What it is
A shallow planted bowl. It catches runoff from roofs, drives, and patios. It drains in 12 to 48 hours.
Not a pond. No standing water.
Water moves through soil and roots on the way down. Pollutants stay behind. The aquifer gets the rest.
Why it works
A residential rain garden absorbs about 30% more water than the same patch of lawn. In cities, that matters. Hard surfaces dominate, storm drains overflow, streams get hammered.
Same infiltration principle as swales on contour. Smaller scale. Sharper aesthetic.
Siting
Walk the yard during a storm. Where does the water go? That is your site.
Keep the bowl 3 m from any foundation. Skip ground above septic lines, utilities, or a high water table. Puddles after rain are a signal, not a problem.
Sizing and soil
Aim for 10 to 20% of the impervious area draining in. A 50 m² roof slope feeds a 5 to 10 m² garden.
Center depth: 15 to 25 cm. Deep enough to hold a real volume. Shallow enough to drain in two days.
Percolation test. Dig a hole 30 cm deep, fill with water, time the drop. If it sits past 48 hours, you have heavy clay. Excavate deeper and backfill with 60% sand, 20% compost, 20% topsoil.
Plant in zones
Plants need to handle a wet weekend and a dry month. Native species win. Deep roots open channels, native bees show up, irrigation drops to zero once established.
Three zones, by moisture:
- Center. Rushes, sedges, blue flag iris, cardinal flower, native ferns. Wet feet tolerated.
- Sides. Black-eyed Susan, bee balm, switchgrass, native asters. Mid-moisture workhorses.
- Edges. Drought-tolerant ground covers and bunch grasses. They stitch the bowl into the lawn.
Skip invasives. Skip heavy feeders. You want roots that filter, not roots that leak nutrients.
Tend it
Year one is the work year. Water through dry spells so roots dive. Mulch 2 to 5 cm with shredded hardwood. Fine mulch and fresh chips float away in the first big storm.
After that, the rhythm is light:
- Scrape sediment from the inlet once or twice a year.
- Pull weeds before they seed.
- Cut perennials back in late winter.
- Divide plants that crowd a zone.
If drainage slows, the inlet is clogged. Scrape the top centimeter, lay fresh mulch, move on. A maintained rain garden runs for decades. It gets better as roots go deeper.
When it goes wrong
Standing water past three days means the soil is too tight or the bowl is too deep. Re-test percolation and amend.
Bare soil around the inlet means scour. Add stone or a splash pad where the downspout lands.
Plants drowning in the center? You picked edge species for a wet zone. Swap them.