Trees Please Hamilton

Green Solutions to Air Pollution

How to Build a Rain Garden 101

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Last Tuesday’s workshop focused on building rain gardens at home, through the expertise of Michael Albanese of AVESI Storm Water Management Solutions.  While we love rain gardens for the stormwater management and beautiful plants, Michael’s most important priority with the construction of rain gardens is to keep the people and properties safe from water damage.

There are three main approaches to rain garden design:

  1. Excavate and replace: excavate all soil at least one metre deep, replace with a pre-made mixture of sand, compost, and top soil. No tilling of the subsoil.
  2. Dig and shape: excavate to desired depth, no outside soil added, shape rain garden
  3. Excavate and amend: Michael’s preferred approach, in which one excavates to the desired depth, till the subsoil, mix in compost, and shape rain garden.

Before you begin, identify the existing grading and drainage of the property, and how the house was designed. Mark out all downspouts, and assess how to redirect them to take advantage of the natural gradient of the property.  Be aware if you notice water pooling somewhere on your property: this indicates poor drainage, and it would be better to put the garden slightly upstream of this point to help it drain a bit before reaching that spot.  Keep the garden about 3 metres from the foundation of the house. Be sure to call Ontario One Call before digging.


As you begin designing, consider the following:

  • How big is the contributing area?  Your garden footprint should be about 10-20% of this size.
  • How big and deep should the garden be? Ponding depth should be between 2-8″, with soil amendments around 6-12″ deep. Your rain garden can be any shape or size that works for your property – there are no set rules.
  • How is water getting in and out?
  • Is the area in sun or shade? What species should you choose to plant?
  • What are the properties of the native soil? How should this be amended?

As much of the soil in Hamilton is dense clay, you need to dig out some of this layer and remediate with a mixture of other soils.  If you dig too deep to replace clay, you create a “bathtub effect”, where a large quantity of water fills up without an overflow drain, and plants sit soaking for longer than 24 hours.  Amend the soil by tilling, adding compost and garden soil.  Choose native, water-tolerant plants with deep roots to help maintain the rain garden over time.  Native wildflowers beloved by pollinators (ex: asters, coneflower, black-eyed Susan, goldenrod, etc.) will have your garden working double duty.  Make sure each plant has room to grow, and avoid species that will drastically outcompete the others.  Top with a shredded mulch (not chipped) 2-4″ deep.


Photo: Michael Albanese

You can slow down water as it makes its way to your rain garden by adding river rock with some liner underneath to help infiltration. You may want to redirect using a rain barrel, with the overflow hose leading to the rain garden.  Take proper care of your rain barrel. Make sure there is a spigot on the bottom high enough off the ground so you can fit your watering can underneath, some sort of overflow management on top, and a filter at the downspout entry.  Always drain the barrel before the next rain to ensure full capacity, and make sure to direct the overflow downhill and away from the house.  Empty and seal your barrel before winter, or just turn it over to protect from damage.

When designing your rain management system, take the “Treatment Train” approach to combine features to handle more rain.  For example, connect your downspout to a rain barrel, which drains into a bioswale, flowing into a rain garden.


Photo: Michael Albanese

Michael’s main message from the evening: it’s easier than you think!  There are many resources out there to help you along the way.  Give it a try and get digging!


Author: treespleasehamilton

A project of the Hamilton Naturalists' Club and Environment Hamilton. Funding by Ontario Trillium Foundation.

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