It was all along a fence in a downtown core in Ottawa, flowering in November.
The example of Asteraceae provided by RHA seems more likely to be close to the correct answer, although it seems to me that the leaves don't look exactly the same as the one showed in Wikipedia for Tanacetum parthenium:
I disagree with @bli. I am pretty sure it is an Asteraceae, and I think I even know what species. It is Matricaria parthenium, also known as Chrysanthemun parthenium or Tanacetum parthenium. It is indigenous in Europe (Palearctic species) and it is also grown in gardens often. It is said to be a medicinal plant that works against fever and migraine. For more info see Wikipedia