Now the fun begins! Bring your rain garden to life with a variety of interesting plants. Choose plants only after taking into account the many factors that contribute to the long-term success of your garden.
Garden design and plant considerations
Natives. Select plants native to your area to reduce maintenance—natives are already adapted to local climate, soil, and pests.
Sun vs. shade. Depending on your rain garden site, choose plants best-suited for full sun, part sun, or shade. With so many options among trees, shrubs, perennials, and grasses, you are bound to find the right plants for your space. Be aware that if you install trees or large shrubs, they will increase the amount of shade on your rain garden as they grow.
Integrated planting. Mix plants of different heights, textures, colors, and bloom seasons. Variety is visually appealing, creates year-round interest, and reduces the chance of disease spread. Design your plant layout on paper first, using colored pencils to draw circles of various sizes to represent different plant groupings. If you purchase your plants, set the containers out in the garden according to your plan and assess their positioning and how they look together. Rework the arrangement until you achieve the visual effect you desire.
Start with small plants. One to three-gallon sizes are easiest to plant and establish quickly. Look for plants with well-established, healthy root systems. You can also start seedlings indoors or get transplants from friends.
Water tolerance. The most water-tolerant plants go in the center of the garden where they may need to survive one or two days of standing water. As you move toward the outer edges of the garden, you can use plants that are less susceptible to damage from wet soil. Place sturdy plants around the water inflow and outflow (weir)—or the points at which water velocity and volume will be greatest.
Putting plants in the ground
Follow good planting practices. Dig the hole for each plant twice as wide—and only as deep—as the container. Mix the soil from the hole with compost, loosen and spread the plant roots, set the plant in the hole so that the crown is at or slightly above the surface of the ground, and backfill the hole with the original soil mix. Firmly tamp down the soil around the plant roots and water thoroughly. Finish the garden with 2 to 3 inches of hardwood mulch, keeping the mulch from piling up against plant stems. Do not let roots of new plantings dry out. Water twice a week until plants are established, which may take up to a year.
A good example—The North Carolina Arboretum
The photo below of a rain garden at The North Carolina Arboretum in Asheville showcases many of the features we’ve discussed in our blogs. While larger in scale than a home garden, the design elements remain the same and provide a learning model for any homeowner thinking of building a rain garden.
This rain garden is strategically located on a downward slope and sized to handle large volumes of rainwater runoff from the garden and street above. Note that the garden basin is carved out well below street level, or the source of the water runoff. After digging the basin, the contractors layered different materials to ensure water infiltration. First, a porous filter fabric, lining the bottom of the hole, separates sediments and prevents them from creeping down into the ground below. Next, a layer of perforated pipe covered with a layer of gravel promotes drainage from the center of the garden. The final layer is a soil mix that allows water to drain quickly and anchors the plants.
The sodded slope at the street helps slow the water down and disperse it as it moves toward the basin. The stone retaining wall hides an underground pipe that empties water into the garden. There is a rock apron laid in front of the pipe to absorb the force of the water, help prevent soil erosion at the inflow point, and collect silt and debris.
This rain garden absorbs water into the ground of at the rate of 1 inch of water per hour. On rare occasions, overflow is routed to another underground pipe hidden beneath the tall shrubs on the downhill side of the garden.
Most of the plants are native to the area and provide a mix of foliage colors—chartreuse, burgundy, blue green, and deep green—as well as a variety of textures—feathery, stiff, upright, dense, and open.
Plantings are grouped according to their water tolerance and are able to withstand dry conditions, as well as periodic submersion.
Dwarf blazing star (Liatris microcephala) is closest to the inflow point and set off by a backdrop of miniature tall bearded Iris (Iris fulva), and switchgrass (Panicum virgatum ‘Prairie Sky’), as well as cardinal flower (Lobelia cardinalis), a showy, red-flowered perennial. Planted in the lowest part of the basin, where most water accumulates, are common rush (Juncus effuses) and swamp hibiscus (Hibiscus moscheutos). Moving outward into the middle zone are a variety of shrubs—arrowwood viburnum (Viburnum dentatum), Virginia sweetspire (Itea virginica ‘Little Henry’), and fothergilla (Fothergilla x intermedia ‘Mount Airy’). The tallest plants anchor the top of the berm on the downhill side of the garden—common winterberry holly (Ilex verticillata), a lavender twist redbud, and three deodara cedars (Cedrum deodara ‘Shalimar’). The hollies and trees provide height, structure, and winter interest, but will never get large enough to shade this sun-loving garden. The berm is finished off with a low-growing, creeping juniper (Juniperus communis ‘Green Carpet’) that provides a transition from rain garden to lawn beyond.
While every rain garden is unique, yours at home—like this one at the Arboretum—will help solve a stormwater runoff problem, benefit the environment, and add to the beauty of the landscape.
For more information about gardens at The North Carolina Arboretum, visit http://www.ncarboretum.org/ .
Watch for the final blog in this series: Rain Gardens—Part IV: Regular Maintenance Keeps It Clean and Healthy.
Resources with plant lists for rain gardens:
Rain Gardens—the Plants
by PennState Extension
Good tree, shrub, and perennial plant list separated by three rain garden zones—wet, middle, and transition.
Rain Gardens: Green Solutions to Stormwater Pollution
by Clemson University, South Carolina, Feb. 2009
Provides A to Z information with good diagrams. Be sure to adapt plant and soil recommendations to N.C. Mountains.
Native Mountain Rain Garden Plants for Shady Areas
by Transylvania County, NC Cooperative Extension
Article written by Beth Leonard, Extension Master GardenerSM Volunteer.