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Ludwigia anastomosans has been previously considered to be an isolated member of L. sect. Myrtocarpus because of its unique tree habit, black water streams habitat, and leaf venation. Recent additional information adds to its distinctiveness and includes flowers lacking a style, whereas all the other species of Ludwigia have one; capsules dehiscing along one of the lines of union between each of the four carpels, whereas in the other species of Ludwigia the capsules are irregularly dehiscent or, if regularly dehiscent, in a very different way; and seeds possessing an enlarged wing-shaped raphe. Ludwigia anastomosans is self-compatible and outcrossing but tardily autogamous when not cross-pollinated, and tetraploid. We have thus concluded that the species is best treated in a section of its own in keeping with the usage of this taxon elsewhere in the family. Section Arborescentes is here newly described.