South American spongeplant

Limnobium laevigatum

Family: Hydrocharitaceae

Other Scientific Names:

Limnobium stoloniferum, Hydromystria laevigata, Hydromystria stolonifera, Salvinia laevigata, Limnobium spongia spp. laevigatum

Other Common Names: frogbit, West Indian spongeplant, Amazon frogbit, smooth frogbit
Weed class: A
Year Listed: 2020
Native to: Mexico, Central America, the Caribbean including Puerto Rico, and south through South America to Buenos Aires, Argentina
Is this Weed Toxic?:

not known to be

Legal listings:

Currently being considered for addition to the Washington State quarantine list (WAC 16-752)

Why Is It a Noxious Weed?

Known only from one location so far in WA, South American spongeplant is an aquatic perennial that can form dense mats on the water surface. Plants can spread vegetatively by producing daughter plants and also by seed, which can be viable for at least 3 to 4 years. In California where it is already listed as a noxious weed, mats of South American spongeplant can outcompete other invasive aquatic plants such as parrotfeather (Myriophyllum aquaicum) and water primrose (Ludwigia species). It also has accumulated on infrastructure, blocking pumps, dams, and gates, as well as recreational water access.

How would I identify it?

General Description

South American spongeplant is a perennial herb that is generally free-floating, though it will also grow rooted in mud in shallow water or on wet shorelines. Young plants resemble duckweed, then they develop into a rosette stage and finally a mature phase with stalked emergent leaves.

The floating leaves have spongy honeycomb-like aerenchymous tissue on the underside, giving the plant its common name and providing floatation. It has small pale green to white flowers, and female flowers stalks bend as the fruit ripens so seeds are released in the water or sediment.

Flower Description

Plants have white, inconspicuous flowers that are around 1/2 inch wide. Flowers are male or female and are on the same plant (monoecious). Flowers have greenish-white sepals, around 5 mm long by 2 mm wide and spreading. Male flowers have petals and 6 stamens, while female flowers typically do not petals and have 3 to 6 divided styles.

Leaf description

Leaves are floating or aerial. Aerial leaves on stalks up to 10.6 inches (27 cm) long, while floating leaves on a stalk up to 3.3 inches (8.5 cm) long. Mature leaves stalks have a stipule at the base, up to 0.9 inch (22 mm) long. Leaf blades 0.8 to 2 inches (20 to 50 mm) long by 0.3 to 1.6 inches (8 to 40 mm) wide. Blades usually have a rounded tip and base. There is typically honeycomb, spongy-like tissue (aerenchymous tissue), up to 0.4 inch (1 cm) thick on underside of floating leaves.

Stem description

Plants have compact stems, up to 0.8 inches (20 mm), that bear its leaves. The runners/stolons branch off the stem.

Fruit Seed Description

Its fruit is a soft capsule, about 9 mm long, that forms up to 100 small seeds. Each capsule on a stem that, after pollination, bends and the capsule mature underwater or in mud.

May Be Confused With

South American spongeplant may be confused with American spongeplant (Limnobium spongia), the only other species in the genus. See pictures of both South American spongeplant and American spongeplant here, with helpful comparison images here and here.

Characteristics used to distinguish the two:

South American spongeplant leaves are typically rounded at the tip and base, fruits are up to 5 mm in diameter, female flowers generally lack petals, male flowers usually have 6 stamens.
American spongeplant leaves are typically heart-shaped; fruits are 4-12 mm in diameter, female flowers have petals, male flowers have 9-12 stamens. Click here for pictures of Limnobium spongia.

When not in flower, South American spongeplant may be confused with European frogs’s-bit (Hydrocharis morsus-ranae), which has been found in Washington. European frogs’s-bit has exclusively floating leaves with typically no spongy tissue on leaf undersides and unbranched roots.

Water hyacinth (Eichhornia crassipes) is an aquatic plant, sold as an ornamental pond plant, and is on our monitor list. It is a free floating plant that has distinct swollen leaf stalks and purple flowers.

Where does it grow?

Plants grow in aquatic habitats such as wetlands, irrigation canals, lakes, reservoirs, rivers, streams, and ponds. It can grow in full sun and shade and prefers water temperatures between 59 and 64.4 degrees F (15 – 18 C) and can tolerate mild salinity. Plants are reported to survive frost where protected, but are thought not to be cold hardy where temperatures regularly drop below freezing. So far only one location of South American spongeplant is known of in Washington state, growing in Pacific County.

How Does it Reproduce?

Plants reproduce both by seed and vegetative production of daughter plants. Seeds remain viable at least 3 to 4 years. Plants spread by water and wind, and on birds, watercraft and in the horticulture plant trade.

How Do I Control It?

General information:

Make sure to clean watercraft, equipment, and boots to prevent spreading plants to new locations.

Manual/Mechanical Control:

Small populations can be removed by hand. Carefully remove all plants and plant parts. Remove plants prior to seed production to prevent further addition of seed on site. Monitor the site and continue hand removal as needed for at least 3 to 4 years as seeds can be viable at least that long.

Herbicide Control:

Use of pesticides in water is regulated in Washington State. All applicators must have an aquatic endorsement on their pesticide applicators license, which is issued by the Washington Department of Agriculture. In addition, coverage under a permit issued by the Department of Ecology is required. See the WA Department of Ecology for details or contact your county noxious weed board. General herbicide information available in the book "Weed Control in Natural Areas in the Western United States".