the initial inspiration that led me to create one!
Resources for Rain Gardens
Soak Up the Rain: Rain Gardens | US EPA
All About Rain Gardens – What They Are & How to Build One (groundwater.org)
RGPlantingDesigns2020.pdf (montgomerycountymd.gov)
Rain Garden Basics | Rain Gardens | Washington State University (wsu.edu)
How To Build a Rain Garden | Rain Gardens | Washington State University (wsu.edu)
Rain Gardens – A how-to manual for homeowners (dc.gov)
An Introduction to Rain Gardens (psu.edu)
https://www.epa.gov/watersense/what-plant
Water Resources Program at Rutgers NJAES
Rain Garden Manual of New Jersey by Rutgers Cooperative Extension Water Resources Program – Issuu
Basic Information about Nonpoint Source (NPS) Pollution | US EPA
Simplified hydrologic cycle (weather.gov)
