How-to Guide
How to Build a Rain Garden
The short answer
A rain garden is a shallow, bowl-shaped planting bed positioned to intercept runoff from a roof downspout or paved area, hold it briefly, and let it soak into the soil instead of rushing into the storm drain. Site it at least 10 feet from buildings with the ground sloping away from the foundation, in a spot that already drains reasonably well — a rain garden is not a pond, and extension guidance is explicit that it should empty within about 12 to 48 hours of a typical storm, which is also why a working rain garden does not breed mosquitoes.
Construction is honest digging: excavate a basin roughly 6 to 8 inches deep with a flat bottom, use the excavated soil to build a low berm on the downhill edge, route the downspout in with a corrugated extension pipe, and plant in moisture zones — species that tolerate periodic standing water in the deep center, wet-dry cycle plants on the slopes, and ordinary well-drained-border plants on the rim. Deep-rooted native perennials are the standard palette because their roots keep the soil absorbing year after year.
Site and size it
- Keep it at least 10 feet from the house and any foundations, on ground sloping gently away — never over a septic field, and call utility-locate before digging.
- Test drainage first: dig a hole, fill it, and confirm the water is gone within a day or two. Slow-draining clay sites need a bigger, shallower garden or an amended base.
- Size is proportional to the roof or pavement area draining into it — extension manuals size home rain gardens as a fraction of the contributing impervious area, with typical home installations in the low hundreds of square feet. Bigger catches more, but any reasonably sized basin helps.
- Full sun widens your plant palette; shade rain gardens work with shade-tolerant natives.
Build and plant
- 1Excavate a shallow basin — Dig 6 to 8 inches deep with a level bottom so water spreads evenly rather than pooling at one end; use the spoil to form a berm on the low side.
- 2Route the water in — Connect the downspout to the basin with a buried or surface extension pipe, armored with stone where it enters to prevent erosion.
- 3Plant by moisture zone — Wettest center: species that tolerate short-term standing water. Slopes: plants that handle wet-dry cycling. Rim: standard border perennials. Native deep-rooted perennials and grasses anchor all three zones.
- 4Mulch and babysit year one — Apply a few inches of shredded-wood mulch (it stays put better than chips in moving water), keep it off plant crowns, and water the plantings through their first season until established.
✔ A properly draining rain garden holds water for hours, not days — if yours still has standing water two days after rain, enlarge it or improve the soil. That drain-down window is what keeps it mosquito-free.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- UMN Extension — Building a rain garden — Rain garden function (capture and infiltrate runoff), siting away from foundations, drainage testing, planting in moisture zones with native perennials, establishment care.
- Penn State Extension — How to Make a Rain Garden — Siting no closer than 10 feet from building/downspout with slope away from the house; routing downspout with corrugated drainpipe extension; basin construction.
- UW-Extension — Rain Gardens: A How-To Manual for Homeowners — Homeowner rain garden sizing relative to contributing impervious area, basin depth in the several-inch range with berm, infiltration within roughly a day or two, zone planting design.