Rue-Anemone (Thalictrum thalictroides syn. Anemone thalictroides). Photographed on the Niagara Escarpment near Dundas, Ontario, May 9th, 2011.
In Canada, rue-anemone is restricted to the Carolinian zone of southern Ontario, and it is not common there. It is a true spring ephemeral, disappearing after it sets seeds and spending the summer as a small tuber. Garden varieties of rue-anemone, with pink or pale blue flowers, or double petals, are lovely in shady rock gardens, but I have found them to be a bit miffy, unless one can give them a nice loose, woodsy soil.
The wild rue-anemone in the photo was growing in close proximity to another low-growing ephemeral woodland anemone, wood anemone (Anemone quinquefolia). Wood anemone is also Carolinian, with a slightly larger range, and is more common. It can form large patches in rich deciduous woods, to charming effect.