Marsh seedbox, water purslane
This smallish native primrose willow is frequent in the marshes, swamps and pond margins of northern Florida, south to the central peninsula. It blooms from spring to fall (Wunderlin, 2003). Of the thirty or so Ludwigia species in Florida, only four have opposite leaves. This species has opposite leaves, and might be confused with Ludwigia repens, except that L. repens has more rounded, spatula-shaped leaves which are reddish underneath.
This species occurs throughout most of the U.S. and Canada (Kartesz, 1999).