Rain Garden for Stormwater: Size, Dig and Plant It Right

A shallow planted basin can soak up far more water than the same patch of lawn, which is why cities across the country now give rebates to homeowners who build one. A rain garden for stormwater catches runoff from a roof, driveway or lawn, holds it for a day or two, and lets it seep into the ground instead of rushing into storm drains, washing out beds or pooling against your foundation.

In short: put it at least 10 feet from the house and well away from any septic system, size it at roughly 10 to 30 percent of the area draining into it, dig it 4 to 8 inches deep with a level bottom, amend the soil so it drains, add a stable overflow, and plant it with deep-rooted, water-tolerant plants. A properly built one drains within 24 to 48 hours, which is too fast for mosquitoes to complete their life cycle. Call 811 before you dig.

How a Rain Garden for Stormwater Works

Water arrives from a downspout, a swale or a pipe, spreads across a flat bottom, and ponds a few inches deep during a storm. Plant roots and amended soil keep the ground open so that water infiltrates instead of running off. Along the way, the soil filters out sediment, nutrients and some pollutants that would otherwise reach streams. In very heavy rain, extra water exits through an overflow into a lawn or drainage path, so the garden never floods your yard.

What separates it from a pond or bog: a rain garden is dry most of the time. If water sits for more than two days, something is wrong with soil, sizing or siting.

Tools and Materials

• Garden spade and flat shovel (a sharp garden spade makes cutting sod and shaping edges much easier)
• Tape measure, stakes and string
• A long straight board and a level, or a line level
• Wheelbarrow and rake
• Compost and coarse sand for soil amendment (if needed)
• Stone or river rock for the inlet and overflow
• Shredded hardwood mulch (heavy enough not to float)
• Pipe or flexible downspout extension to carry water to the garden
• Plants, chosen for wet base, moist sides and dry rim

Step 1: Choose the Site

Siting is the most important decision. Follow these rules:

• At least 10 feet from building foundations, farther for homes with basements, so you are not adding water near walls.
• Well away from septic tanks and drainfields, commonly 25 feet or more; check your local health department’s setback. Never put a rain garden over a drainfield.
• Away from wells and not over buried utilities.
• Downhill from the water source, on a slope under about 12 percent. Steeper slopes need terracing or a different approach.
• Not where water already sits for days. Those spots have poor infiltration or a high water table.
• Not under large trees, where digging damages roots.

Full or partial sun widens your plant choices. Also check HOA rules and any local stormwater program, which may offer free design help or rebates for a stormwater rain garden.

Step 2: Do a Perc Test

Dig a hole about 8 inches deep and 6 inches wide where the garden will go. Fill it with water, let it drain, then fill it again and measure the depth. Check again after a few hours. You want the water to drop at least half an inch per hour, ideally more. If the hole still holds water after 24 hours, the soil is heavy clay or the water table is high. Choose another spot, plan heavier soil amendment, or consult a landscape professional about an underdrain.

Step 3: Size It to the Drainage Area

Measure the area sending water to the garden. For a downspout rain garden, that is the roof footprint feeding that downspout. Add any paved areas that drain the same way.

• Sandy, fast-draining soil: garden about 10 to 20 percent of drainage area.
• Loamy soil: about 20 to 25 percent.
• Clay soil: about 25 to 30 percent or more, or a deeper amended basin.

Example: a downspout draining 500 square feet of roof, on loamy soil, needs roughly 100 to 125 square feet of garden, such as 10 by 12 feet. Shallower gardens need more area; deeper ones need less, but do not exceed about 8 inches of ponding depth in a home garden. Long, kidney or crescent shapes oriented across the slope catch water best.

Step 4: Dig the Basin

1. Call 811 a few business days ahead and wait for utility marks.
2. Outline the shape with string or a hose and remove sod.
3. Dig 4 to 8 inches deep for ponding, plus several more inches if you are amending soil. On a slope, dig the uphill side and use that soil to build a berm on the downhill side.
4. Level the bottom. Set a board across the basin with a level on top and check in several directions. A sloped bottom lets water run to one end and overflow early.
5. Shape the sides at a gentle angle, about 3 to 1, so they do not erode.
6. Compact and seed or mulch the berm so it holds in storms.

Lift with your legs, take breaks, and use a helper or small excavator for large gardens; moving a few cubic yards of soil by hand is real work.

Step 5: Soil Mix, Inlet and Overflow

If your perc test was slow, replace the top 8 to 12 inches with a mix of roughly 50 to 60 percent coarse sand, 20 to 30 percent topsoil and 20 to 30 percent compost. Avoid fine sand, which can cement with clay.

At the inlet, line the entry with stone to slow water and prevent erosion. Route the downspout through a buried pipe or surface swale. At the low point of the berm, build an overflow lined with rock that sends excess water toward lawn or an approved drainage path, never toward your home or a neighbor’s property.

Step 6: Plant and Mulch

Plant in three zones: water-tolerant species in the bottom, plants tolerating both wet and dry on the sides, and drought-tolerant plants on the rim and berm. Native sedges, rushes, switchgrass, swamp milkweed, blue flag iris, joe-pye weed and asters are common choices in many regions; ask your local extension office for a list. Plant about 12 to 18 inches apart for fast coverage, then add 2 to 3 inches of shredded hardwood mulch. Water weekly the first season until roots establish.

Seasonal Maintenance

A rain garden needs less care than a lawn once established, but it is not maintenance free. In the first two growing seasons, weed every few weeks so young plants are not crowded out, and water during dry spells longer than a week. Each spring, cut back last year’s dead stems, rake out sediment that collected near the inlet, and top up mulch to 2 to 3 inches without burying plant crowns.

After major storms, walk the garden. Look for washouts at the inlet, gaps in the berm and debris blocking the overflow, and check that the basin empties within two days. Clean gutters and downspout screens in fall so leaves do not wash in and smother plants. Over time, divide crowded perennials and replace anything that struggled in its zone; plants that sulk in the wet bottom often thrive on the rim.

Pairing With Rain Barrels and Other Practices

A rain barrel at the downspout stores the first portion of each storm for watering, then its overflow can feed the rain garden. That combination lets a smaller garden handle a larger roof. Other practices that work alongside a rain garden include redirecting downspouts onto lawn rather than pavement, replacing a section of driveway with permeable pavers and planting trees, which intercept rain before it reaches the ground.

Rain Garden Drainage Troubleshooting

• Water stands more than 48 hours: soil is compacted or too clayey. Loosen and amend, add deep-rooted plants, or reduce inflow.
• Erosion at the inlet: add more stone, a splash pad or a wider spreader.
• Mulch floats away: use heavier shredded hardwood rather than bark nuggets.
• Overflow too often: the garden is undersized; expand or add a rain barrel upstream.
• Sediment buildup: rake it out in spring and stabilize bare soil uphill.
• Mosquitoes: a sign that water is ponding too long; fix drainage.

Cost Ranges

As typical ranges, a DIY rain garden installation costs roughly $3 to $6 per square foot for plants, compost, sand, stone and mulch. Professionally built gardens often run $10 to $20 per square foot or more, depending on excavation, soil replacement, piping and plant size. Many municipalities offer rebates that offset a share of the cost.

When to Call a Licensed Professional

Many homeowners handle building a rain garden themselves. Call a landscape architect or civil engineer if your yard has steep slopes, very poor infiltration, a basement with water issues, nearby retaining walls or septic systems, or if you plan to connect underground pipes to municipal storm sewers, which often requires a permit. A licensed plumber or drainage contractor should handle any work tied to foundation drains or sump discharge lines. Check whether your city requires permits for altering drainage near property lines.