Amazon frogbit

Limnobium laevigatum
Aquatic
Whorled
Simple
White
Green

Frogbit is a floating freshwater plant from Central and South America. It has been kept and traded for use in fish ponds, aquariums and water features. It can rapidly invade and smother waterways and is a serious biosecurity threat. Plants can form into large mats of runners and adult plants can develop very quickly. Juvenile plants have a great capacity for distribution in that they are small and can be easily and quickly carried along by water currents.

Common names 
Also known as: South American spongeplant, smooth frogbit, West Indian spongeplant,
Family 
Hydrocharitaceae
Deciduous 
No
Native/Exotic 
Exotic
Origin 
Fresh water habitats of tropical and subtropical Central and South America
Notifiable 
No
Council declaration 
Class E - Early detection and eradication
Known distribution 

Has been reported in Queensland waterways

Habitat 

L. laevigatum originates from fresh water habitats of tropical and subtropical Central and South America. It occurs in rivers, ponds, lakes, canals and other aquatic habitats. In Puerto Rico it is reported to grow in shallow ponds, sluggish shaded rivers, fresh-water ditches and swamps at or near sea level.

Habit 

The growth habit of L. laevigatum is similar to that of most other floating plants. The floating rosettes send runners out into the water, the ends of which form juvenile plants. A large mat of runners and adult plants can develop very quickly.

A large mat of runners and leaves can develop very quickly, smothering waterways with a similar impact to that of salvinia 

Stem and leaves 

L. laevigatum is a floating or emergent aquatic herb. Leaves are subcircular, floating, glabrous and glossy above, with a thick layer of air-filled spongy tissue beneath, base rounded or shallowly cordate. Juvenile plants grow in rosettes of floating leaves that lie prostrate upon the water surface. A distinguishing character of the juvenile plant is the presence of spongy aerenchyma tissue upon the underside of the leaf. Mature plants grow up to 50 cm tall and have emergent leaves borne on petioles that are not swollen or inflated like the spongy leaf stalks of water hyacinth. The plant produces stolons which bear ramets which may be the main source of new plants where flowers are unknown.

Flowers and fruits 

Flowers are small, white, and unisexual. Female flowers have an inferior ovary. The fruit is a fleshy, berry-like capsule 4–13 mm long and 2–5 mm in diameter, borne on a recurved pedicel, developing in mud or under water. The fruit contains up to 100 seeds. The seeds are 1 mm long, ellipsoid, and hairy The small, floating seeds easily disperse via water and wind once produced.

Reproduction and dispersal 

L. laevigatum can reproduce sexually through flower pollination and seed production. The species is monoecious (there are separate male and female flowers on the same plant) and plants are autogamous (pollen from male flowers pollinate female flowers on the same plant). After pollination the pedicel of female flowers bends downward, forcing the fruit to develop in the water or in the mud. The seeds can germinate underwater. The survival time of seeds is not known. But at a pond in Redding, California seedlings of this species were appearing despite populations being almost completely suppressed for three years. This suggests that there may be a seed bank. It can also reproduce vegetatively through fragmentation of stolon segments which connect rosettes.

Similar species 

water hyacinth.