I am just outside of Toronto.
Is there a specific definition of botanic gardens? There is a park 3.5 inches from me called Rhododendron Gardens that has all kinds of botany growing there all through the summer. A lot of it is labeled and educational n' shit, but it is just a plain old free public park with lots non-botanical areas for bike riding and picnicking and playing frisbee-football, so I don't know if it meets the formal definition of botanical garden.
There is a big honking botanical garden in Burlington which is about twenty minutes in the opposite direction of Toronto. It is definitely a Botanical Garden. A quick google check reveals it being called the biggest (baddest?) botanical garden in Canada. The Royal Botanical Gardens.
I haven't been there since I was a kid. I don't know about Toronto.