How do rain gardens help reduce flooding and pollution of waterways?

House and rain garden.
*Image source: City of Calgary*

Rain gardens are a fantastic way to enhance your landscape while supporting the environment. They are designed to collect and absorb rainwater runoff from roofs, driveways and other impermeable surfaces, reducing the impact on local waterways, preventing erosion and reducing the risk of flooding.

SLOW IT DOWN, SOAK IT UP, KEEP IT CLEAN!

Joe Pye weed is an excellent plant choice for rain gardens. *©D. Irvine*

Every time it rains, water flows from roofs, driveways and impermeable surfaces into the streets, picking up chemicals like fertilizers, pesticides and oil. This polluted runoff makes its way into local streams, rivers and lakes, posing a significant threat to water quality, causing erosion, degrading habitats and increasing flood risks. But we can help slow this water down and soak it into the soil before it has the chance to reach our waterways - with rain gardens!

Benefits of Rain Gardens

Slow it down:

By reducing the rush of stormwater flowing downstream, rain gardens help reduce soil and stream bank erosion.

Soak it up:

By soaking it into the ground, rain gardens help reduce the risk of flooding and sewer overflows.
Rain gardens help keep the groundwater replenished and keep streams and rivers flowing.
Soaking rainwater into the ground will help keep it out of the community infrastructure, reducing management costs.

Keep it clean:

Plants and soil help filter pollutants from stormwater, improving the quality of water entering our rivers and streams.
Rain gardens provide habitat for local wildlife, including birds, butterflies and beneficial insects, and enhance the beauty of your yard too!

Designing Your Rain Garden

When designing a rain garden, consider the following:

Distance: Position rain gardens at least 4 meters away from your home, garage or septic system (and your neighbours’ too!), and 15 meters from any steep slope.

Size: The size of your rain garden should be 10-20% of the roof or driveway area it drains, known as the directly connected impervious area.

Rainwater entry: Protect against erosion at water entry points with stones or gravel to slow down water flow.

Growing medium: Add sand and compost to improve infiltration.

Plants: Choose hardy, wet and drought-tolerant, native perennials for their aesthetic and functional benefits, like soil aeration and habitat for pollinators. Follow the Credit Valley Conservation Authority’s recommendations for choosing plants for each zone (high, middle, low).

Overflow: Ensure an overflow system directs excess rainwater away from your home and neighboring properties without causing erosion.

Installing Your Rain Garden

Preparing the area: Position your rain garden at least 4 meters from any foundation to ensure safety and efficiency. Water can be directed to the garden from a downspout, rain barrel overflow, or naturally low area on your property. It's important to estimate how much water your garden will need to handle during a large storm to size it appropriately. Dig your garden to a depth of 85 cm, sloping the edges away from foundations.

Optional sand base: Add 20-25 cm of clean sand to the hole's bottom for extra water storage.

Soil, compost, and sand mix: Mix the excavated soil with compost and sand to enhance nutrients and water infiltration. Fill the hole with this mix, leaving about 25 cm from the top for plants, mulch, and a 10-15 cm depression for water collection.

Berm: Create a berm, a bit of a raised edge around your garden, to contain water and mulch during heavy rain.

Inlet and overflow: Place some small stones around the inlet to slow the water as it enters the garden. Designate a low point in the berm to direct overflow water away from foundations using a small stone channel to prevent soil erosion. Direct excess water toward a second garden or lawn.

Native plants: Choose plants that can withstand wet and dry conditions. An example of suitable native perennials, shrubs and grasses can be found here.

Optional river stones: These enhance water flow to the subsoil and add visual appeal to larger gardens.

Mulch: Apply 10-15 cm of natural, undyed mulch after planting to absorb water and keep soil moist.

Maintaining Your Rain Garden

Just like any garden, your rain garden will require some maintenance to look good and function properly. A few things to consider;

Erosion and debris: Fast flowing water can cause some erosion to the soil, so make sure exposed soil is protected with mulch. Add or move rocks as needed to reduce velocity of water at the inlet and overflow sites of the garden. Keep the inlet and overflow areas clear of debris, like leaves and sticks to ensure better water absorption.

Weeding: Be sure to remove any invasive plants that might arrive by seed, before they choke out your native plants.

Watering: New plantings require consistent waterings during the establishing season. Be sure to water well during dry spells. Water deeply, but do not overwater.

With the perfect mix of sand, soil, native plants, and a bit of creativity, you’re not only helping the environment but also creating a beautiful and functional space in your yard.