At least two of your three photos have been misidentified here. The second and third photo which show large yellow flowers are not Swamp False Loosestrife (Ludwigia palustris) and the common name used here is inappropriate for a plant described in Herbaceous Plants of MD (Brown and Brown, 1984) as "Stems glabrous, floating or prostrate, soft and weak. Leaf blades ovate to oval, 5-30 mm [These look much longer, possibly as much as 75-100mm.] long; tips acute, bases abruptly narrowed to a petiole that often equals the blade in length. Flowers sessile [The flowers in your two photos have long peduncles.], solitary and axillary, about 2 mm wide [these look over 25 mm wide]; petals lacking or minute, reddish if above water [these petals are large and bright yellow]." The first photo appears to be an earlier season photo of the same species with the floating stems that are just starting to turn upright into flowering stems.
This is most likely one of the Primrose Willows (Jussiaea spp. now placed by some in the Genus Ludwigia). There are two Exotic species in this group which appear to be becoming Invasive in the Potomac River near Washington, DC. They are Creeping Primrose Willow aka Creeping Water Primrose (Jussiaea repens aka Ludwigia peploides) and Showy Primrose Willow aka Showy Water Primrose (Jussiaea uruguayensis aka Ludwigia grandiflora ssp. hexapetala). The latter has "Stems creeping or floating, rooting at nodes, with flowering portions erect and branching, sparsely to densely villous. Leaves to 17 x 4.5 cm ... upper leaves acute ..." [From Flora of VA (Weakley et al, 2012)]. There is a Native species, Upright Primrose Willow (Jussiaea decurrens aka Ludwigia decurrens), which grows upright and has no floating stems. In my opinion this is probably Showy Primrose Willow although Weakley says the flowering stems are supposed to be villous.
Please get this identified correctly and post it under the correct name. This is probably not any of the species which were originally in the Genus Ludwigia.
The three photos by plantladylin appear to be correctly identified as Swamp False Loosestrife (Ludwigia palustris).
The common name here needs to be changed and your three photos need to be removed from this page and placed on the appropriate page when correctly identified. If your plants are Showy Primrose Willow, as I think, please contact the park where they are growing and have them removed and then keep the area monitored for a recurrence of this potentially Invasive species.
Joe Metzger
