Rain Gardens

What is a Rain Garden?

A rain garden is a shallow garden with enhanced soils and a rock reservoir designed to collect rainwater from roadways, parking lots or other hard surfaces.  They can be planted with a combination of shrubs, grasses and flowers.

Rain gardens provide stormwater control at the source, or where the rain falls.  Rain gardens are designed to mimic nature in an urban setting, increasing infiltration to native soils and removing pollutants prior to releasing surface runoff to the Thames River.  Rain gardens can also provide an aesthetic landscape feature. 

Why Rain Gardens?

  • Limit the amount of water that enters the storm sewer
  • Reduces the potential for flooding drainage problems and stream bank erosion by slowing the water down and reducing flow into the waterways
  • Removes pollutants that run from yards, parking lots and roads straight into the waterways
  • Restores and recharges groundwater
  • Are low maintenance. They are planted with beautiful, hardy plants that require little to no watering
  • Attracts birds, butterflies and beneficial insects, such as mosquito-consuming dragonflies
  • Enhance the beauty of the surrounding neighbourhood

Consider building a Rain Garden at home.

 A rain garden only requires four elements:

  • A source of stormwater runoff, such as a downspout or sump pump discharge
  • An absorbent soil mix with lots of compost
  • Full or partial sun
  • Native plants that are both water-tolerant and drought-tolerant. Properly constructed rain gardens are designed to allow overflow in a large rain event and hold standing water for no more than 48 hours

Fun Fact!

Compared to a similar sized patch of lawn, a rain garden allows for 30% more water to soak into the ground!

General Building Guide

Place the rain garden in full or partial sun at least 10 feet from the house to ensure water won’t seep into the ground around the home, but no more than 30 feet from the downspout.

Your rain garden should not be placed over a septic tank or where water pools in your yard – this is a slow infiltration spot.

Soil in the rain garden is a special soil mix that is a combination of compost and sand which allows the water to infiltrate quickly.  There should be about 24 inches thickness of rain garden soil added which leaves room for plants.

Create an inlet for water to travel into the rain garden; at the inlet into the garden add small stones to slow the water entering which helps reduce erosion.

Create an overflow; the overflow should be created on the downhill side of the garden; add small stones at the overflow as was done at the inlet.

Plant rain garden friendly plants; those that like water and to be dry.

Ensure plants are mulched with at least 2-4 inch layer.

How large should your Rain Garden be?

Measure the footprint of your house (area of first floor); identify the percentage of water runoff that will feed into the garden from the nearest downspout (most houses have four downspouts, each collecting 25% of the runoff); multiply the footprint by percentage of roof runoff to determine roof drainage area; and divide roof drainage area value by six. This calculation sizes a garden to hold one 1 inch of roof runoff in a garden that will be 6 inches deep but dug to about 33 inches to allow room for amended soil. A deeper rain garden can be smaller. Keep your soil type in mind when deciding on the size of your garden. A garden with clay soil absorbs water more slowly than one with sandy soil, and therefore should be larger. The garden should be level so that water is spread evenly across its length. Don't forget to call 811 before you dig!

Use the following steps to measure the size of the area draining to your garden 

    1. Measure the footprint of your house. This is the area taken up by your house if looking down from above. Multiply the length by the width of the house, the answer will be square feet (or square meters if you are using metric measurements).
    2. Estimate how much of the area actually drains to the area where you want to install the garden. Typically, gutters drain to both ends of a house, so the length can be cut in half, but this is not always the case.
    3. If you are collecting runoff from your driveway or a section of road, just estimate the size of the impervious surface draining to the spot where you want to put the garden, and continue to step 4.
    4. Divide this area by 6. This calculation sizes the garden to hold one inch of runoff from the drainage area, in a garden 6 inches deep. The result is the area in square feet (or square meters) that you need for your rain garden. See the example below.
    5. You can make the garden and shape you like, as long as it is roughly the size that you calculated above. Ovals or kidney-shaped gardens tend to look nicer than square or rectangular gardens, but it’s up to you.