Closely resembles Bog Goldenrod and Ohio Goldenrod - almost a cross between these two species. But mostly looks like an Ohio Goldenrod with large flowers. Only occurs at a few sites in Ontario.
Similar species: Ohio Goldenrod (Solidago ohioensis) - has smaller flowers, and fewer flowers; much more common. Bog Goldenrod (Solidago uliginosa) - leaves are somewhat clasping the stem.
Houghton's Goldenrod is extremely rare in Ontario, being found mainly in only one place (a very small area of the Bruce Peninsula). In fact, it very rare outside of Ontario as well. This Goldenrod closely resembles Ohio Goldenrod (Solidago ohioensis).
These pictures of its flowers were taken in mid-August.