There is a natural garden by-law. It does not apply to city-maintained property at all.
In respect of private property, it permits an intentionally naturalized garden. There are some guidelines on what that means. It doesn't come with a specific plant list; but it is not a blanket pass for neglect.
The City absolutely have power to take action where it poses any risk to the public. That certainly includes sight lines, but is not limited to it. A woman was cited not so long ago for leaving a dead racoon carcass on her lawn.
In general, if a private owner keeps a clear edge/border (not necessarily mowed, just a defined limit such that the nature does not encroach the public ROW and there are no other issues with it from a public safety stand point, they will get fairly wide latitude.
Again, all of that is for a private lawn, not a City boulevard.