Rain Garden & Bioswale Construction Checklist

Locate the rain garden at least 10' from the foundation in a naturally low-lying area where stormwater runoff collects, or at the end of a downspout discharge. Size the garden at 10-30% of the contributing impervious drainage area (roof, driveway, patio). Perform a percolation test: dig a 12" deep hole, fill with water, and time the drainage. The soil should drain at least 1" per hour; if slower, the soil must be amended or an underdrain installed. Avoid areas over septic systems, utilities, or where the water table is within 2' of the surface.

1. Measure distance from the foundation to verify at least 10' separation (15-25' is ideal)

2. Calculate the contributing drainage area and size the rain garden at 10-30% of that area

3. Perform a percolation test and confirm drainage rate is at least 1" per hour

4. Verify the site is not over a septic system, utility lines, or high water table

5. Confirm the site receives runoff naturally or can be connected via a swale or downspout extension

- Placing too close to the foundation, causing water to seep toward the basement

- Undersizing the garden, leading to frequent overflow and erosion at the outlet

- Building in clay soil without soil amendment or underdrain, creating a standing-water pond instead of a rain garden

- Not testing percolation rate, resulting in a garden that holds water for days and breeds mosquitoes

Design the rain garden as a shallow bowl shape, 6-12" deep, with a flat bottom for even water infiltration. Build a berm 6-8" high on the downslope side to retain water. Include an overflow outlet at the maximum ponding depth. Select native plants that tolerate both wet and dry conditions: sedges and rushes for the center (wettest zone), black-eyed Susan, cardinal flower, and switchgrass for the middle zone, and native grasses or asters for the outer edges (driest zone). Avoid invasive species.

1. Verify the design includes distinct wet, middle, and dry zones with appropriate plant selections for each

2. Confirm the maximum ponding depth is 6-12" (deeper gardens are harder to maintain and may require a permit)

3. Check that the berm height matches the designed ponding depth plus 2-3" of freeboard

4. Verify an overflow outlet is included in the design at the maximum water level

5. Confirm all selected plants are native to the region, non-invasive, and tolerant of both wet and dry conditions

- Selecting plants that cannot tolerate periodic flooding (most standard garden plants will drown)

- No overflow outlet, causing uncontrolled spilling that erodes the berm

- Garden too deep (over 12"), making maintenance difficult and creating a safety concern

- Using non-native or invasive plants that spread beyond the garden and become a management problem

Excavate to the design depth (6-12") and create a flat bottom for even water infiltration across the entire garden floor. Build the berm on the downhill side using the excavated soil, compacting it in 4" lifts to 6-8" final height. Shape the sides with gentle 3:1 slopes for easy maintenance and mowing. Do NOT compact the bottom of the rain garden, as compaction reduces infiltration. If an underdrain is needed, install perforated pipe in a gravel bed at the bottom before backfilling with amended soil.

1. Verify the bottom is flat and level across the entire garden area (use a level or string line)

2. Confirm the berm is compacted firmly and shaped to the correct height (6-8" above the garden bottom)

3. Check that side slopes are gentle enough for safe mowing and access (3:1 or gentler)

4. Verify the bottom is NOT compacted (a screwdriver should push in easily by hand)

5. If an underdrain is installed, confirm it has positive slope to the outlet and is bedded in washed gravel

- Compacting the bottom during excavation (especially with heavy equipment), destroying infiltration capacity

- Berm not properly compacted, allowing water to seep through or erode the berm during storms

- Uneven bottom causing water to pool in one area instead of spreading across the full garden

- Excavating too deep on one side, creating a tilted basin that does not function as designed

Replace native soil with a rain garden soil mix for optimal infiltration and plant growth: 60% coarse sand, 20% topsoil, and 20% compost by volume. This mix provides rapid drainage while retaining enough moisture and nutrients for plant roots. Install a minimum 18" deep layer of amended soil in the garden basin. The amended soil mix should drain at least 1" per hour. If native soil has very poor drainage (heavy clay), add a 4-6" gravel reservoir layer beneath the amended soil to increase storage volume.

1. Verify the soil mix ratio is approximately 60% sand, 20% topsoil, 20% compost (test by feel and visual inspection)

2. Confirm the amended soil layer is at least 18" deep across the entire garden area

3. Perform a percolation test on the amended soil: fill a hole with water and verify it drains at least 1" per hour

4. If a gravel reservoir was installed, confirm it is 4-6" of clean washed gravel with no fines

5. Check that the finished soil surface is at the correct depth below the berm top to provide the designed ponding depth

- Too much topsoil or compost and not enough sand, creating a mix that drains too slowly

- Amended soil layer too shallow, limiting root growth and water storage capacity

- Using fine play sand instead of coarse concrete sand, which compacts and reduces drainage

- Not mixing the components thoroughly, leaving pockets of clay or pure sand that drain unevenly

Plant from the center of the garden (wettest zone) outward to the edges (driest zone), selecting species appropriate for each moisture zone. Space plants 1-2' apart for perennials and grasses, allowing room for mature spread. Plant at the same depth as the nursery container (do not bury crowns). Apply 3" of shredded hardwood mulch across the entire garden surface to suppress weeds, retain moisture, and prevent erosion during establishment. Avoid dyed mulch and rubber mulch, which can leach chemicals into the filtered water.

1. Verify plants are placed in the correct moisture zone (wet-tolerant species in the center, dry-tolerant at edges)

2. Confirm plant spacing allows for mature growth without overcrowding (check plant tags for spread)

3. Check that plants are installed at the correct depth with the root crown at soil level

4. Verify mulch layer is approximately 3" deep and does not cover plant crowns

5. Water all plants thoroughly after installation and plan for regular watering during the first growing season

- Planting moisture-zone species in the wrong location (dry plants in the center or wet plants on edges)

- Mulch piled against plant stems, promoting crown rot and disease

- Not watering during establishment; rain gardens need supplemental water until plants are established (first year)

- Using too few plants, leaving bare soil exposed to erosion during the first growing season

Install an overflow outlet at the maximum designed water level (typically 6-12" above the garden bottom, at the top of the ponding zone). Route the overflow to a storm drain, dry well, or daylight outlet at a lower elevation. Use a 4" or 6" PVC pipe set through the berm, covered with a flat grate or screen to prevent clogging from mulch and debris. The overflow must handle the design storm volume without allowing uncontrolled erosion of the berm. Stabilize the outlet with riprap or gravel to prevent scour.

1. Verify the overflow pipe is set at the correct elevation (at the maximum designed ponding level)

2. Confirm the overflow routes to an approved outlet (storm drain, dry well, or daylight) with positive slope

3. Test the overflow by filling the garden with a hose; water should exit cleanly through the overflow pipe without eroding the berm

4. Check that the grate or screen is securely attached and has openings large enough to pass water but small enough to block mulch and debris

5. Verify the outlet is stabilized with riprap or gravel to prevent erosion where water discharges

- No overflow provided, causing uncontrolled overtopping and berm erosion during heavy storms

- Overflow pipe set too low, causing the garden to drain before water has time to infiltrate

- Grate or screen clogged with leaves and mulch, blocking the overflow during storms when it is needed most

- Overflow outlet causing erosion at the discharge point due to lack of energy dissipation