Green Infrastructure for Montreal Homes: Rain Gardens, Swales, and Permeable Surfaces

Montreal's combined sewer system carries both wastewater and stormwater in the same pipes. During heavy rain, the system overflows and untreated water goes straight into the St. Lawrence River. Every property that slows down and absorbs stormwater runoff helps cut those overflows.

Green infrastructure means landscaping elements built to manage stormwater naturally: absorbing it, filtering it, and releasing it slowly instead of sending it straight to the storm drain. These solutions work at the residential scale and often improve the look of your yard in the process.

Why it matters for Montreal homeowners

Beyond the environmental benefit, green infrastructure solves practical problems:

• Basement moisture. Properties with clay soil (the dominant soil type across Montreal, Laval, and the South Shore) drain slowly. Water that pools near your foundation seeps in. Green infrastructure moves water away from the house and into the soil further from the building.
• Yard flooding. Low spots in yards that turn into seasonal ponds can be converted into functional rain gardens that absorb water by design.
• Municipal requirements. Montreal increasingly encourages permeable surfaces in new construction and renovation projects. Some boroughs require a minimum percentage of permeable area on residential lots.
• Property value. Well-designed green infrastructure looks like intentional, premium landscaping. Because that's exactly what it is.

Rain gardens

What they are

A rain garden is a shallow, planted depression designed to collect and absorb stormwater runoff from roofs, driveways, and other impervious surfaces. The depression is typically 4 to 8 inches deep and filled with an engineered soil mix that drains faster than the native clay soil surrounding it (Source: Sustainable Technologies Evaluation Program).

How they work

Runoff flows into the garden through a swale, downspout extension, or surface grading. The engineered soil absorbs the water, and plants take up moisture and nutrients. Excess water percolates into the subsoil over 24 to 48 hours. During heavy storms, the garden temporarily ponds before draining.

Sizing for Montreal

A rain garden should be roughly 20% to 30% of the impervious area it serves. For example, if your roof is 1,000 square feet and you direct one downspout (serving approximately 250 square feet of roof) to a rain garden, the garden should be 50 to 75 square feet.

Best plants for Montreal rain gardens

Plants in a rain garden must tolerate both wet feet during storms and dry conditions between rains. Native species adapted to Quebec's Zone 5 climate are ideal:

• Joe-Pye weed (Eutrochium maculatum): 4 to 6 feet tall, pink flower clusters, thrives in wet soil. Native to Quebec.
• Blue flag iris (Iris versicolor): 2 to 3 feet tall, blue-purple flowers, tolerates standing water. Quebec's floral emblem.
• New England aster (Symphyotrichum novae-angliae): 3 to 5 feet tall, purple flowers in fall. Attracts pollinators.
• Switchgrass (Panicum virgatum): 3 to 5 feet tall, adds structure and movement. Tolerates wet and dry conditions.
• Cardinal flower (Lobelia cardinalis): 2 to 4 feet tall, brilliant red flowers. Attracts hummingbirds.
• Turtlehead (Chelone glabra): 2 to 3 feet tall, white or pink flowers. Tolerates wet shade.

Installation basics

1. Location: Place the rain garden at least 10 feet from the house foundation and away from septic systems and utilities.
2. Excavation: Dig to 18 to 24 inches deep. The bottom 12 inches are filled with the engineered soil mix (60% sand, 20% compost, 20% topsoil).
3. Overflow: Include an overflow outlet (a gravel-filled channel or pipe) for storms that exceed the garden's capacity.
4. Mulch: Top with 2 to 3 inches of shredded hardwood mulch to suppress weeds and retain moisture during dry periods.

Bioswales

What they are

A bioswale is a shallow, vegetated channel that moves stormwater from one point to another while filtering it through soil and plant roots. It's basically a landscaped drainage ditch that looks like a garden feature instead of a utility.

Residential applications

• Driveway edges. A bioswale along the low side of a driveway intercepts runoff and filters it before it reaches the street or neighboring property.
• Property line channels. Where water flows from an uphill neighbor onto your lot, a bioswale along the property line captures and manages the flow.
• Downspout channels. A swale connecting a downspout to a rain garden or dry well moves water across the yard while filtering it.

Design specifications

• Width: 18 to 36 inches for residential applications.
• Depth: 6 to 12 inches.
• Side slopes: Gradual (3:1 ratio or gentler) for stability and mowing access.
• Check dams: Small stone berms every 10 to 15 feet slow the flow and create ponding areas for additional infiltration.
• Vegetation: Dense plantings of native grasses and sedges that tolerate periodic inundation. Blue joint grass, fox sedge, and soft rush are well-adapted options.

Permeable paver systems

What they are

Permeable pavers are interlocking concrete or stone units installed with gaps between them that allow water to pass through the surface and into a gravel reservoir beneath. The water then slowly infiltrates into the subsoil.

How they differ from standard pavers

Standard interlock pavers are set on a compacted gravel base with tight joints filled with polymeric sand. The surface is effectively impervious, and water runs off to the edges.

Permeable pavers use a different base system: open-graded (no-fines) gravel that stores water in the voids between stones. The joints are filled with small angular gravel rather than sand. Water passes between the pavers, through the gravel joints, and into the reservoir base.

Performance

Research from the Sustainable Technologies Evaluation Program shows that permeable paver systems effectively remove total suspended solids, total phosphorus, nitrogen, and metals from stormwater (Source: STEP). They handle rainfall events that would cause runoff on standard surfaces.

Montreal considerations

Freeze-thaw. Permeable pavers handle Montreal winters well because water drains through rather than pooling on the surface and freezing. The open-graded base also provides frost protection.

Maintenance. The gravel joints can clog with sediment over time, reducing permeability. Annual vacuuming with a commercial sweeper restores infiltration rates. Avoid applying sand or deicing salt directly on the surface.

Cost. Permeable paver installations cost approximately 20% to 30% more than standard paver installations due to the specialized base system and joint material. The trade-off is reduced need for separate drainage infrastructure.

Best applications

• Driveways. Permeable pavers on driveways reduce runoff from one of the largest impervious surfaces on a residential lot.
• Walkways. Lower traffic loads mean less compaction of the joint material and better long-term performance.
• Patio edges. A permeable paver border around a standard patio captures edge runoff.

Dry wells

What they are

A dry well is an underground chamber (either a perforated barrel, a stack of concrete rings, or a gravel-filled pit) that collects stormwater and allows it to slowly percolate into the surrounding soil.

When to use them

Dry wells work well where rain gardens aren't practical (insufficient space, too close to the foundation, or poor soil infiltration). They handle the discharge from downspouts, French drains, and surface drains.

Sizing

A typical residential dry well for a single downspout is 3 to 4 feet in diameter and 3 to 4 feet deep. For larger drainage areas, multiple dry wells connected by pipes provide additional capacity.

Combining green infrastructure elements

The most effective approach combines multiple elements into a connected system:

1. Roof runoff flows through a downspout into a bioswale.
2. The bioswale channels water across the yard to a rain garden.
3. The rain garden absorbs most of the water. Overflow exits through a pipe to a dry well.
4. The driveway uses permeable pavers, handling its own runoff independently.

This layered approach manages stormwater from the entire property and reduces the load on the municipal system during heavy rain events.

Cost overview

These costs include materials, plants, and professional installation. DIY-friendly elements like small rain gardens can be completed at lower cost with a willingness to do the digging yourself.

Getting started

Green infrastructure works best when you plan it alongside your overall landscape design. Grading, drainage, and plant selection all need to line up with your hardscaping, fencing, and other landscape elements.

Montreal Paysagement Pro designs and installs residential green infrastructure across Montreal, Laval, and the South Shore. Call 514-900-3867 for an estimate by phone, photo, or video.