How to Shape a Rain Garden and Create the Ideal Soil Mix

A rain garden is a beautiful, sustainable addition to the home landscape—returning rainwater from roofs and other impervious surfaces to the aquifer instead of sending it downstream through storm drains. Rain gardens drain water onsite, cleansing it as it infiltrates. First, determine how to site and size your rain garden, then learn shaping and soil mix creation.

Lisa Wilcox Deyo Landscape Architecture

Why Soil Matters

Rain gardens function by slowly infiltrating water instead of runoff into city storm drains or property flooding. They act like shallow, permeable containers draining through flat bottoms back into soil. Soil is the true functional foundation.

Soil must be graded to act as a container and have the right composition for slow infiltration. Once the soil foundation is set, plants maintain soil function over time.

Creating Sustainable Landscapes, LLC

1. Test Existing Soil Infiltration Rate

Once you've chosen your location, test existing soil infiltration—it influences your soil mix. DIY test (or hire a contractor):

1. Dig a hole ≥1 foot deep at the intended location.
2. Fill with water to saturate base soil, let sit 45 minutes.
3. Refill completely and observe for 48 hours.
4. If water drains within 48 hours, existing infiltration is adequate. If not, excavate existing soil, provide subdrainage, or consider an alternate location.

Pennsylvania Landscape & Nursery Association

2. Lay Out the Shape

Before digging, call your local utility authority to flag utility lines. Mark the garden layout with nontoxic, low-VOC spray paint—easiest full-scale visualization. Alternatively, use a staked rope.

Shown: Orange spray paint marks rain garden outline as landscape staff begin digging.

3. Set Inflow and Overflow Points

Samuel H. Williamson Associates

Direct water from impervious surfaces via rock/vegetated swale or underground pipe. Here, a drainpipe connects to the roof downspout leading downhill to a flattened rain garden area.

A swale moves water across the surface and can integrate with rain garden design. Remember: swales collect additional water en route, increasing volume the rain garden must accommodate.

The inflow source (where water enters via swale or pipe) should be higher than the overflow point—a small outlet allowing excess water escape during major storms, set at the top of ponding depth.

Falon Land Studio LLC

Pennsylvania Landscape & Nursery Association

Shown: Section of rain garden's soil layers and water flows.

4. Excavate and Amend

Rain gardens drain slowly, shouldn't hold water >48 hours. Critical: soil mix allows adequate infiltration. If existing soil failed the infiltration test, two approaches:

1. Complete removal: Excavate existing soil 18–24 inches deep, replace with bioretention mix of compost and sand. Necessary for very poor infiltration (per test). Also install subdrainage below rain garden.
2. Amend existing soil: Mix in compost and sand. More common and less expensive.

Whichever method, rain garden base should be 4–8 inches below surrounding ground, amended soil 1–2 feet below that.

Creating Sustainable Landscapes, LLC

5. Grade the Rain Garden

Grading contours and slopes ground for water movement. Rain garden should function like a shallow, wide container with porous bottom. Checklist for correct grading:

• Flat base: Level base for even drainage. Use a level to verify.
• Containment: On sloped sites, create a berm on the low edge if needed—ridge around lower side contains water, prevents splashing over.
• Sloped sides: 2:1 slope—for every 2 inches horizontal run, side rises 1 inch.

Shown: Grading eventually blends into surrounding landscape as plants mature.

6. Get the Right Soil Amendments

Photo by the Wisconsin Department of Natural Resources

Rain garden soils act like sponges—soaking up water while letting it pass to native soil below. They need high organic content and sand for coarseness and texture. Ideal mix: 35–40% compost, 60–65% coarse sand. Once proper mix is created, continuous amendment isn't necessary.

Shown: Front-yard rain garden with mature plants.

Next: How to choose plants for your rain garden.