A rain garden is a shallow depression in the landscape, planted with water-tolerant species, designed to capture and absorb stormwater runoff from roofs, driveways, and lawns. It is not a pond. It is not a swamp. It is a engineered garden that holds water for 24 to 48 hours after a storm, then drains completely through the soil.
The first rain garden I built was a failure. I dug a hole, planted some irises, and directed a downspout into it. Within a month, it was a stagnant puddle that bred mosquitoes and killed the plants. The problem was that I skipped the engineering. A rain garden is a hydrological system, and if you do not understand the water flow, the soil, and the sizing, you get a mud pit instead of a garden.
This guide covers the technical design of a rain garden, from site selection through plant selection. The calculations and specifications come from stormwater management guidelines and from building three rain gardens in different soil types over the past eight years. The goal is to give you the information to build one that works the first time.
What a Rain Garden Actually Does
Before building one, you need to understand the hydrology. A rain garden intercepts stormwater that would otherwise run off your property, carrying fertilizer, oil, and sediment into storm drains and streams. The garden captures this water, holds it briefly, and allows it to infiltrate into the soil.
The Water Cycle Problem
In a natural landscape, rain falls on soil and plants, and most of it infiltrates into the ground. In a developed landscape, rain falls on roofs, driveways, and compacted lawns, and most of it runs off. A typical suburban lot generates 10 to 20 times more runoff than the same area in its natural state. This runoff causes flash flooding, erodes stream banks, and carries pollutants into waterways.
A rain garden reverses this locally. By capturing the runoff from a roof or driveway and allowing it to infiltrate, the garden restores the natural water cycle on a small scale. A properly sized rain garden can absorb 30 to 80 percent of the runoff from the impervious surface it serves.
The Infiltration Principle
The key to a rain garden is that it drains. The water does not stay. It infiltrates into the soil within 48 hours, which is fast enough to prevent mosquito breeding (mosquitoes need 7 to 10 days of standing water to complete their life cycle). If your rain garden holds water for more than 48 hours, something is wrong with the soil, the sizing, or the design.
The infiltration rate depends on the soil type. Sandy soils drain at 1 to 4 inches per hour. Loam soils drain at 0.3 to 1 inch per hour. Clay soils drain at less than 0.1 inch per hour, which is too slow for a rain garden without amendment. This is why soil testing (covered in the next section) is the first step in design.
What a Rain Garden Is Not
A rain garden is not a retention pond. Retention ponds hold water permanently. A rain garden is not a wetland. Wetlands have saturated soil year-round. A rain garden is dry most of the time, wet for a day or two after storms, and planted with species that tolerate both conditions.
A rain garden is not a rainwater harvesting system. It does not store water for later use. It infiltrates water into the ground, where it recharges groundwater and feeds streams through base flow (the slow seepage that keeps streams running between storms).
Site Selection and Percolation Testing
The location of the rain garden determines whether it works. The three factors are: where the water comes from, where it can go, and what the soil is like.
Locating the Water Source
A rain garden captures water from a specific impervious surface, usually a roof. Identify the downspout or surface that generates the most runoff, and plan the garden downslope from it. The garden needs to be at least 10 feet from the house foundation (to prevent water from infiltrating back toward the basement), and at least 5 feet from any property line.
The path from the downspout to the garden can be a shallow swale (a grassed channel), a buried pipe, or a surface channel. The swale is the simplest and cheapest. A buried pipe (4-inch corrugated plastic) is cleaner-looking but requires excavation and a consistent slope.
Slope and Overflow
The garden should be on a slope of 2 to 5 percent. Less than 2 percent and the water does not flow to the garden. More than 5 percent and the garden needs a berm (a raised edge on the downslope side) that becomes impractically large.
Every rain garden needs an overflow path. In a storm larger than the garden was designed for, the excess water needs somewhere to go that is not your basement or your neighbor’s yard. Design the overflow as a low spot in the berm that directs water to a safe discharge point (a lawn, a swale, or a storm drain). The overflow should be at the elevation where the garden is full, so excess water leaves before it backs up toward the house.
Percolation Testing
This is the step most people skip, and it is the one that determines whether the garden works. You need to know how fast your soil drains.
The simple percolation test: Dig a hole 12 inches deep and 12 inches wide in the proposed garden location. Fill it with water and let it drain completely (this saturates the surrounding soil). Fill it again and measure how long it takes to drain the second time.
- If the water drains in 12 hours or less (1 inch per hour or faster), the site is excellent for a rain garden.
- If the water drains in 24 hours (0.5 inch per hour), the site is acceptable but the garden should be sized larger.
- If the water drains in 48 hours (0.25 inch per hour), the site is marginal. Consider amending the soil or choosing a different location.
- If the water takes more than 48 hours to drain, the soil is too clay-rich for a rain garden without significant amendment or an underdrain.
Do the test in the exact location where the garden will be, because soil varies across a yard. Do the test in the wet season (spring or fall), because drainage rates are slower when the water table is high.
If the percolation test fails, you have three options. Move the garden to a better-draining location. Amend the soil by excavating the clay and replacing it with a sand-compost mix (expensive but effective). Install an underdrain (a perforated pipe at the bottom of the garden that carries excess water to a discharge point). The underdrain option is essentially converting the rain garden into a biofiltration system, which still provides water quality benefits but reduces the infiltration benefit.
Sizing Calculations for Your Roof
A rain garden that is too small overflows in every storm. A rain garden that is too large takes up yard space unnecessarily and may not receive enough water to support the plants. The sizing calculation is straightforward but requires measuring your roof.
Measuring the Drainage Area
Calculate the area of roof that drains to the downspout serving the garden. For a simple gable roof, measure the length and width of the roof section that slopes toward the downspout. For a hip roof or a complex roof, estimate the area of each section that contributes to the downspout.
Example: A house is 40 feet wide and 30 feet deep, with a gable roof running the width. Each side of the roof is 20 feet wide (from ridge to eave) and 40 feet long, for an area of 800 square feet per side. If the garden serves one downspout that drains one side of the roof, the drainage area is 800 square feet.
If the downspout serves only part of the roof (for example, one of two downspouts on one side), divide the roof area by the number of downspouts. Two downspouts on the 800-square-foot side means each serves 400 square feet.
The Sizing Formula
The size of the rain garden depends on the drainage area, the soil infiltration rate, and the depth of the garden. The standard formula from stormwater guidelines is:
Garden Area (sq ft) = Drainage Area (sq ft) / Sizing Factor
The sizing factor depends on the soil type and the garden depth:
- Sandy soil, 6-inch deep garden: sizing factor = 0.15 (garden is 15% of drainage area)
- Sandy soil, 8-inch deep garden: sizing factor = 0.10
- Loam soil, 6-inch deep garden: sizing factor = 0.25
- Loam soil, 8-inch deep garden: sizing factor = 0.20
- Clay soil, 6-inch deep garden: sizing factor = 0.40
- Clay soil, 8-inch deep garden: sizing factor = 0.30 (or use an underdrain)
Example calculation: Drainage area is 800 square feet. Soil is loam. Garden depth is 6 inches. Sizing factor is 0.25. Garden area = 800 / 0.25 = 200 square feet. A 200-square-foot garden (say, 10 by 20 feet, or 14 by 14 feet) handles the runoff from an 800-square-foot roof area in loam soil.
For clay soil, the same roof area needs a 320-square-foot garden (800 / 0.40), which is large. If you do not have the space, the options are to reduce the drainage area (redirect some downspouts elsewhere), increase the garden depth (to 8 inches, which reduces the factor to 0.30), or amend the soil to improve infiltration.
Garden Depth
Most home rain gardens are 4 to 8 inches deep. The depth is the vertical distance from the bottom of the garden to the surrounding grade (the overflow elevation). Deeper gardens hold more water per square foot, but they are harder to mow around and can be a tripping hazard.
A 6-inch depth is the standard for home gardens. It holds enough water to be effective (6 inches of water over 200 square feet is 100 cubic feet, or about 750 gallons) and is shallow enough to be safe and easy to maintain.
The Berm
The berm is the raised edge on the downslope side of the garden that holds the water in. The berm should be 6 to 12 inches high (high enough to hold the design depth plus a few inches of freeboard), 2 to 3 feet wide at the base, and compacted to prevent erosion. Cover the berm with grass or mulch to stabilize it.
Include a notch or low spot in the berm for overflow. The overflow should be at the design depth (6 inches for a 6-inch garden), so water exits before it reaches the top of the berm. Line the overflow with stone to prevent erosion.
Excavation and Soil Amendments
Once the garden is sized and located, the construction is straightforward but labor-intensive. Plan for a full weekend of digging for a 200-square-foot garden.
Excavation
Mark the garden outline with a hose or paint. Excavate to the design depth (6 inches for a 6-inch garden), plus an additional 6 to 12 inches if you are amending the soil. Save the excavated soil for the berm.
The bottom of the garden should be flat and level. A level bottom ensures even water distribution. If the garden is on a slope, the uphill side will need to be excavated deeper than the downhill side to create a level bottom. Use a level on a long board to check.
Shape the sides of the garden with gentle slopes (3:1 or flatter) to prevent erosion and to make mowing easier. Steep sides erode and are hard to maintain.
Soil Amendment
If your soil is loam or sandy and passes the percolation test, you may not need to amend. Simply loosen the top 6 to 8 inches of soil with a fork (do not rototill, which destroys soil structure) and mix in 2 to 3 inches of compost.
If your soil is clay or compacted, amend it. Remove the clay to a depth of 18 to 24 inches. Backfill with a mix of 50 percent sand, 30 percent compost, and 20 percent native soil. This “bioretention soil mix” drains well (1 to 2 inches per hour) and supports plant growth. The cost of the mix (delivered in bulk) runs 30 to 50 dollars per cubic yard, and a 200-square-foot garden needs 4 to 6 cubic yards.
Do not add gravel to the bottom of the garden. This is a common mistake. Gravel does not improve drainage (the water sits on top of the clay beneath the gravel). It just reduces the soil volume available for plants. The amendment should be mixed into the native soil, not layered beneath it.
The Inlet
The inlet is where water enters the garden from the downspout or swale. The inlet should be stabilized with stone to prevent erosion from the incoming water flow. A 2-foot wide apron of 1 to 3 inch stone, extending 3 feet into the garden, dissipates the energy of the incoming water and prevents it from scouring a channel.
If the water enters via a pipe, the pipe should discharge onto the stone apron, not directly onto soil. If the water enters via a swale, the swale should flare out as it enters the garden to slow the flow.
Plant Zones: Wet, Mesic, and Dry
A rain garden has three distinct zones based on how long water sits in each area. The plants in each zone must be adapted to the moisture conditions of that zone. Planting a dry-zone plant in the wet zone kills it. Planting a wet-zone plant in the dry zone stunts it.
Zone One: The Wet Zone (Bottom)
The bottom of the garden is the wettest area. It is inundated with water after every storm and may stay saturated for 24 to 48 hours. Plants here must tolerate periodic flooding and saturated roots.
This zone is typically the central 30 to 50 percent of the garden floor. The soil here is the amended bioretention mix, which drains well but is wet more often than the sides.
Zone Two: The Mesic Zone (Sides)
The sides of the garden, from the bottom up to the surrounding grade, are the mesic zone. This area is wet after storms but drains within hours. The soil here is moist but not saturated, and it dries between storms.
This zone is typically the sloped sides of the garden. Plants here must tolerate periodic wetness but also periods of drought (the sides can be dry between storms in summer).
Zone Three: The Dry Zone (Top and Berm)
The top edge of the garden and the berm are the dry zone. This area is never inundated. It receives water only from rain and from splash, and it dries quickly. Plants here must be drought-tolerant.
The berm, in particular, is well-drained (it is built of compacted soil and is elevated), and it can be very dry in summer. Plant the berm with deep-rooted species that can access moisture below the surface.
Plant Selection Table
| Zone | Moisture Condition | Light: Full Sun | Light: Part Shade | Light: Full Shade |
|---|---|---|---|---|
| Wet (Bottom) | Saturated 24-48 hrs | Blue flag iris, swamp milkweed, joe-pye weed, cardinal flower | Turtlehead, marsh marigold, great blue lobelia | Ferns (cinnamon, royal), jack-in-the-pulpit |
| Mesic (Sides) | Moist, drains in hours | Black-eyed Susan, purple coneflower, bee balm, New England aster | Columbine, wild geranium, foamflower | Wild ginger, solomon’s seal, ferns (marginal wood) |
| Dry (Top/Berm) | Well-drained, drought-prone | Little bluestem, butterfly weed, yarrow, coreopsis | Pennsylvania sedge, creeping phlox | Ivy, pachysandra (use natives where possible) |
Plant Selection Principles
Choose native plants. Native species are adapted to your local climate, soil, and rainfall patterns. They require less maintenance (no supplemental watering after establishment, no fertilizer) and they support local wildlife (pollinators, birds). Non-native species may survive but they do not provide the same ecological benefits and some become invasive.
Select for bloom time. A well-designed rain garden blooms from spring through fall. Plan the plant list so that something is in bloom at all times. Spring bloomers (marsh marigold, columbine) give way to summer bloomers (bee balm, black-eyed Susan) and fall bloomers (New England aster, goldenrod).
Plan for mature size. Plants in a rain garden grow fast because of the consistent moisture. Space plants based on their mature spread, not their current size. A garden that looks sparse at planting will fill in within 2 years. Overplanting leads to crowding and disease.
Use a mix of grasses, sedges, and forbs (flowering plants). Grasses and sedges provide structure (their fibrous roots also help maintain soil infiltration) and winter interest. Forbs provide color and nectar. A ratio of roughly 60 percent forbs to 40 percent grasses/sedges is a good starting point.
Plant Selection Table
Expanding on the table above, here are specific recommendations by region. Choose plants appropriate for your hardiness zone (check the USDA zone map for your location).
Northeast and Midwest (Zones 4-6)
Wet zone: Blue flag iris (Iris versicolor), swamp milkweed (Asclepias incarnata), joe-pye weed (Eutrochium purpureum), cardinal flower (Lobelia cardinalis), great blue lobelia (Lobelia siphilitica), turtlehead (Chelone glabra).
Mesic zone: Black-eyed Susan (Rudbeckia hirta), purple coneflower (Echinacea purpurea), bee balm (Monarda fistulosa), New England aster (Symphyotrichum novae-angliae), wild bergamot (Monarda fistulosa), oxeye sunflower (Heliopsis helianthoides).
Dry zone: Little bluestem (Schizachyrium scoparium), butterfly weed (Asclepias tuberosa), yarrow (Achillea millefolium), coreopsis (Coreopsis lanceolata), Pennsylvania sedge (Carex pensylvanica).
Southeast (Zones 7-9)
Wet zone: Southern blue flag (Iris virginica), swamp hibiscus (Hibiscus coccineus), cardinal flower, lizard’s tail (Saururus cernuus), cinnamon fern.
Mesic zone: Stokesia (Stokesia laevis), black-eyed Susan, purple coneflower, blazing star (Liatris spicata), spiderwort (Tradescantia ohiensis).
Dry zone: Muhly grass (Muhlenbergia capillaris), coreopsis, yarrow, little bluestem, gaura.
Pacific Northwest (Zones 7-9)
Wet zone: Pacific iris (Iris douglasiana), slough sedge (Carex obnupta), camas (Camassia quamash), monkey flower (Mimulus guttatus).
Mesic zone: Oregon sunshine (Eriophyllum lanatum), yarrow, Douglas aster (Symphyotrichum subspicatum), penstemon.
Dry zone: Idaho fescue (Festuca idahoensis), kinnikinnick (Arctostaphylos uva-ursi), Oregon grape (Mahonia aquifolium).
Planting Technique
Plant in spring or early fall. Fall planting is ideal in most regions because the plants establish roots over winter and are ready for growth in spring. Plant at the same depth as the plant was in its pot. Space plants according to their mature size (typically 12 to 24 inches apart for most forbs, 18 to 36 inches for grasses).
Water thoroughly after planting and keep the garden watered (1 inch per week) for the first growing season. After establishment (one year), the plants should survive on rainfall alone. Mulch with 2 to 3 inches of shredded hardwood mulch (not dyed mulch, not gravel) to suppress weeds and retain moisture. Replenish the mulch annually for the first 2 years.
Maintenance and Long-Term Performance
A rain garden is not maintenance-free, but it is low-maintenance compared to a traditional garden. The first 2 years require the most attention as the plants establish. After that, the garden largely takes care of itself.
First-Year Maintenance
Water weekly during the first growing season (1 inch per week, including rainfall). The plants are establishing root systems and are vulnerable to drought. Weeds are the main challenge in the first year, because the garden is disturbed soil and weeds colonize quickly. Weed every 2 weeks for the first season. Mulch heavily (3 inches) to suppress weeds.
Do not fertilize. The compost in the soil mix provides enough nutrients. Fertilizing promotes lush, weak growth that is susceptible to disease and that does not develop the deep roots the plants need to survive in the rain garden environment.
Ongoing Maintenance
After the first year, maintenance drops to a few tasks per year:
Spring: Cut back last year’s growth (leave 4 to 6 inches of stubble for nesting bees). Remove dead plant material. Check the inlet and overflow for sediment buildup and clear as needed. Replenish mulch to maintain a 2-inch layer.
Summer: Weed as needed (less every year as the plants fill in). Monitor for pest damage. Do not water (the plants are established and the rain garden is designed to be wet, not irrigated).
Fall: Leave the dead stalks for winter interest and wildlife habitat. The seed heads feed birds. The stems provide overwintering sites for beneficial insects. Cut back in spring, not fall.
Sediment Management
Over time, sediment from the runoff accumulates in the garden, particularly at the inlet. This sediment reduces the garden’s storage capacity and can smother plants. Inspect the inlet annually and remove accumulated sediment. Every 3 to 5 years, you may need to remove a layer of sediment from the entire garden floor and re-mulch.
If the garden’s infiltration rate drops (water stands for more than 48 hours), the soil may be clogged with fine sediment. Core-aerate the soil (use a broadfork to create deep holes) to restore infiltration. If that does not work, the top 4 to 6 inches of soil may need to be removed and replaced with fresh bioretention mix.
Performance Monitoring
A well-designed rain garden should capture water from storms up to its design size (typically a 1-inch or 1.5-inch storm) and drain within 48 hours. Monitor the garden during storms:
- During a storm, water should flow in through the inlet and fill the garden to the design depth.
- After the storm, the garden should drain within 24 to 48 hours.
- If water stands for more than 48 hours, check for clogged soil, insufficient overflow, or a high water table.
If the garden overflows frequently (more than 2 to 3 times per year for a garden sized for a 1-inch storm), the garden may be undersized, or the drainage area may be larger than calculated. Measure the drainage area again and consider enlarging the garden or redirecting some flow.
Long-Term Plant Management
Plants in a rain garden grow vigorously. After 3 to 5 years, some species may dominate and crowd out others. Thin aggressive species (divide and replant or give away divisions). Replace plants that do not thrive (some species will not be suited to your specific conditions, despite being native). Allow the garden to evolve toward a stable plant community.
The rain garden I built eight years ago is now a mature landscape feature. It captures runoff from a 900-square-foot roof section, drains within 30 hours of even the heaviest storms, and supports a community of native plants that bloom from April through October. The maintenance is a half-day in spring (cutting back, mulching, checking the inlet) and occasional weeding. The payoff is a garden that manages stormwater, supports wildlife, and looks better every year as the plants mature. That is what a properly designed rain garden does, and the design is what makes the difference between a garden and a mud pit.
