How rain garden sizing actually works
The method matches infiltration capacity to incoming runoff volume. From Wisconsin DNR and EPA Soak Up the Rain:
- Step 1: Pick the design rainfall (typically 1 inch in 24 hr — the "first-flush" event that drives most pollutant loading).
- Step 2: Multiply impervious area by sizing factor based on soil (a sandy loam factor of 0.25 means rain garden is 25% of the impervious area it serves).
- Step 3: Subtract for distance from downspout (longer runs lose volume to absorption en route).
- Step 4: Garden depth determines volume capacity; 6–9 inches is the operational range.
For sites with clay loam, the practical recommendation is to either amend the bottom 18 inches with sandy-loam compost mix (1:1 sand to compost with native soil) or move to a structural underdrain.
Plants to use
Rain garden plants need to handle both wet feet (4–48 hours after a storm) and drought between storms. Our full rain garden plant guide covers regionally appropriate species. Common bulletproof picks:
- Bottom zone (wettest): Iris versicolor, Lobelia cardinalis (cardinal flower), Asclepias incarnata (swamp milkweed)
- Mid zone: Rudbeckia fulgida (black-eyed Susan), Echinacea purpurea (coneflower), Panicum virgatum (switchgrass)
- Edge zone: Schizachyrium scoparium (little bluestem), Solidago spp. (goldenrod), Aster novae-angliae (New England aster)