Asian marshweed (Limnophila sessiliflora)

Synonyms: Ambulia sessiliflora, Hottonia sessiliflora, Stemodia sessiliflora

Common Names: Ambulia

Description:

Asian marshweed (Limnophila sessiliflora) is a rooted underwater plant from southern and Southeast Asia, sold in the aquarium trade as "ambulia" [1]. Released or dumped from aquariums, it has escaped into warm U.S. waters ? notably in Florida ? where it grows fast and forms dense underwater stands [1]. MISIN tracks it because it is a U.S. Federal Noxious Weed: it is illegal to sell or move across state lines, and it can crowd out native water plants and clog canals and waterways [1].

Habit:

Asian marshweed is a rooted aquatic plant whose soft stems grow from the bottom up to the surface, sometimes reaching 12 feet long [1]. Its underwater leaves are its signature: they grow in whorls ? rings of many leaves circling the stem ? and each leaf is finely divided into thread-like segments, giving the plant a soft, feathery look [1]. Where stems reach the surface and grow into the air, the leaves change to small, undivided, lance-shaped leaves in whorls of about five to eight [1]. Small stalkless flowers ? blue-violet, pink, or lavender, with petals fused into a little tube ? sit tucked against the stem in the leaf joints near the tips [1]. It is easiest to spot in the warm months when the feathery stems fill the water and reach the surface [1].

At-a-glance cues:

1. Underwater leaves in whorls (rings) around the stem, finely divided and feathery [1]
2. Many leaves per node ? not paired like fanwort [1]
3. Small, stalkless blue-violet to pink flowers tucked in the leaf joints near the stem tips [1]
4. Rooted soft stems growing up to the surface, sometimes 12 feet long [1]

Leaves:

1. Arranged in whorls (rings) around the stem; the plant has two leaf forms [1]
2. Underwater leaves finely divided into thread-like segments (feathery), in whorls of about 6 to 10 or more, roughly ? to 1? inches long [1]
3. Above-water (emergent) leaves undivided and lance-shaped, in whorls of 5 to 8, up to about 2 inches (5 cm) long [1]
4. Dark green [1]

Stems:

1. Soft, herbaceous aquatic perennial, rooted in the bottom sediment [1]
2. Stems grow up to about 12 feet (3.7 m) long, reaching from the bottom to the surface, with the tips sometimes emerging into the air [1]
3. Stem fragments root and grow readily, so broken pieces start new plants [1]

Flowers:

1. Small, blue-violet to pink or lavender, with five petals fused into a short tube [1]
2. Stalkless (sessile), borne singly in the leaf joints toward the upper part of the stem [1]
3. Appear in the warm season near the water surface [1]
4. Each flower has both male and female parts (perfect) [1]

Fruit and seeds:

1. Produces small seed capsules; a single flower may yield 200 to 300 tiny seeds [1]
2. Seeds germinate at very high rates (reported around 96%) [1]
3. The plant also spreads without seed, by rooting from broken stem fragments [1]

Habitat:

Asian marshweed grows in fresh water ? lakes, ponds, rivers, springs, streams, canals, and ditches ? and on damp soil at the water's edge [1]. It tolerates a wide range of conditions, including low light, which helps it spread [1]. It is native to southern and Southeast Asia, including India, Sri Lanka, and the Philippines, and was brought to the United States through the aquarium trade [1]. In warm U.S. waters, especially in Florida, it has escaped and become invasive [1][2].

Reproduction:

Asian marshweed spreads in two ways, and both are easy to picture [1]. It breaks into pieces easily, and each broken stem fragment can root and grow into a new plant ? so it travels on boats, trailers, and gear, and when aquarium or water-garden plants and their bits are dumped into a waterway [1]. It also makes seeds: a single flower can produce 200 to 300 of them, and they sprout at very high rates, so a population can build quickly [1].

Because a single fragment can start a new infestation, the key to control is removing the whole plant and every loose piece ? and, for a federally regulated plant like this, catching it early and reporting it before it spreads [1].

Impact and Damage:

Asian marshweed is not poisonous to people or pets ? its harm is to the water itself [1]. It grows rapidly and forms dense underwater stands that fill the water column from the bottom up to the surface, sometimes to a depth of 12 feet [1]. Those thick stands shade out and outcompete native underwater plants, taking over the habitat that native plants, fish, and other aquatic life depend on [1]. The dense growth also clogs canals, irrigation ditches, and flood-control channels, and has fouled pumping and power-station intakes in Florida [1]. Because it spreads fast, tolerates a wide range of conditions, and is hard to remove once established, it is regulated at the federal level as a noxious weed [1].

Similar species:

Asian marshweed is mistaken for other feathery underwater plants. The whorls of many finely divided leaves around the stem are the best clue.

Fanwort (Cabomba caroliniana) is the closest aquarium look-alike, with similar finely divided underwater leaves. But fanwort's leaves are paired ? two per joint, on opposite sides of the stem ? while Asian marshweed has many leaves in a ring at each joint [1].

Parrotfeather (Myriophyllum aquaticum) is another feathery invasive with a similar growth habit. Its above-water leaves are a waxy, powdery blue-green and very deeply divided, unlike the dark green leaves of Asian marshweed [1].

Eurasian watermilfoil (Myriophyllum spicatum) also has whorled, feather-divided underwater leaves and forms dense underwater stands. Its leaves are divided into many thread-like pairs along a central rib, and it lacks the small stalkless tubular flowers tucked in the leaf joints [1].

Coontail (Ceratophyllum demersum) is a native underwater plant with whorled leaves, but its leaves are stiff and forked with a rough, slightly toothed feel, not soft and feather-divided, and it has no roots [1].

Monitoring and rapid response:

Signs to watch for:

1. Rooted underwater stems with whorls of soft, finely divided, feathery leaves, reaching toward the surface [1]
2. Small stalkless blue-violet to pink flowers tucked in the leaf joints near the stem tips [1]
3. Dense underwater stands filling a pond, canal, or slow stream [1]
4. New growth where aquarium plants or water may have been dumped [1]

What you can do:

1. Never dump aquarium plants, water, or water-garden plants into any pond, lake, stream, ditch, or storm drain ? this is the main way it arrives [1]
2. If you think you have found it, report it right away ? early detection is the most useful step for a federally regulated plant [1]
3. In a small pond you own, rake or pull out the plants and collect every loose fragment, since each piece can re-root [1]
4. Bag and dispose of all plant material on land ? never return pieces to the water [1]
5. Clean, drain, and dry boats, trailers, nets, and gear before moving between waters so fragments do not hitch a ride [1]
6. Natural area crews and lake managers should treat any find as a rapid-response priority, remove entire stands and fragments, and coordinate with the state aquatic invasive species program [1]

When to call a professional:

1. Any find of this federally regulated plant ? contact your state aquatic invasive species program or land manager for guidance [1]
2. Large stands, or any infestation in flowing or large water bodies that cannot be fully cleared by hand [1]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Texas Invasive Species (Texas Invasives). Limnophila sessiliflora (Asian marshweed). texasinvasives.org
2. USDA NRCS PLANTS Database. Limnophila sessiliflora (Vahl) Blume (LISE3). plants.usda.gov

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