Rain Gardens: A Low Tech Approach to Stormwater Management

by Angela Nelson, CESCL

Thanks to Angela Nelson and 2020 Engineering for sharing their expertise on this emerging approach to stormwater management. At the end of the article, we offer a list of plants we believe would be well suited to rain garden plantings based on our experience of field growing them in our sandy loam soil.

What They Are

Raingardens are landscaped depressions designed with specific soil characteristics that are gaining popularity as stormwater filtration and retention devices in lieu of more traditional civil engineering methods. They are a part of a new approach to stormwater management termed Low Impact Development (LID). In low impact development, the landscape component of the project is integral to the stormwater management design, enhances water storage, and attenuates storm flows (Low Impact Development Technical Guidance Manual for the Puget Sound). Rain gardens are aesthetically pleasing, providing beautification to commercial and residential landscaping while at the same time treating and slowing down stormwater flows in a way that mimics natural systems.

Benefits of Rain Gardens

There are many benefits to using raingardens. In residential settings they provide a natural system to soak up stormwater from roofs and driveways, decreasing flooding in yards and neighborhoods. Because water is slowed down through filtration, water levels found in adjacent ditches and streams are reduced during peak storm events, reducing erosional processes and damage associated with high flows. In a commercial setting, rain gardens create an aesthetically pleasing area around parking lots and walkways. They filter oil, grease and gasoline out of the runoff from parking lots, decreasing pollution loads to conveyance systems that eventually lead to streams, rivers, lakes and the Puget Sound. In an urban setting, rain gardens provide habitat for insects, birds and small mammals.

Environment They Create

As we begin to consider the most suitable plant species for this type of garden, we must take into account the growing environment that they create. The name “rain garden” would have one believe that it is a wet environment typically inundated with water. In actuality, rain gardens are engineered to infiltrate large amounts of water in very short periods of time. The porous nature of the soil matrix required creates dry soil conditions most months of the year.

In Western Washington, the typical rain event for a day (outside of the rain forest) is between .05 and .3 inches of rain. Rain gardens are designed to filter 1-2 inches of stormwater per hour. Their size is calculated as a percentage corresponding to the amount of impervious area generating runoff to them. On average, the rain garden bottom is around eight percent of the size of the impervious surface (roof, parking lot, road way). Large impervious areas draining to rain gardens result in significant flows of water through these filter systems. Ponding conditions that last more than a day in a rain garden are a result of significant storms on the magnitude of a hundred year storm event.

In order to achieve proper drainage, soils for rain gardens should be comprised of approximately 60-65% loamy sand and 35-40% compost. Clay content of soils should be less than 5% (Low Impact Development Technical Guidance Manual for the Puget Sound). Typical compost amended soil depth of a rain garden is between 18-24 inches deep. The compost amended soil is placed on top of existing native soils that have high to moderate drainage capacity. In Western Washington, existing soils suitable for infiltration from rain gardens would be Type A and B soils as classified by the Natural Resources Conservation Service. In areas where existing native soils have poor drainage capacity, engineered drain rock reservoirs can be constructed below amended soils to allow for the slower infiltration into native soils.

This soil moisture regime is most conducive to plants designated facultative – that is plants found in mesic rather than wet conditions. Rain gardens have been divided into three zones characterized by soil moisture and periodic inundation. Zone 1 is located in the bottom of the rain garden where soils are often saturated during the rainy season. Zone 2 is found along the side slope of the raingarden that may be exposed to occasional standing water for short periods of time and Zone 3 is the area along the top of the raingarden where soils are well drained, without irrigation, and dry outside of the wet season. Water does not often distribute evenly over the rain garden during typically rain events. Depending upon flow design and rain garden location, Zone 1 does not always greatly differ from Zone 2 in soil saturation.

rain garden cross section

Native Plants for Raingardens

As native plants gain popularity in the urban landscape, the opportunity exists to groom a wide variety of native perennials and ground covers to achieve the standards of beauty normally ascribed to ornamental varieties. Rain gardens open up a new growing paradigm for our beloved natives. As more and more cities are encouraging or mandating the use of native plants in home gardens and commercial landscaping, people desire plant options beyond the usual limited selection of shrub species installed. An expanded palette of native annuals, perennials and shrubs to choose from is becoming available.

raingarden plan view

When creating a planting plan for the rain garden consider a scheme of native plants that includes ornamental native species. Native perennials such as penstemons, lily’s, lupines, columbines, and asters can be cultivated to provide bright colors in the garden spring through fall. Below is a suggest list.

Native plants installed in rain gardens do benefit from maintenance and grooming. Watering plants a few times a month in July, August, and September can keep natives more vibrant into the fall, staving off dormancy and prolonging blooming. Their natural resistance to drought during the summer months will ensure plant survival but some additional water will help the garden to thrive. Native shrubs such as Lonicera involucrata, Symphorocarpus albus, Physocarpus capitatus, and Rosa nutkana respond well to being cut back in the fall. Pruning promotes growth and blooming in these species. Take care with Cornus stolonifera, it is susceptible to a fungus that enters the plant after pruning and should be left alone.

list of suggested plants

Rain Garden Maintenance

In considering the long term maintenance and care of a rain garden, mycorrhizal mushrooms could play a significant role in plant and soil health. The relationship between plants and fungus has been well documented. Plants with mycorrhizal fungal partners have the ability to absorb more nutrients and resist disease. Mycorrhizal mushrooms also have the ability to filter and break down polluted sediment and stormwater. In Paul Stamet’s book, Mycelium Running, How Mushrooms Can Help Save the World he writes extensively on mycofiltration and mycoremediation.

Through his experimentation, he has demonstrated that the Oyster mushroom (Pleurotus ostreatus) has the ability to break down hydrocarbons in contaminated soils. He writes “The roots of mushrooms, called mycelium, produce enzymes that unlock wood fibers, which are composed of strings of carbon-hydrogen molecules in the form of cellulose and lignin. Similarly oil and most petroleum products are held together by these same molecular bonds. The mushroom mycelium breaks these bonds, and then re-constructs the oil into carbohydrates, fungal sugars, that make up the mushroom’s physical structures.” The application of mycoremediation in rain gardens has not yet been explored. The possibility for long term care of rain garden soils through inoculation with fungus is probable and needs more research.

Rain gardens provide us with a new concept in stormwater treatment and new venue for native plant cultivation. Enjoy the process of designing these gardens for stormwater management at your home, as part of a neighborhood plan, or in residential and commercial developments.