Native to: Eastern and Southeastern Asia
Currently invasive in Texas, Georgia, and Florida, Limnophila likely escaped cultivation in Florida where it was first recorded from Hillsborough County in 1961. In addition to being listed on the Federal Noxious Weed List, it is regulated in 5 states including Florida. Little is known about its introduction, but it has been documented being sold in the aquarium trade.
Habit: Submerged to emergent perennial aquatic herb, has two distinctly different forms of leaves, submersed and emersed. Stems grow to 12 feet, with several inches erectly emersed leaves in whorls along the stem.
Leaves: Mostly to 1.5 in. (5-40 mm) long. Emersed leaves dark green, more-or-less lance-shaped; in whorls of 5-8 leaves about the stem (also reported as 4-12, Gilbert 1984); margins appear to be torn irregularly (crenate-serrate to variously lacerate). Submersed leaves are finely divided and feathery, segments opposite; ovate, elliptic to broadly lanceolate; in whorls of 6-10 leaves (and more) about the stem.
Flowers: Small, sessile (without stalks); solitary in leaf axils (angle where leaf meets the stem) in the apical (uppermost) parts of the stems, above the water (aerial); corolla (petals) 5-10 mm long, blue, violet, pink or lavender, upper lip white or pink with 2 blue dots, 3-lobed, lobes ovate; calyx 4-7 mm long, hairy (pubescent); pedicels stout (not slender).
Fruit: Capsules, ellipsoid, 3.5-5.5 mm long, green-brown when submersed, dark brown when emersed. Can contain up to 300 seeds.
Distribution in Florida: Reported from Central and South Florida as well as a few counties in the Panhandle.
Fast-growing with high seed germination rates and able to regrow from fragments, Limnophila can form dense stands from the bottom to the top of the water column altering the aquatic system functions. Infestations may outcompete native aquatic vegetation, reduce oxygen causing fish kills, and limit recreational activities. It also clogs irrigation and flood-control canals and is a major problem in paddy rice fields of India, China, Japan and the Philippines.
Transporting plant fragments on boats, trailers, and in livewells is the main introduction route to new lakes and rivers. Always clean boats and equipment thoroughly before leaving waterways.
Never dump or release water from aquarium systems with live plant material into waterways.
Not recommended. The action of mechanical harvesters and chopping machines serves to help spread this invasive plant, which re-grows from leaf fragments.
More research is needed. Registered aquatic herbicides provide very limited control of this species; however, high levels of 2-4,D reportedly kills this plant (Mahler 1980). Reach out to your local UF IFAS Extension for further assistance with management recommendations.