Rain Gardens

Rain gardens are a combination of absorbent landscaping and soil infiltration.  These approach provides a garden area in a gentle depression to collect water from impervious areas.  The top garden layer of absorbent soil filters and traps water while supporting a variety of moisture tolerant plants.  All the excess water infiltrates through the organic soil layer to an underlying rock pit to store and further infiltrate water.  This is a nice blend of the two approached to provide a visually appealing and active garden area, and providing extra ability to store water.

Rain gardens provide a large degree of flexibility as they can be sited around the yard and close to individual downspouts or separate gardens for the home and garage.  Rain gardens do however require more space than the infiltration chambers described in the suggested tools, and would be expected to range from about 10m (100ft2) to 36m2 (390ft2) for a range of roof areas and soil infiltration capacities.  The Tool Sheet linked below provides a range of sizes.


  • Rain gardens should be setback from deep foundations but are not regulated by the Building Code, 3m (10’) is a suggested setback.
  • Rain gardens should be a minimum of 1m (3’) from side yards and should located away from drained retaining walls on property side boundaries.
  • Rain gardens need an overflow and an underdrain is also recommended as shown in the typical section.
  • Any foundation drains shall bypass the infiltration chamber and flow directly to the storm sewer.

Simple rain gardens could cost as low as $2,500 and would offset other landscaping costs.  For sites where the groundwater is high, or the depth of good infiltrating soil is small, a rain garden provides a very shallow approach to rainwater management.

While rain gardens can manage all of the rainwater from your site, they require more space, and will need much more on-going plant maintenance. The  first three rainwater management approaches are all underground and would allow use of the surface area for other uses in contrast to rain gardens.

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