large-flowered primrose-willow

Ludwigia grandiflora, the water primrose, is an aquatic plant of the order Myrtales.

It is closely related and easily confused with Ludwigia hexapetala. The two species can be distinguished at a chromosomal level, because L. grandiflora is hexaploid and L. hexapetala is decaploid. However, they can be distinguished morphologically. L. grandiflora has villous hairs, smaller flowers and smaller pollen grains. Some authorities consider that these differences are too slight to consider these different species and so separate these taxa as two varieties or two subspecies.