A rain garden is a shallow planted depression that receives runoff from a roof, driveway, patio, or other hard surface. It gives rainwater time to slow down and soak into the soil instead of immediately moving across the property. Planted with native perennials, grasses, sedges, and shrubs, it can also add flowers, structure, and wildlife habitat to the landscape.
It is not a pond edge or a permanently soggy garden bed or spring. It should hold water temporarily after a storm and then drain, generally within 24 to 48 hours. That cycle of occasional flooding followed by drier conditions is one reason choosing the right plants matters.
Why Fall Is a Good Time to Build a Rain Garden
By fall, you have had a full growing season to notice how rain moves through your property. Instead of guessing where water travels, go outside during a good rain and watch it. Look for water leaving downspouts, crossing the lawn, washing soil from a slope, or concentrating along the edge of a driveway or patio.
Fall also brings cooler planting weather while the soil is still warm. Hardy perennials planted now can begin settling their roots into the surrounding soil before winter and will already be in place when spring growth begins.
Choose the Site Carefully
A low area that receives runoff can be a good candidate for a rain garden, but an area that stays saturated for days may not be. Groundwater seepage, a high water table, or an existing wetland calls for a different approach.
Before digging the garden, test how quickly the soil drains. Dig a hole about a foot deep in the proposed area, fill it with water, and observe it over the next day. If the water remains after 24 hours, investigate the drainage further or choose another location.
Keep the rain garden away from structures and underground systems. NC State Extension recommends locating rain gardens at least 10 feet from a building foundation and at least 25 feet from a wellhead or septic system drain field. Have underground utilities located before excavation.
Follow the Water
The best location is usually somewhere between the source of runoff and the place the water would otherwise leave the landscape. A downspout is an easy example. Rather than allowing roof water to rush across the lawn, the downspout can direct it toward a shallow basin located a safe distance from the house.
Pay attention to both ends of the system. The inlet needs to accept water without eroding the soil, and the garden needs a safe overflow route for unusually heavy storms. A small area of stone at the inlet can help slow fast-moving water. The overflow should lead away from buildings and toward a stable area where it will not create a new erosion problem.
Build a Shallow Basin, Not a Deep Hole
For a typical residential rain garden, NC State Extension recommends a shallow depression about 4 to 6 inches deep. The bottom should be relatively level so water spreads across the garden instead of pooling in one deep corner.
On a gentle slope, soil removed from the uphill side can be used to build a low berm along the downhill edge. The berm helps hold runoff in the basin long enough for it to infiltrate. Cover disturbed soil promptly with plants or mulch so the new garden does not become an erosion source itself.
A broad, shallow garden is usually more useful than a narrow, deep pit. The goal is to spread water out and give it time to enter the soil.
Working With Clay Soils
Heavy clay does not automatically rule out a rain garden, but it makes the drainage test especially important. If the site drains adequately but the soil needs improvement, compost can be incorporated into the existing soil to improve structure and pore space.
Avoid trying to fix clay by mixing in sand. NC State Extension specifically cautions against this because some combinations of sand and clay can create a very dense soil. Work with the soil you have, improve its structure when needed, and make sure the finished garden can drain before planting it heavily.
Plant According to Moisture Zones
A rain garden does not have one uniform moisture level. The lowest part of the basin usually stays wet longest after a storm. Slightly higher areas drain sooner, while the upper edge may become fairly dry between rains.
You can use those differences to grow a much more interesting garden. Before planting, watch how water behaves in the finished basin during a rain. Notice where it lingers and which areas drain first.
Plants for the Lower Basin
The lowest areas need plants that tolerate temporary inundation as well as periods without standing water.
Mad-dog Skullcap (Scutellaria lateriflora) is a compact native perennial that handles moist and wet soil well. White Turtlehead (Chelone glabra) is another strong choice for the lower basin and provides late-season flowers. Great Blue Lobelia (Lobelia siphilitica) naturally suits moist sites and adds vivid blue flowers in late summer.
Fringed Sedge (Carex crinita) adds a grassy layer and a dense root system to wetter portions of the planting. Wild Mint (Mentha arvensis) is at home in moist soil and can spread into a fragrant ground layer, while Sweet Grass (Hierochloe odorata) brings fine grassy texture to sunny or partly shaded areas that stay on the moist side. Sweet Black-Eyed Susan (Rudbeckia subtomentosa) adds height and late-season flowers where the lower basin grades into the middle zone. In a larger rain garden, Buttonbush (Cephalanthus occidentalis) can provide a woody anchor where there is enough room for a shrub.
Plants for the Middle Zone
The middle portion receives plenty of moisture during storms but generally drains sooner than the bottom.
Tufted Hair Grass (Deschampsia cespitosa) is useful here as a clump-forming grass that adds fine texture between flowering plants. Shining Blue Star (Amsonia illustris) brings pale blue spring flowers and substantial foliage. Mistflower (Conoclinium coelestinum) adds lavender-blue flowers late in the season, and New York Aster (Symphyotrichum novi-belgii) extends the flowering season into late fall. Green and Gold (Chrysogonum virginianum) can form a low ground layer in partly shaded portions of the garden. Where there is room for a taller, spreading plant, Thimbleberry (Rubus parviflorus) adds larger foliage and a different layer of structure. Nash's Blue-eyed Grass (Sisyrinchium) can fill smaller spaces along the transition between zones.
Plants for the Upper Edge
The upper edge spends less time under water and may dry considerably between storms. Plants with broad moisture tolerance fit well here.
Yarrow (Achillea millefolium) handles a wide range of conditions and can connect the rain garden to the surrounding planting. Dense Blazing Star (Liatris spicata) adds upright summer flowers, while Goldsturm Orange Coneflower (Rudbeckia fulgida 'Goldsturm') provides long-lasting yellow-orange color in a sunny garden. The faster-draining edge also opens up room for Bradbury's Bee Balm (Monarda bradburiana), Early Sunflower (Heliopsis helianthoides), and Purple Giant Hyssop (Agastache scrophulariifolia) for a longer succession of flowers and more height. In part shade, Cherokee Sedge (Carex cherokeensis) and Nodding Fescue (Festuca subverticillata) add a finer-textured grassy layer without requiring the wetter conditions of the basin bottom.
These zones do not need to look like three rings. Let the plantings overlap and repeat species throughout the areas where their moisture preferences fit.
Plant Densely
A rain garden benefits from more than a few widely spaced plants surrounded by mulch. Leaves and stems slow water at the surface, while roots help stabilize soil and create pathways for water deep below ground. As the planting fills in, it also leaves less open soil for weeds.
Small starter plants are especially useful for this kind of project because they make it practical to install more individual plants instead of spending the entire planting budget on a few large pots.
Measure the planting area before ordering. A 50-square-foot rain garden can use dozens of plants when they are spaced according to their mature size. Repeat plants in groups rather than buying one of every species. Repeating groups of three, five, or more creates continuity, and grasses and sedges can be woven between flowering perennials to fill the ground layer.
Plan for Overflow
A rain garden is not meant to contain every drop from the largest possible storm. During heavy rainfall, excess water needs a safe way to leave the basin.
Choose the overflow route before you finish construction. It should carry water away from buildings and neighboring property without cutting a channel through exposed soil. Stone or established vegetation can help stabilize the outlet.
Watch the First Big Rain
Once the garden is established, go outside during the next substantial rain and see how it performs. Water should enter without washing soil away, spread through the shallow basin, and leave through the planned overflow if the basin fills.
After the storm, the standing water should gradually disappear. If water remains for more than 48 hours, revisit the drainage, the amount of runoff entering the garden, or the garden's size and construction.
A Garden Designed Around the Rain
A rain garden can solve a practical landscape problem while adding another planted area to the yard. Instead of moving runoff away as quickly as possible, you are giving some of that water a safe place to slow down and soak into the ground.
Start by watching your property during a storm. Find the runoff, test the soil, choose a safe location, and build the basin before choosing the plants. Once you understand how water moves through the site, the planting plan becomes much easier.
Our favorite rain gardens include native perennials, grasses, sedges, and shrubs suited to the changing moisture conditions found in rain gardens. Fall is a good time to get them established so the garden is ready to grow and protect soil when spring returns.
