I’m in Toronto and see this weed everywhere, at the park in my garden, and everywhere in between. I’ve gone through the Ontario government weed site and cannot seem to find what this is.. looking for your expertise!
Without a picture of the flowers along with the leaves it is usually difficult to give a positive identification as to what the plant may be.
That being said the plant in question appears to be Solidago canadensis (Canada goldenrod).
S. canadensis is a perennial plant that will return year after year and spreads via underground rhizomes. It has plumes of yellow flowers that bloom in late summer and fall which provide vital sources of pollen and nectar for bees and other insects.