South American spongeplant
Limnobium laevigatum
Washington State Classification
Class A — Eradication Required Statewide
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.
Quick Reference
Identification
Growth Habit
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.
Leaves
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.
Flowers
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.
Fruit & Seeds
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.
Impact
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.
Ecology & Spread
Habitat
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. Click here to see a county-level distribution of South American spongeplant in Washington.
Spread Mechanisms
Reproduction
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.
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