A good rain garden either works or fails on a handful of numbers, not on how pretty the flowers look. Solid good rain garden design comes down to four things. You need the right depth, enough distance from your house, drainage within 2 to 4 days, and plants matched to the wet center. Get those right and the flowers take care of themselves.
The basic rain garden requirements are simple once you see what the basin is for. It is a shallow bowl that catches the roof and driveway runoff from your property. It holds that water for a short while, then lets it soak into the ground. The water should never sit there for long, and you want it gone well before the next storm rolls in.
Here is the mechanic that matters most. The basin holds runoff only temporarily so the soil can filter it and pull it down before the next storm arrives. If your water lingers past four days, you have a swamp, not a rain garden. That standing water also hands mosquitoes the still pool they need to breed, and most species can hatch in about a week. Quick drainage is what keeps a rain garden clean instead of buggy.
Start with your rain garden depth, since it sets everything else. A basin of 6 to 8 inches (15 to 20 cm) holds a useful amount of water without becoming a pond that takes a week to clear. Dig it too shallow and it overflows in a real storm. Dig it too deep and you drown the plants and drain too slow. That 6 to 8 inch window is the sweet spot for most yards.
Keep your garden at least 10 feet (3 meters) from the house. Any closer and the water you collect can seep toward your foundation, which is the exact problem you are trying to solve. Place it where roof or driveway runoff already flows, on a gentle low spot, so gravity does the work for you. You should never put it over a septic field or right above a buried utility line.
Size your basin with a quick rule. Take the area that drains into it and divide by 20. So a roof section and driveway that shed 400 square feet of runoff need a garden of about 20 square feet. This keeps the bowl big enough to swallow a normal storm without spilling over the edge. If you are not sure how much area drains your way, walk the yard during a hard rain and watch where the water runs.
Before you dig anything, run the simple infiltration test. Dig a 6 inch hole, fill it with water, and watch the clock. If it has not drained within 24 hours, the spot has heavy clay or a high water table, and it will not work as a rain garden. Pick a new site or amend the soil with sand and compost first.
Finish by pairing your design with zoned native plants. The center of the basin floods then dries out, so it needs species that handle both, like blue flag iris or swamp milkweed. The drier edges take plants that prefer firmer ground. Match each plant to its zone and your garden runs itself for years. Native roots also dig deep, which keeps that soil draining fast long after you plant them.
Read the full article: Best Rain Garden Plants by Moisture Zone
