Rain Gardens for Clay Soil in London, Ontario: Siting and Sizing Guide
A rain garden is a shallow, planted basin that catches roof water from a downspout and lets it soak into the ground. It can work in London, Ontario's clay soil if you site it properly: at least 3 m from the house, in a spot that drains, and with an improved soil mix. The City of London's guidance also limits ponding to 48 hours and gives a simple sizing formula, which this guide walks through.
Many London yards have the same problem. Downspouts dump water beside the foundation, the clay can't absorb it fast enough, and the lawn stays soggy for days. A rain garden won't fix a badly graded lot, but it can turn a wet corner into a planted bed that handles roof runoff. Fall is a good time to plan one: you can watch where water goes during autumn rain, test your soil before freeze-up, and have the bed ready for spring planting.
What a rain garden does (and doesn't do)
A rain garden:
Collects water from a roof downspout or a small paved area.
Holds it briefly in a shallow basin.
Lets it soak in through improved soil, so less runoff reaches the street and storm sewers.
Supports plants that tolerate both wet spells and dry summers.
A rain garden is not:
A pond. Water should be gone within 48 hours, and that limit also keeps mosquitoes from breeding.
A fix for a basement leak or a yard that slopes toward the house. If water is running toward your foundation, start with lot grading.
A replacement for proper downspout setbacks. See our post on downspouts and sump pumps in London.
Where to put a rain garden in London
The City of London's stormwater guidance sets out practical siting rules:
At least 3 m from your house so water doesn't soak back toward the foundation.
No more than 9 m from the downspout, so the water reaches it.
Full or partial sun.
Not over a septic system, and not where water already sits for days. A permanently wet spot means the soil underneath isn't draining.
Away from utilities. Call Ontario One Call (1-800-400-2255) for free locates before you dig. Allow about 5 working days.
Also respect your lot's approved grading. Don't fill or block a drainage swale between houses to build a garden.
Test your soil first: the clay question
Clay is the main reason rain gardens fail in southwestern Ontario. Before you commit to a location, run two quick tests. Both come from the Essex Region Conservation Authority's rain garden manual.
Infiltration test
Dig a hole about 15 cm deep where the garden will go.
Fill it with water and let it drain, then fill it again.
If the water is gone within 24 hours, the site can work.
Ribbon test
Take a moist handful of soil from garden depth.
Squeeze it between your thumb and finger to push out a ribbon.
If the ribbon is longer than 2.5 cm before it breaks, the soil is heavy clay and may not drain well enough.
If your soil fails, you have three options: find another spot, amend the soil with sand and compost and test again, or switch to a different approach such as regrading or a dry well designed by a professional.
How big should a rain garden be?
The City of London uses a simple formula:
Rain garden area = first-floor footprint × share of roof draining to that downspout ÷ 6
For example, a home with a 100 m² footprint, where one downspout drains about a quarter of the roof, works out to about 4.2 m² (100 × 0.25 ÷ 6).
Other guides size rain gardens differently. The Essex Region manual suggests about 15% of the drainage area, with typical gardens of 10–30 m² and a ponding depth of 10–20 cm. If you're in London, the City's method is the one to follow. Elsewhere, use the method from your local guidance.
Building it: step by step
Excavate the basin. Remove sod and dig out the native clay to make room for the soil mix. The City's guidance calls for roughly 60 cm of a compost and sand mix.
Build a level bottom so water spreads evenly instead of pooling at one end.
Make a berm on the downhill side using the excavated soil.
Bring the water in. Extend the downspout with a pipe or a rock-lined swale. Add stone where water enters to prevent erosion.
Add an overflow to a safe spot on your property, away from the house and neighbours, for heavy storms.
Plant and mulch. Use shredded wood mulch that won't float away, and keep it thin where water enters.
Our mulch page and topsoil page list the materials we deliver, and the material calculators help estimate volumes.
Plant choices for southwestern Ontario
Choose hardy, preferably native plants that tolerate wet feet in spring and dry soil in August. Common rain garden choices in Ontario include:
Basin bottom (wettest): swamp milkweed, blue flag iris, sedges, Joe Pye weed
Middle slopes: New England aster, black-eyed Susan, switchgrass, bee balm
Top edge (driest): little bluestem, wild bergamot, purple coneflower
Shrubs for larger gardens: red osier dogwood, winterberry, ninebark
Check with a local native plant nursery for plants suited to your soil and light.
Rain garden vs. other drainage fixes
Downspout soaking one corner of the yard: Rain garden?: Yes, if soil drains; Better first step: Extend downspout, then rain garden
Water pooling against the foundation: Rain garden?: No; Better first step: Regrade to slope away from the house
Whole backyard soggy for days: Rain garden?: Usually not alone; Better first step: Grading plus subsurface drainage
Water from a neighbour's lot: Rain garden?: Not alone; Better first step: Check the approved grading plan
Sandy soil, good drainage: Rain garden?: Yes, works very well; Better first step: Rain garden
Maintenance
Water new plants during their first summer.
Pull weeds in spring and early summer.
Top up mulch once a year, but don't bury plant crowns.
Clear leaves and sediment from the inlet each fall.
After a big storm, check that water drains within 48 hours. If it doesn't, the soil mix may need work.
Local angle: London's clay and the Thames
London's storm sewers drain to local creeks and the Thames River. Every litre of roof water that soaks in at home is a litre that doesn't carry lawn fertilizer, road salt or sediment downstream. On heavy clay lots, the success of a rain garden depends on the soil preparation, so the digging and soil mix matter more than the plant list.
FAQ
Do rain gardens work in clay soil?
They can, but only if water drains within 24 hours in an infiltration test. On heavy clay, you usually need to dig out native soil and replace it with a compost and sand mix, or choose a better-draining spot.
How far from the house should a rain garden be in London?
The City of London recommends at least 3 m from the house and no more than 9 m from the downspout, in full or partial sun.
How do I size a rain garden?
London's formula is first-floor footprint × share of roof draining to that downspout ÷ 6. A 100 m² footprint with one quarter of the roof draining to one spot gives about 4.2 m².
Will a rain garden breed mosquitoes?
A properly built rain garden drains within 48 hours, which is shorter than mosquito breeding cycles. Standing water longer than that means the soil isn't draining and needs fixing.
Is there a City of London rebate for rain gardens?
We couldn't find a current City of London or UTRCA rain garden rebate. Check with the City before you build, since programs change.
Get help with grading, excavation and soil
SPM Landscape & Excavation has served London since 2006. We are WSIB covered and carry $1 million in liability insurance. We can excavate the basin, deliver topsoil and mulch, and fix grading so roof water goes where it should. Call (519) 474-0008 or email spmlandscape@gmail.com.
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