India and Malaysia
Reported as an expanding problem in south Florida canals in 1980 (Vandiver); now replacing the well-known hydrilla as the most serious weed in these waterways (Sutton 1995), clogging irrigation and flood-control systems and interfering with navigation (Woolfe 1995).
Perennial aquatic herb with squarish stems ascending to creeping, mostly submersed, usually rooted in substrate; also roots freely at floating nodes. Leaves opposite, to 8 cm (3 in) long (aerial leaves smaller) and to 2 cm (0.8 in) wide, usually broader toward tip; sessile, with bases joined at node by ciliated flanges of tissue, the cilia (hairs) easily observed, to 1.5 mm long. Flowers small, solitary in uppermost leaf axils, nearly hidden by leaves, calyx 5-lobed, corolla bluish white, 2-lipped; 2 fertile stamens. Fruit a narrow capsule, splitting lengthwise to release tiny round seeds. (Langeland and Burks, 1998)
May be confused vegetatively with small, opposite-leaved natives sometimes found submersed, such as Ludwigia repens and Diodia spp., but these without flanges at nodes (Ludwigia) or with flat-bristled flanges (Diodia). The native marsh species, Hygrophila lacustris (Schlecht. & Cham.) Nees is larger (aerial leaves to 15 cm long) and erect in habit, with larger flowers in axillary clusters along upper stems.
Appeared in the aquarium trade in 1945 as “oriental ludwigia” (Innes 1947). First collected in Florida near Tampa as an escapee from cultivation in 1965, but naturalized populations on east coast, especially one near the town of Miramar in Broward County, first brought to public and scientific attention in the late 1970s (Vandiver 1980, Les and Wunderlin 1981).
Found in Florida from Miami-Dade and Lee counties north into the Panhandle (Wunderlin et al. 1995, Schardt 1997).
Very difficult to control. Effective herbicide has yet to be found. New insect biological controls are being pursued.