Rain Gardens and Bioswales: Siting, Soil and Plants

A rain garden looks like an ordinary garden bed and does a job no ordinary bed does: it catches the water running off a roof, a driveway or a street and lets it soak away slowly, cleaner than it arrived. The one idea that matters most is that a rain garden is designed around the water first and the plants second, and the plants chosen must take both a flood and a drought.

This page covers what rain gardens and bioswales do, where they can and cannot go, sizing and soil, choosing plants that take wet and dry, and looking after them. It is written for home gardeners, schools, community groups and municipalities planning a small one. It draws on the water agencies named below and on HortOS, a free directory of Canadian wholesale plant growers with their published availability and trade prices. Street-scale bioswales are engineered by the municipality, and this page does not replace their design rules.

Quick facts

What it does	A shallow planted depression catches runoff, slows it, filters it through soil and roots, and lets it soak away.
Bioswale	A bioswale is a planted channel that carries runoff along as well as soaking it in, often beside a road or parking lot.
Siting	Away from building foundations, septic systems and underground services, on gentle ground, where water already flows.
Soil	A free-draining but water-holding mix is the heart of it. A mix that drains too fast starves the plants.
Plants	A variety of plants that take both standing water and long dry spells, mostly natives.
Where the growers are	Growers listing these plants are on the province hubs: Ontario, Quebec, British Columbia.

What a rain garden does

The Sustainable Technologies Evaluation Program describes rain gardens as planted installations that capture runoff in an amended soil, with a surface depression of about 150 mm, and calls more highly engineered versions with drainage systems bioretention facilities.

A bioswale does the same along a line. It is a shallow planted channel that carries runoff from a road, path or parking lot, slowing it and soaking some of it in on the way to a drain. Small check dams of rock across a swale hold water back in steps so more of it soaks in.

Siting and sizing

The Toronto and Region Conservation Authority's guide says a rain garden should be at least three metres from any building foundation, and never within four metres of a septic bed, or fifteen metres if uphill of one. The ground should slope 1 to 5 per cent. It suggests digging about 85 cm and filling about 60 cm with a soil mix, leaving about 25 cm for plants, mulch and pooling water.

Watch where water runs in heavy rain before choosing the spot, and plan where the overflow goes when the garden is full. A garden that sends its overflow toward a house or a neighbour's fence does more harm than good.

Soil and the build

The soil does most of the work. It must let water through quickly enough that the surface does not stay flooded, and hold enough moisture that the plants survive between storms. Most designs replace the top layer of heavy soil with a mix of sand, compost and loam over a gravel layer, and cover it with organic mulch, which traps fine sediment and keeps roots cool better than gravel.

Leave a shallow lip of rock or a stone splash pad where water enters, so it does not scour the bed, and keep the overflow at a level that stops water ponding for more than a day or two.

Choosing the plants

The plants must take standing water after a storm and dry spells between. The STEP guide lists trees, shrubs, grasses and perennials suited to rain gardens, and native species from that list are a sound place to start.

Think of the garden in three zones. The base holds water longest and suits plants that take wet feet for days. The sloping sides are wet only after storms. The rim and the ground around it stay dry most of the time and suit the hardiest plants. Group three or more of each species together, so the planting reads as a design rather than a collection.

The table lists plants of the kind rain gardens and bioswales use, with how many growers listed each on HortOS on 25 October 2026. Counts are the nurseries each plant's page on HortOS showed as growing it on 25 October 2026. Median trade prices are not published for Canada yet.

Of the plants in the table, red osier dogwood (Cornus sericea) had the most nurseries growing it, 36, and meadowsweet (Spiraea alba) the fewest of those shown, 8. Order each zone's plants together, so the garden is planted in one go while the soil is fresh. Stormwater fee credits and rain garden rebates in Canadian cities lists what some cities pay toward one.

A rain garden for a school or community group

A rain garden is a good project for a school or community group, because the result is easy to see. After a storm, children can watch water run off a roof into the garden, pool for a few hours and drain away while the gutter beside the street runs full. A simple rain gauge and a photograph after each storm turn it into a lesson.

Agree the site with whoever owns the ground and looks after the drains, usually the school's facilities staff or the municipality. Ask them about the downspout or path the water will take, the services under the ground, and who will look after the garden over the summer break. A small garden built well and looked after teaches more than a large one that floods its neighbour.

Looking after it

• Water while establishing. New plants need watering through their first dry season, even in a garden built to catch water.
• Clear the inlet. Check where water enters after storms, and clear leaves, litter and sediment.
• Weed by hand. Avoid sprays in a garden designed to drain into waterways.
• Top up mulch. Replace mulch washed away by storms, and keep the surface level so water spreads.
• Replace failures. Replant gaps with the species that have thrived in that part of the garden.

What to watch for

• Too close to a building. Water soaking in beside foundations can find its way inside. Keep the garden well away.
• Unmarked services. Pipes and cables run under many front yards and boulevards. Have them marked before digging.
• A mix that drains too fast. Plants in pure sand die in the first dry spell. Use a mix that holds some water.
• Plants for one condition. Plants that only like wet feet die in summer, and dry-garden plants drown in winter. Choose for both.
• No overflow plan. Every garden fills in a big storm. Send the overflow where it does no harm.

Getting it right

1. Watch where water runs in heavy rain, and choose a spot well away from buildings and septic systems.
2. Have underground services marked, and ask the municipality before working on a boulevard or street.
3. Size the garden for the area draining to it, and plan the overflow.
4. Build a free-draining but water-holding soil over gravel, with organic mulch on top.
5. Plant a variety of species that take wet and dry, the wettest-tolerant in the base.
6. Water through the first dry season, clear the inlet after storms, and weed by hand.

A word of caution

The siting and sizing figures on this page are the agencies' named here, and the municipality's stormwater rules decide what may connect to a drain. Changes to stormwater pipes may need a licensed plumber. The counts are what HortOS printed on 25 October 2026, and the live pages change as growers update their lists.

Sources

• Toronto and Region Conservation Authority, A complete guide to building and maintaining a rain garden: rain garden siting, depth and soil.
• Sustainable Technologies Evaluation Program, LID SWM Planning and Design Guide: Rain gardens: rain garden design and plants.
• Canadian Wildlife Federation: gardening for wildlife and pollinators.