Ambulia (Limnophila sessiliflora)

Assessment ID: 4680
Jurisdiction: North Territory
Methodology: Northern Territory Weed Risk Management System - aquatic
Management area: Darwin Region

Notes: Technical report available here https://territorystories.nt.gov.au/10070/971428/0/0

Status: PUBLISHED
Last updated: 18/6/2025 4:43:38 pm AEST

Questions answered: 38 / 46

Weed Risk: 163.56 (High)
   Invasiveness (raw/adjusted): 8.00 / 6.664
   Impacts (raw/adjusted): 7.00 / 3.682
   Potential distribution (raw/adjusted): 6.00 / 6.666

Feasibility of Control: 223.69 (High)
   Control costs (raw/adjusted): 4.00 / 3.636
   Current distribution (raw/adjusted): 10.00 / 10.000
   Persistence (raw/adjusted): 8.00 / 6.152

Result: Protect priority sites

Invasiveness

A1. What is the ability of the plant to establish amongst intact native environments?
Answer: Very High

Comments: 
Findings from the competition experiment showed that ambulia was significantly reduced in area occupied when grown with hydrilla in the warm treatment. Under the same conditions, the cover of cabomba is significantly reduced when grown in competition with ambulia and the area of ambulia is also reduced in competition with cabomba, indicating that the two species are competitively similar and dominance of one species over the other is more likely to result from chance events (Champion et al. 2008).

Limnophila sessiliflora is another high volume species in the aquarium trade (traded as L. heterophylla). It has replaced cabomba since the designation of that species as a WONS. It has naturalised in several countries and is federally banned from USA. It has also been reported from a field site in New South Wales, but not confirmed as such. Champion et al. 2008 Areas commonly invaded by L. sessiliflora are slow-moving freshwater ecosystems such as lakes, ponds, swamps, streams, ditches, and canals (Koncki & Aronson 2015).

These three species are well known as noxious invaders of freshwater habitats in the southeastern United States as well as globally, and none are persistent in the northeastern United States. All three species appear to be limited to the southern United States because they require a warmer climate (Koncki & Aronson 2015).

The outbreak in Palmerston Durack community lakes has been growing rapidly (NT WRTC 2022).

A2. What is the reproductive ability of the plant?

(a) Time to seeding
Answer: 2-3 years

Comments: 
It seems unlikely that it could grow from a seed to an emergent plant and set seed in one year (PPQ 2020).

(b) Annual production of viable seed /m2 or /plant
Answer: High

Comments: 
Unlike the other species, ambulia produced viable seed in Florida (~300 seed/flower) (Champion et al. 2008).

Limnophila sessiliflora has the ability to reproduce sexually, with each flower able to produce as many as 300 seeds with up to a 96% germination rate (Koncki & Aronson 2015).

Each flower of Limnophila sessiliflora may set 200-300 seeds with a germination rate as high as 96% (Ramey 2001).

(c) Vegetative reproduction
Answer: Frequent

Comments: 
In addition to sexual reproduction, it can reproduce through stem fragmentation (Koncki & Aronson 2015).

It is highly likely that mutilation would benefit the plant as it does for other aquatics such as hydrilla. Langeland 1996 Reproduces asexually (regrows from plant fragments) (Ramey 2001).

Fast growing, it reproduces by both seeds and stem fragments (Scher et al. 2015).

A3. Do propagules of the plant have properties that allow them to be dispersed long-distance by natural means?

(a) Flying animals (birds, bats)
Answer: No

Comments: 
No specific information available. Based on form and habit, the WRTC agreed that dispersal was likely only via water and not via animals or wind (NT WRTC 2022).

(b) Other wild animals
Answer: No

Comments: 
No specific information available. Based on form and habit, the WRTC agreed that dispersal was likely only via water and not via animals or wind (NT WRTC 2022).

(c) Water
Answer: Yes

Comments: 
L. sessiliflora is found frequently in riparian habitats, this species inhabits areas much farther from streams than A. philoxeroides and S. molesta. The inability of L. sessiliflora to establish in the habitats closest to streams could limit how rapidly it will disperse to nearby locations. Previous studies have indicated that rivers can be important conduits for dispersal for species that have a high capacity to float and reproduce vegetatively (Koncki & Aronson 2015).

Fast growing, it reproduces by both seeds and stem fragments (Scher et al. 2015).

Spencer and Bowes (1985) describe the fragments as easily dispersed, presumably on the water (Spencer & Bowes 1985).

(d) Wind
Answer: No

Comments: 
No specific information available. Based on form and habit, the WRTC agreed that dispersal was likely only via water and not via animals or wind (NT WRTC 2022).

A4. How likely is long-distance dispersal by human-assisted means?

(a) Deliberate spread by people
Answer: Unlikely

Comments: 
Limnophila sessiliflora is another high volume species in the aquarium trade (traded as L. heterophylla). It has replaced cabomba since the designation of that species as a WONS. It has naturalised in several countries and is federally banned from USA. It has also been reported from a field site in New South Wales, but not confirmed as such (Champion et al 2008).

Although it is and has been in trade for some time, there have only been two known instances of it occurring in the wild, and one is unconfirmed (NT WRTC 2022).

(b) Accidentally by people and vehicles
Answer: Common

Comments: 
Limnophila sessiliflora is another high volume species in the aquarium trade (traded as L. heterophylla). It has replaced cabomba since the designation of that species as a WONS. It has naturalised in several countries and is federally banned from USA. It has also been reported from a field site in New South Wales, but not confirmed as such (Champion et al 2008).

(c) Contaminated produce
Answer: Unlikely

Comments: 
No specific information available. Based on habitat, form and habit, the WRTC agreed that spread via contaminated produce, or domestic/farm animals was unlikely (NT WRTC 2022).

(d) Domestic/farm animals
Answer: Unlikely

Comments: 
No specific information available. Based on habitat, form and habit, the WRTC agreed that spread via contaminated produce, or domestic/farm animals was unlikely (NT WRTC 2022).

Impacts

B1. What is the plant's competitive potential?
Answer: Medium

Comments: 
Findings from the competition experiment showed that ambulia was significantly reduced in area occupied when grown with hydrilla in the warm treatment. Under the same conditions, the cover of cabomba is significantly reduced when grown in competition with ambulia and the area of ambulia is also reduced in competition with cabomba, indicating that the two species are competitively similar and dominance of one species over the other is more likely to result from chance events (Champion et al 2008).

Limnophila sessiliflora is another high volume species in the aquarium trade (traded as L. heterophylla). It has replaced cabomba since the designation of that species as a WONS. It has naturalised in several countries and is federally banned from USA. It has also been reported from a field site in New South Wales, but not confirmed as such (Champion et al 2008).

Significant decreases in area were seen in Limnophila sessiliflora when grown in competition with Hydrilla verticillata under warm conditions, indicating that this species is superior in competition, and capable of displacing L. sessiliflora. Under the same conditions, the cover of C. caroliniana is significantly reduced when grown in competition with L. sessiliflora and the area of L. sessiliflora is also reduced in competition with C. caroliniana, indicating that the two species are competitively similar and dominance of one species of the other is more likely to result from chance events. The outcome of competition is more definite under cool conditions; competition with L. sessiliflora reduces the area occupied by C. caroliniana but L. sessiliflora is unaffected by C. caroliniana. Further support for the increased competitive ability of L. sessiliflora under cool conditions may be taken from the reduction of H. verticillata cover when growing with L. sessiliflora under cool conditions (Champion et al 2008).

More references available at https://territorystories.nt.gov.au/10070/971428/0/0

B2. What is the plant’s potential to modify the existing fire behaviour and alter the fire regime?
Answer: No potential

Comments: 
No specific information available. Based on habitat, form and habit, the WRTC agreed that there is no potential for impact on fire behaviour (NT WRTC 2022).

B3. What is the plant’s potential to restrict the physical movement of people, animals, vehicles, machinery and/or water?
Answer: Low

Comments: 
It is unlikely to actually prevent the movement of a boat, and is more likely to just break up and move aside for it (NT WRTC 2022).

It can outcompete both native plants and the invasive Hydrilla verticillata. It can clog canals and pump and power stations (PPQ 2020).

It may form dense stands from the bottom to the top of the water (Ramey 2001).

Its stems form dense stands, clogging canals and pumping and power stations in Florida. It is a major weed of rice fields in Asia. Fast growing, it reproduces by both seeds and stem fragments. Limnophila sessiliflora is resistant to chemical control and is reported to outcompete Hydrilla (Scher et al. 2015).

It can also produce dense mats on the water surface (Spencer & Bowes 1985).

B4. What is the plant’s potential to negatively affect the health of animals and/or people?
Answer: None

Comments: 
If it covers the water surface, it can shade out submerged species and deplete oxygen in the water, killing fish (PPQ 2020).

It can also produce dense mats on the water surface (Spencer & Bowes 1985).

B5. Does the plant potentially have negative effects on natural and cultural values?

(a) Reducing habitat quality for native animals?
Answer: Medium

Comments: 
If it was to establish in Fogg Dam Conservation Reserve it would fill the water column and replace the existing structure, including native food sources. It could potentially impact on multiple vulnerable and endangered species, including;
  - Monochoria hastata
  - Gulf snapping turtle - Elseya lavarackorum
  - Largetooth sawfish - Pristis pristis
  - Mertens’ water monitor - Varanus mertensi
  - Mitchell’s water monitor - Varanus mitchelli.
It could potentially impact on multiple sites of natural or cultural significance including;
  - Kakadu National Park
  - Garig Gunak Barlu National Park (Cobourg Peninsula) (NT WRTC 2022).

It can also produce dense mats on the water surface (Spencer & Bowes 1985).

(b) Threatened species or communities
Answer: More than one

Comments: 
It could potentially impact on multiple vulnerable and endangered species, including;
  - Monochoria hastata
  - Gulf snapping turtle - Elseya lavarackorum
  - Largetooth sawfish - Pristis pristis
  - Mertens’ water monitor - Varanus mertensi
  - Mitchell’s water monitor - Varanus mitchelli (NT WRTC 2022).

(c) Sites of natural or cultural significance
Answer: More than one

Comments: 
It could potentially impact on multiple sites of natural or cultural significance including;
  - Kakadu National Park
  - Garig Gunak Barlu National Park (Cobourg Peninsula) (NT WRTC 2022).

It can also produce dense mats on the water surface (Spencer & Bowes 1985).

B6. Is the plant presumed to have negative effects on environmental health?

(a) Soil chemistry/stability
Answer: No

Comments: 
No specific information available. The NT WRTC agreed that if an impact existed on soil chemistry or stability, it would have been identified during the USDA weed risk assessment (NT WRTC 2022).

(b) Water quality
Answer: Yes

Comments: 
Areas commonly invaded by L. sessiliflora are slow-moving freshwater ecosystems such as lakes, ponds, swamps, streams, ditches, and canals (Koncki & Aronson 2015).

Limnophila sessiliflora is a rooted herb, with both submersed and immersed plant parts, that can form a dense mat up to 3 m (9.8 ft) in depth (Koncki & Aronson 2015).

It takes over the water column and impacts on ecology and water quality (NT WRTC 2022).

It can outcompete both native plants and the invasive Hydrilla verticillata. It can clog canals and pump and power stations (PPQ 2020).

The submerged leaves are adapted to shade and will continue to take up carbon dioxide with little light (Spencer & Bowes 1985).

(c) Hydrology
Answer: No

Comments: 
Areas commonly invaded by L. sessiliflora are slow-moving freshwater ecosystems such as lakes, ponds, swamps, streams, ditches, and canals (Koncki & Aronson 2015).

It is fairly soft, so it’s more likely to break and spread than actually block passage (NT WRTC 2022).

It can also produce dense mats on the water surface (Spencer & Bowes 1985).

Potential distribution

C2. How many broad habitat types in the NT will the plant potentially naturalise in (up to 5)?
Answer: Four

Comments: 
No specific information available. The NT WRTC agreed that it has the potential to naturalise in tropical wetlands, tropical floodplains, arid and semi-arid wetlands, and in permanent water bodies (NT WRTC 2022).

C3. What is the potential of the plant to occur throughout its favoured habitat in the NT (as identified in question 2)?
Answer: Some

Comments: 
Its favoured habitat is permanent water bodies. Extreme depth in the wet season could impede its growth in some places, however fragmentation caused by the running water could aid in colonising new areas (NT WRTC 2022).

Limnophila sessiliflora is a submerged to emergent perennial aquatic herb that is primarily a weed of shallow water in natural areas. Limnophila sessiliflora is an aquatic plant and most likely to grow in still, shallow water (PPQ 2020).

Control costs

A1. How detectable is the weed?

(a) Distinguishing features
Answer: Non-descript

Comments: 
Limnophila sessiliflora is almost identical to the closely related and native species Limnophila brownii, and was not immediately recognised even at the herbarium until it was fully keyed out by a botanist (NT WRMC 2022).

(b) Active growth period
Answer: >8 months

Comments: 
It can grow all year as it is not affected by lack of rain (NT WRMC 2022).

(c) Height at maturity
Answer: <0.5 m

Comments: 
It grows predominantly below the surface of the water and has only small emergent stems or terrestrial growth (NT WRMC 2022).

A2. What is the general accessibility of infestations at the optimum treatment time?
Answer: High

Comments: 
The only known infestation is easily accessible in the Palmerston Durack community lakes, with both vehicle and boat access all year round (NT WRMC 2022).

A3. How expensive is control of the weed in the first year of targeted control, for an infestation that has reached maximum weed density?

(a) Chemical costs
Answer: Very high

Comments: 
Management of L. sessiliflora has been virtually unsuccessful because no current management strategies have significantly reduced plant populations, and aquatic herbicides have resulted in minimal reduction of the plant (Koncki and Aronson 2015).

To treat aquatic weeds you can’t just treat individual plants, you need to treat the entire water body as whole. This area would cost roughly $48k/ha. You would need to also spot-spray the land-based plants on the edges (NT WRMC 2022).

It is shade-tolerant, and herbicides have a limited effect on it (Spencer and Bowes 1985).

(b) Labour cost
Answer: Medium

Comments: 
Treatment with carfentrazone is extremely easy, with tablets ready to use, you can just throw them into strategic locations from the bank, and then follow up with some backpack spot spraying around the edges. Labour and access will be easy due to the current location, but if it got into Darwin River or somewhere else it would be almost impossible (NT WRMC 2022).

(c) Equipment cost
Answer: Medium

Comments: 
OPTIONS ARE WRONG, SHOULD BE - [LOW]

Treatment with carfentrazone is extremely easy, with tablets ready to use, you can just throw them into strategic locations from the bank, and then follow up with some backpack spot spraying around the edges. Labour and access will be easy due to the current location, but if it got into Darwin River or somewhere else it would be almost impossible (NT WRMC 2022).

A4. What is the general community perception of this weed within the region?
Answer: Low

Comments: 
Limnophila sessiliflora is almost identical to the closely related and native species Limnophila brownii, and was not immediately recognised even at the herbarium until it was fully keyed out by a botanist. The general public is not aware of it as a weed (NT WRMC 2022).

Current distribution

B1. What is the current pattern of the species’ distribution across the weed management region?
Answer: Restricted

Comments: 
It is only known to be present in one lake in the Durack community lakes in Palmerston (NT WRMC 2022).

Persistence

C1. How long will it take to reach the maintenance period?
Answer: High

Comments: 
OPTIONS ARE WRONG, SHOULD BE - [1-2 years]

Seeds can remain viable in the soil for up to 30 years (Les 2017 cited in PPQ. 2020).

C2. What is the minimum time period for reproduction of sexual or vegetative propagules?
Answer: <1 growing season

Comments: 
Vegetative reproduction is possible almost immediately (NT WRTC 2022).

C3. What is the maximum longevity of sexual or vegetative propagules?
Answer: >5 years

Comments: 
Seeds can remain viable in the soil for up to 30 years (Les 2017 cited in: PPQ 2020).

C4. What is the threat of reinfestation from outside the management region?

(a) Long-distance (>100m) dispersal by natural means
Answer: Rare

Comments: 
There are no specific characteristics produced for long-distance dispersal other than fragmentation moved by running water (NT WRTC 2022).

(b) Long-distance (>100m) dispersal by human means
Answer: Rare

Comments: 
Coloured rocks found in the local community lakes suggest that occasional instances of fish tanks being dumped out are occurring and could be a source of new infestation (NT WRTC 2022).

References

Champion, P. D., Burnett, D. A. and Petroeschevsky, A., 2008, Risk assessment of tradeable aquatic plant species in Australia (AUS1008/001), National Institute of Water and Atmospheric Research, Perth, Australia. 163 pp. Koncki, N.G., and Aronson, M.F.J., 2015, Invasion Risk in a Warmer World: Modelling Range Expansion and Habitat Preferences of Three Nonnative Aquatic Invasive Plants, Invasive Plant Science and Management, 2015 8:000–000. Langeland, K. A., 1996, Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), "The Perfect Aquatic Weed", Castanea, 61(3): 293-304. Les, D. H., 2017, Core Eudicots: Dicotyledons V "Asterid" Tricolpates, pages 493-1127 in Aquatic Dicotyledons of North America: Ecology, Life History, and Systematics. CRC Press, Boca Raton, FL. NT WRTC, 2022, 19 October- Meeting Minutes, NT Weed Risk Technical Committee, NT Government, 19 October 2022. Petroeschevsky, A. and Champion, P., 2008, Preventing further introduction and spread of aquatic weeds through the ornamental plant trade, Sixteenth Australian Weeds Conference. PPQ, 2020, Weed risk assessment for Limnophila sessiliflora Blume (Plantaginaceae) – Ambulia, United States Department of Agriculture, Animal and Plant Health Inspection Service, Plant Protection and Quarantine (PPQ), Raleigh, NC. 18 pp. Ramey, V., 2001, Limnophila sessiliflora, University of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL. Last accessed 17/10/20202, https://plants.ifas.ufl.edu/plantdirectory/limnophila-sessiliflora/. Scher, J. L., Walters, D. S. and Redford, A. J., 2015, Federal Noxious Weed Disseminules of the United States, United States Department of Agriculture, California Department of Food and Agriculture. http://idtools.org/id/fnw/factsheet.php?name=14631. Spencer, W., and Bowes, G., 1985, Limnophila and Hygrophila: A Review and Physiological Assessment of Their Weed Potential in Florida, Journal of Aquatic Plant Management 23: 7-16.