Common St. Johnswort [Hypericum perforatum] photographed by Don Sinclair at his farm in Chatham, Ontario on 30 June 2013. It’s a member of theMangosteen (Clusiaceae) family. Newcomb’s Wildflower Guide, pages 268-267, says that it blooms in summer and fall grows, commonly found in fields and waste places.
USDA Plants Listing: https://plants.usda.gov/home/plantProfile?symbol=HYPE