Stormwater runoff is a growing environmental concern, causing pollution, flooding, and erosion in urban and suburban areas. A well-designed rain garden is an effective and beautiful solution to this problem. By capturing and filtering runoff from roofs, driveways, and other impervious surfaces, rain gardens help recharge groundwater, prevent flooding, and create a thriving habitat for pollinators and native plants.
We’re going to dive DEEP into how to design a rain garden. We’ll also talk about the tactical steps you take to build. But people underestimate how important doing your research is when designing a rain garden.
Or maybe you’re not underestimating it, and that’s why you’re here.
Anyways, let’s get into it.
Take These Nine Preparation Steps
Choosing the Right Location
The primary considerations are water flow, distance from structures, and soil drainage capacity.
Distance from Foundations: Place the rain garden at least 10 feet away from your home to prevent water from seeping into the foundation.
Water Flow Patterns: Identify areas in your yard where water naturally collects and flows. Ideally, the rain garden should intercept runoff from driveways, rooftops, or sidewalks.
Soil Drainage Capacity: You’ll want to conduct a percolation test so that the selected site drains effectively. Avoid locations with persistently soggy or compacted soil unless amendments are made.
Utilities and Septic Systems: Before digging, check for underground utility lines. The last thing you want to do is build over a septic system. In fact, stay at least 50 feet away from septic.
Sunlight: While rain gardens can function in partial shade, areas that receive full sun provide the best environment for a wide variety of native plants.
Climate and Region: Choose a location that aligns with your region’s rainfall patterns and temperature fluctuations. Areas that experience high rainfall events may require additional overflow management strategies.
Not sure what a rain garden is? Then start here.
Measuring the Slope
The slope of your yard plays a critical role in the effectiveness of your rain garden. A properly measured slope helps determine how water will flow into and through the garden, ensuring optimal drainage and preventing erosion.
Tools Needed: Use a long straight board, a level, and a measuring tape to accurately determine the slope.
How to Measure
Place a 10-foot-long board on the ground, positioning one end at the highest point and the other at the lowest point of the intended rain garden location.
Level the board to check that it’s perfectly horizontal.
Measure the vertical distance from the lower end of the board to the ground using a measuring tape.
Divide this measurement by the board’s length (10 feet) and multiply by 100 to determine the percentage of slope.
Ideal Slope for a Rain Garden
A 1-2% slope is optimal, allowing water to flow into the rain garden naturally while preventing excessive pooling. If the slope is greater than 12%, additional modifications such as terracing, berms, or infiltration trenches may be required to slow water movement and prevent erosion.
Adjusting for Sloped Terrain
If your site is too steep, consider placing the rain garden in a flatter section or adding a series of smaller rain gardens connected by vegetated swales to control water flow. Terracing and strategic placement of large stones can also help slow down water movement in steep areas, improving infiltration rates and reducing erosion risks.
Soil Testing
Testing your soil is essential to determine its ability to absorb and filter stormwater. Different soil types influence how quickly water infiltrates and whether amendments are needed to improve drainage.
Conduct a Percolation Test
Dig a hole 12 inches deep and 6 inches wide in the selected area.
Fill the hole with water and let it drain completely.
Refill the hole and measure how long it takes for the water to drain.
If the water drains within 12-48 hours, the soil is suitable for a rain garden.
If drainage is too slow, amend the soil with sand, compost, or loamy topsoil to improve infiltration.
Soil Types
Clay soils drain slowly and may require additional excavation and soil amendments to prevent standing water.
Sandy soils drain too quickly, necessitating organic matter like compost to retain moisture.
Loamy soils are ideal for rain gardens as they balance drainage and moisture retention.
Soil Amendment
Soil amendment refers to the process of improving soil quality by adding materials that enhance its structure, drainage, and nutrient content.
To start, you can incorporate organic compost to enhance nutrient content and soil structure.Mix sand and loamy topsoil in clay-heavy areas to improve permeability. For sandy soils, add a layer of mulch to retain moisture and reduce rapid infiltration.
Determining the Size and Depth
The size and depth of your rain garden depend on the amount of runoff it needs to handle and the drainage capacity of your soil. To determine size, measure the area of impervious surfaces (like roofs and driveways) that direct water into the garden.
A general rule is that a rain garden should be 10-20% of the contributing drainage area.
For depth, consider your soil’s drainage rate.
If your soil drains quickly (sandy), a depth of 6-8 inches is ideal. If it drains slowly (clay-heavy), a shallower depth of 4-6 inches prevents water from stagnating.
Use a percolation test to assess drainage and adjust depth accordingly. Incorporating an overflow area ensures excess water can safely drain without flooding. Proper sizing and depth optimize water retention and filtration while maintaining plant health.
Plotting the Dimensions
The dimensions of your rain garden determine its effectiveness in managing stormwater and supporting plant life. Plan carefully so that water is evenly distributed and absorbed efficiently.
Outlining the Shape
Use stakes and string or a garden hose to mark the perimeter of the garden.
A curved or kidney-shaped design blends naturally with the landscape and enhances aesthetics.
Determining Width and Length
The garden should be at least 10 feet long and 5-10 feet wide, depending on available space and runoff volume.
A longer, shallower design maximizes water absorption while preventing overflow.
Ensuring Even Water Distribution:
Position the lowest point of the garden at the center to allow water to collect and infiltrate evenly.
Adjust the garden’s contour to follow the natural flow of runoff from the contributing areas.
By carefully plotting dimensions, you create a rain garden that efficiently manages stormwater while complementing the surrounding landscape
6. Creating Berms and Overflow Areas
Let’s talk berms and overflow areas.
Berms and overflow areas help manage excess water, especially during heavy rainfall.
Construct berms around the rain garden using excavated soil from the digging process. Shape the berms to be at least 6 inches high to contain and direct water flow. Compact the soil firmly and cover it with mulch or native grasses to prevent erosion.
And for overflow areas.
Identify a safe overflow route for excess water during extreme rain events. Direct overflow towards a vegetated swale, dry well, or permeable landscape to prevent erosion and flooding. Line the overflow path with rocks or gravel to slow water velocity and reduce soil displacement.
Use deep-rooted native plants along berms to stabilize the soil.
7. Soil Preparation
Do you want your rain garden to work for you 24/7? Then heed these tips as good soil preparation will keep your rain garden working efficiently. Water needs to infiltrate to provide nutrients for plant growth and this is the main driver of that process.
Number one, soil mix.
The ideal rain garden soil consists of 30-40% sand, 30-40% loamy topsoil, and 20-30% organic matter. This combination ensures proper drainage, prevents compaction, and provides adequate nutrients for plants.
Here’s how to prepare the soil:
Excavate the Garden Basin – Remove existing soil to the desired depth (typically 4-8 inches).
Loosen the Subsoil – Use a garden fork or tiller to break up compacted soil, improving permeability.
Amend the Soil – Mix sand, compost, and loamy topsoil in the appropriate ratios.
Spread and Level the Soil – Ensure even distribution to create a slightly concave surface for water retention.
8. Choosing Native Plants
The best rain gardens are full of native plants. They are adapted to local conditions and require minimal maintenance. Native plants thrive in wet and dry conditions, support local wildlife, and improve soil health.
We did a deep dive article on the best rain garden native plants for each region.
Best Native Plants for Rain Gardens
Zone 1 (Lowest, Wettest Area):
How to Identify: Zone 1 is the lowest part of the rain garden, where water pools after rain events and drains slowly. If your garden has standing water for extended periods after rainfall, you are in Zone 1.
Regions: Found in areas with frequent heavy rain and poor drainage, including parts of the Midwest, the Pacific Northwest, and the Gulf Coast.
Plants:
Blue Flag Iris (Iris versicolor) – Thrives in standing water, improves filtration.
Swamp Milkweed (Asclepias incarnata) – Attracts monarch butterflies, tolerates wet conditions.
Soft Rush (Juncus effusus) – Provides erosion control and supports wildlife.
Zone 2 (Middle, Occasionally Wet Area):
How to Identify: This zone experiences occasional saturation but drains within 12-24 hours. If the soil remains damp but does not retain standing water for long, this is your zone.
Regions: Found in transitional areas with moderate rainfall, such as the Mid-Atlantic, Great Lakes region, and parts of the Southeast.
Plants:
Joe-Pye Weed (Eutrochium purpureum) – Tall, pollinator-friendly plant.
Black-Eyed Susan (Rudbeckia hirta) – Drought-resistant and visually appealing.
New England Aster (Symphyotrichum novae-angliae) – Late-season bloomer beneficial for bees.
Zone 3 (Edges, Drier Area):
How to Identify: Zone 3 consists of the edges of the rain garden, where the soil is well-drained and rarely experiences prolonged wet conditions. If your area dries out quickly after rain, you are in Zone 3.
Regions: Common in drier climates and higher elevation regions, such as the Southwest, the Rocky Mountains, and parts of the Southern Plains.
Plants:
Little Bluestem (Schizachyrium scoparium) – Hardy grass that stabilizes soil.
Yarrow (Achillea millefolium) – Drought-tolerant and supports pollinators.
Butterfly Weed (Asclepias tuberosa) – Attracts butterflies and thrives in dry conditions.
Planting Tips:
Arrange plants according to their moisture tolerance—wet-loving plants in the center, drought-tolerant ones on the edges.
Space plants one foot apart to give the roots the best shot at proper growth and coverage.
Use a mix of perennials, grasses, and shrubs for biodiversity and resilience.
Mulch around plants to retain moisture and reduce weed competition.
9. Adding Shrubs for Erosion Control
Shrubs play a vital role in stabilizing soil and preventing erosion within a rain garden. Their deep root systems help anchor soil, reducing runoff velocity and enhancing infiltration. Properly chosen shrubs also add aesthetic appeal, provide habitat for wildlife, and contribute to long-term rain garden sustainability.
Step one, identify erosion-prone areas:
Observe where water flows most quickly during heavy rain. Check for exposed roots, washed-out soil, or sediment deposits. Identify slopes or steep sections where soil displacement is more likely.
Best Shrubs for Erosion Control
Redtwig Dogwood (Cornus sericea) – Thrives in wet conditions, provides year-round interest with bright red stems.
Buttonbush (Cephalanthus occidentalis) – Attracts pollinators, excels in damp soils, and offers strong root stabilization.
Winterberry Holly (Ilex verticillata) – Adaptable to both wet and dry conditions, adds seasonal color and wildlife benefits.
Summersweet (Clethra alnifolia) – Handles fluctuating moisture levels well, provides fragrant flowers for pollinators.
Planting Tips
Place shrubs along berms and at the edges of rain gardens to reinforce soil structure.
Space them based on their mature size to ensure adequate coverage and root development.
Use mulch around shrubs to further stabilize soil and retain moisture.
Regularly monitor for signs of erosion and adjust plantings as needed.
Checklist: Materials You’ll Need
Shovel and spade: Essential for digging the basin and shaping berms.
Measuring tape and stakes: Helps accurately mark garden dimensions.
Level and straight board: Ensures proper slope measurement for water flow.
Garden hose or spray paint: Used to outline the rain garden before digging.
Percolation test supplies (bucket, water, timer): Determines soil drainage capacity.
Wheelbarrow: Makes transporting soil, mulch, and plants easier.
High-quality soil mix (sand, loam, compost): Improves drainage and supports plant health.
Native plants and shrubs: Provides erosion control and pollinator habitat.
Mulch (shredded bark, leaf mulch): Retains moisture and prevents weed growth.
Decorative rocks or gravel: Helps with erosion control in high-flow areas.
Gloves and protective gear: Keeps hands safe while working with soil and plants.
Rain gauge: Monitors rainfall to assess garden effectiveness.
Watering can or hose: Essential for establishing plants during the first few years.
Weed removal tools: Helps manage invasive plants without disturbing native species.
Compost bin (optional): Provides an ongoing source of organic matter for soil enrichment.
Start Designing Your Rain Garden Today!
A well-designed rain garden is not only a practical solution for stormwater management but also a stunning landscape feature that supports biodiversity. By following these steps, you can transform your yard into an eco-friendly oasis that benefits the environment and enhances your home’s aesthetic.
At Aquabarrel, we’ve spent 20 years helping homeowners and professionals optimize rainwater management with high-quality products like downspout adapters and rain barrel solutions. Whether you’re designing your first rain garden or improving an existing one, our expertise can guide you to success.
Ready to get started? Explore our rainwater harvesting solutions at Aquabarrel.com and take the first step toward sustainable water management today.