Rain Garden Design: Siting, Sizing, Soil and Plants
How to design a rain garden: where to put it, how to run a percolation test, size it from the drainage area, shape the berm and overflow, and plant its zones.
· 9 min read · OpenLandscape3D editorial
A rain garden is a shallow, planted depression that collects runoff from a roof, driveway or lawn and lets it soak into the ground over a day or two. It is not a pond and it is not a bog. Done well, it holds water briefly after a storm, drains completely, and looks like a well-kept perennial bed the rest of the time. Done poorly, it is a muddy hole that stays wet for a week or sends water toward the house.
Most of the work happens before you plant. Rain garden design depends on where the water comes from, how fast your soil drains, and where the overflow goes when a storm is bigger than the garden. This guide covers those decisions in order, then the planting.
Step 1: Choose the site
Follow the water first. Walk the property during or just after a steady rain and note where downspouts discharge, where sheet flow crosses the lawn, and where water collects. Then pick a spot that meets these conditions:
- At least 3 m (10 ft) from building foundations, and farther if the house has a basement or the ground slopes toward it. The garden should be downhill of the building, never uphill.
- Not over or just uphill of a septic tank or drain field, and well away from any private well. Your local health department sets the required setbacks.
- Clear of buried utilities. In the U.S., contact 811 a few business days before you dig so the utilities can mark their lines. Gas, electric and telecom lines are often shallower than people expect.
- Not under the drip line of large trees, where digging cuts roots and roots compete for the amended soil.
- On a gentle slope. Gardens are easiest on fairly gentle slopes. Steeper ground needs a taller downhill berm or terraced cells, and very steep sites may be better served by other measures.
- Not where water already stands for days. A low spot that stays soggy usually means slow soil or a high water table, which is the opposite of what a rain garden needs.
Full sun to part shade gives the widest plant choice. A sun and shade map of the yard helps you decide which plants will suit the spot.
Step 2: Test how fast the soil drains
A percolation test tells you whether the site can handle a rain garden, and how deep to make it. You need a shovel, a ruler and a hose.
- Dig a hole about 20–30 cm (8–12 in) deep and roughly 15–20 cm (6–8 in) wide where the garden will go.
- Fill it with water and let it drain completely. This wets the surrounding soil so the next reading is realistic.
- Fill it again and note the water depth and the time.
- Check the depth every hour (or after a few hours if it is slow) and work out the drop per hour.
How to read the result (these are working guides, and local manuals vary):
| Drop per hour | What it suggests |
|---|---|
| More than about 25 mm (1 in) | Well-drained soil; a standard garden works well |
| About 13–25 mm (0.5–1 in) | Workable; keep the ponding depth modest |
| Less than about 13 mm (0.5 in) | Slow; make it shallower, amend more deeply, or reconsider the site |
| Water still standing after 24–48 hours | Generally not suited to infiltration without professional design |
Do the test in two or three spots if the garden will be large, since soil can change over short distances. Your county or state extension service can also identify your soil series and may offer soil tests.
Step 3: Size the garden from its drainage area
The garden's size depends on how much area drains into it. Measure the roof section that feeds the downspout (its footprint, not the sloped roof surface), plus any paving or lawn that sheds water toward the spot.
A common rule-of-thumb approach
Many guides size a residential rain garden to hold the runoff from a modest storm, often around 25 mm (1 in) of rain falling on the hard surfaces that drain to it. The steps are:
- Runoff volume = drainage area × design rainfall depth.
- Garden area = runoff volume ÷ ponding depth.
Worked example
A downspout drains a roof section of 60 m² (about 650 sq ft).
- Volume from 25 mm of rain: 60 m² × 0.025 m = 1.5 m³ (about 54 cu ft, or roughly 400 US gallons).
- With a ponding depth of 15 cm (6 in): 1.5 m³ ÷ 0.15 m = 10 m² (about 108 sq ft).
That is roughly a 2 × 5 m (7 × 16 ft) garden, or about one-sixth of the drainage area. A larger garden captures more of each storm. A smaller one still helps, but overflows more often.
Caveats
- This is a sizing estimate, not an engineering calculation. It ignores rain soaking into the soil during the storm, which is a big deal in sandy soil and a small one in clay.
- Many extension manuals use their own size factors based on soil type and depth. Where your state or municipality publishes a rain garden manual, use its method.
- If you want a stormwater credit, need a permit, or plan to take runoff from a large paved area or a neighbour's property, the local stormwater program's rules override any rule of thumb.
- Long, narrow gardens set across the slope usually catch sheet flow better than round ones.
Step 4: Set the depth, berm, inlet and overflow
Depth
Ponding depth (from the overflow to the bottom of the basin) is typically 10–20 cm (4–8 in) for home gardens. Use the shallower end on slow soils so the garden drains within about 24–48 hours. Water that drains in that time does not give mosquitoes long enough to complete their life cycle. Keep the bottom level so water spreads out instead of pooling in one corner.
Berm
On sloping sites, use soil from the uphill side to build a low, compacted berm along the downhill edge and the ends. The top of the berm should be level and higher than the overflow point. Give it gentle side slopes (about 3:1, horizontal to vertical, is a common target) and plant or mulch it so it does not erode.
Inlet
Get water into the garden without cutting a channel. Options include:
- A downspout extension or buried solid pipe from the downspout
- A shallow grassed or stone-lined swale
- A splash pad of flat stones or cobbles where the water enters
Overflow
Every rain garden overflows sometimes. Decide where the excess goes. Usually that is a stone-armoured low notch in the berm, set at the ponding depth and pointing toward a lawn, swale or storm drain, away from the house and the neighbours' property. Never let overflow run back toward a foundation.
Step 5: Prepare the soil
On reasonably well-drained loam, loosening the bottom and mixing in compost may be enough. On heavier soils, many designers dig out 30–60 cm (1–2 ft) and backfill with an engineered mix. Published bioretention mixes are mostly sand, with smaller shares of topsoil and compost, and recipes vary by jurisdiction. Two practical points:
- Adding a small amount of sand to clay can make it denser, not looser. If you amend, either commit to a properly blended mix or rely mainly on compost.
- Avoid compacting the bottom. Keep machines and foot traffic off the basin floor, and do not work wet soil.
Finish with 5–8 cm (2–3 in) of shredded hardwood mulch, which stays put better than bark nuggets, which float. Keep mulch thinner near the inlet, where flow is strongest.
Step 6: Plant by zone
A rain garden has three moisture zones. Plants in the bottom must tolerate brief flooding and dry spells. Plants on the edge must handle ordinary garden conditions.
| Zone | Conditions | Plant traits |
|---|---|---|
| Bottom | Wet for hours to a day or two after storms, then may dry out | Tolerates brief flooding; deep or fibrous roots |
| Sides | Moist, briefly wet near the base | Adaptable; tolerates both wet and dry spells |
| Edge and berm | Normal garden moisture, often drier | Drought-tolerant once established |
The examples below are native to parts of each region. Natural ranges and site tolerances vary, so confirm with your local extension service or native plant society before you buy.
Northeast
- Bottom: blue flag iris (Iris versicolor), swamp milkweed (Asclepias incarnata), white turtlehead (Chelone glabra), fox sedge (Carex vulpinoidea)
- Sides: cardinal flower (Lobelia cardinalis), spotted Joe Pye weed (Eutrochium maculatum), winterberry (Ilex verticillata), sweet pepperbush (Clethra alnifolia)
- Edge: switchgrass (Panicum virgatum), New England aster (Symphyotrichum novae-angliae), black-eyed Susan (Rudbeckia hirta)
Southeast
- Bottom: southern blue flag (Iris virginica), soft rush (Juncus effusus), swamp rose mallow (Hibiscus moscheutos)
- Sides: cardinal flower (Lobelia cardinalis), Virginia sweetspire (Itea virginica), river oats (Chasmanthium latifolium)
- Edge: pink muhly (Muhlenbergia capillaris), American beautyberry (Callicarpa americana), lanceleaf coreopsis (Coreopsis lanceolata)
Southwest
In arid regions the same idea is usually called a water-harvesting basin. Basins there are often deeper, mulched with rock or wood chips, and planted with species that tolerate both occasional inundation and long dry periods.
- Bottom: deergrass (Muhlenbergia rigens), giant sacaton (Sporobolus wrightii), desert willow (Chilopsis linearis) in larger basins
- Sides: chuparosa (Justicia californica) in low deserts, Parry's penstemon (Penstemon parryi)
- Edge: desert marigold (Baileya multiradiata), brittlebush (Encelia farinosa) in low deserts
Plant in groups of three or more of each species rather than one of everything. Groups read better from a distance and hold the soil better. The plant library lists moisture tolerance and a rain-garden tag for each species in the pilot regions.
Step 7: Maintain it
- First one to two growing seasons: water during dry spells until plants establish. Weed regularly, because bare, moist soil invites weeds.
- After big storms: check the inlet and overflow for erosion, and replace displaced stone or mulch.
- Each spring: cut back last year's dead stems (leaving some stems through winter helps overwintering insects), top up mulch, and remove sediment that has collected at the inlet.
- If it stops draining in 48 hours: look for a sediment crust or compaction on the bottom. Loosening the surface and removing fine sediment often restores it.
How OpenLandscape3D helps
In the web planner, you can draw the rain garden as a bed and give that area its own moisture override. The garden then becomes a wet micro-site within an otherwise ordinary yard. The OL-WATER check flags plants whose moisture needs don't match the area, and OL-SUN does the same for light. You can mark low points with spot elevations (terrain). The planner does not yet calculate cut and fill, runoff or irrigation, so do the sizing above by hand. The plant schedule export gives you a count by zone for ordering.
Frequently asked questions
Will a rain garden attract mosquitoes?
A properly sized garden on suitable soil drains within about 24–48 hours. That is not long enough for mosquitoes to complete their development from egg to adult. If water stands longer, the soil or the depth needs attention.
Can I put a rain garden in clay soil?
Often yes, with care. Keep it shallow, make it larger in area, and consider replacing the top 30–60 cm (1–2 ft) with a well-blended mix. If a percolation test shows water still standing after a day or two, ask your extension service or a professional before building.
How far from the house should a rain garden be?
A common minimum is 3 m (10 ft) from the foundation, with the garden downhill of the building. Go farther for basements, steep grades or expansive clay soils, and check any local requirements.
Do I need a permit?
Most small residential rain gardens do not need one. Rules vary, and permits or approvals may apply near wetlands or streams, in rights-of-way, or where you are moving runoff across property lines. Check with your municipality.
Should the plants be native?
Natives adapted to your region are a sound default, because many tolerate the wet-then-dry cycle and support local insects. Well-behaved non-native perennials can work too. Avoid anything on your state's invasive species list, since rain gardens often drain toward streams.