Dame's Rocket or Dame's Violet, Hesperis matronalis
Brassicaceae or Mustard Family

Medium tall plant, 2-3' tall.  Leaves long, pointed, toothed.  Flowers large and showy, white, pink, violet or deep purple, with four petals.  Very pretty but alien invasive weed.  Common in disturbed areas along roads and the Riverway, and near bridges.  Blooms in late spring.

At first glance can be easily confused with smooth phlox; however, smooth phlox has  five petals, and leaves that are not toothed and are opposite each other on the stem.