Native Rain Garden Plants

What it is and what it does

A rain garden is a shallow depression, typically four to eight inches deep, positioned to receive runoff from a roof, driveway, or paved area. Water collects in it during a storm and infiltrates into the ground over the following day or two.

The point is that it drains. A rain garden that holds water for a week is a failed rain garden, and it will breed mosquitoes, which need about seven to ten days of standing water to complete their cycle. Draining within forty-eight hours avoids that entirely.

The purpose is to keep stormwater on site rather than sending it into a storm drain, carrying lawn chemicals, oil, and sediment with it. In many municipalities there are rebates for installing one.

Test your drainage before you dig

Dig a hole about a foot deep, fill it with water, let it drain completely, then fill it again and time how fast the level drops.

Half an inch per hour or better is fine. Much slower than that and you have heavy clay or a high water table, and a basin will hold water too long. The answers there are to make the basin shallower and wider, to excavate deeper and backfill with a sand and compost mix, or to choose a different location.

Site it at least ten feet from the foundation and downslope of it. Avoid the septic field, and call before you dig for utility locates.

Three zones, three plant lists

This is the part that gets skipped, and it is why plants fail in rain gardens.

The bottom of the basin is wet periodically and dry between storms. Plants there need to tolerate both. Cardinal flower, great blue lobelia, blue vervain, boneset, swamp milkweed, and cinnamon fern all handle the swing.

The slopes are transitional, wet briefly and drier overall. Joe-Pye weed, Culver's root, switchgrass, and dense blazing star suit that band.

The rim is essentially ordinary garden conditions and takes ordinary plants: little bluestem, coneflower, aromatic aster.

Planting a bottom-zone species on the rim leaves it thirsty; planting a rim species in the bottom drowns it. Both are common.

Shrubs anchor it

Buttonbush, winterberry, red-twig dogwood, and elderberry all grow in wet ground and give a rain garden structure that perennials alone do not. Buttonbush in particular grows in standing water and carries the heaviest butterfly traffic of anything on this list.

Mulch with shredded hardwood rather than bark nuggets, which float and wash out. Water through the first season regardless of rainfall, since a new planting has no roots to reach the moisture.

Sources: US Environmental Protection Agency (rain garden design and stormwater guidance); university extension rain garden manuals, including University of Wisconsin and Rutgers; Lady Bird Johnson Wildflower Center, University of Texas at Austin (species moisture tolerance); US Centers for Disease Control and Prevention (mosquito breeding and standing water).