Stormwater Planters: Compact Runoff Solutions for Small Gardens

No room for swales or rain gardens? Stormwater planters manage roof runoff in containers, functioning like miniature rain gardens. Ideal for small spaces—they sit close to buildings in any configuration, handle significant rooftop runoff, and prevent flooding or municipal drain overload.

How They Work

Stormwater planters use bioretention—plants and soil slowing, filtering, and intercepting runoff. Similar to rain gardens but smaller, with constructed walls, layered soil/stone, and filter fabric. Can be flush or raised.

Construction Details

Reinforced concrete sides typically. Contents: rain garden plants, ≥18″ soil depth, filter fabric, ~10″ drainage rock at base. Two modes: infiltration (water seeps into ground) or filtration (water cleansed, then directed to subdrainage). Choice depends on site soil infiltration rate—poor infiltration (common in urban lots) favors filtration mode.

Diagram courtesy of Louisiana DEQ and Bayou Land Resource Conservation & Development Council

Sunken planters work best when roof water disperses over a sloped concrete pad before entering as a sheet (not a gush). Water pools on surface during storms, slowly infiltrating layers.

Photo by Louis Cook for Philadelphia Water Department

Sizing & Placement

Best locations: intercepting runoff from roofs, driveways, impermeable surfaces. Cities use them along sidewalks/roads for sheet flow capture.

Simple sizing formula for single downspout:

Roof area draining to downspout × 4% = planter square footage

Example: 500 sq ft roof → 20 sq ft planter (5×4 ft, 2×10 ft, etc.). These handle disproportionate runoff for their footprint.

Directing Roof Water In

Rain chains (pictured) or downspouts deliver water. Rain chains disperse flow naturally. Always provide a dispersal surface (concrete splash block or stones) at the entry point.

Critical: Overflow mechanism required. Direct excess water away from structures or into subdrainage—similar to rain barrel overflows.

Photo by Mark Hogan