Common Yellow Woodsorrel (Oxalis stricta)
"Common yellow wood sorrel grows in a variety of habitats from open forest to fields and gardens. It can be a difficult garden weed. It is reported from all the contiguous United States except Oregon, California, Nevada, and Utah, but likely occurs in those states as well. It is also reported from Newfoundland Labrador west to Manitoba, and British Columbia.
This species may flower throughout the year depending on the part of the country in which it is found. It most commonly flowers spring to fall. Bees, ants and butterflies visit the flowers. The leaves fold at night and open again during the day. The plant contains oxalic acid which gives it a sharp taste. Some people use the leaves in salads, but consuming large amounts may contribute to kidney stones...."