Quick Facts

Quick facts

  • Lagarosiphon (Lagarosiphon major) is a very aggressive weed of still and slow-moving waterways in New Zealand and Europe; it is currently not known to be present in Australia.
  • Quarantine control, public education and the co-operation of the aquarium and garden plant industries will help to prevent its spread into Australia.
  • If it became established, eradication would be extremely difficult because it can rapidly spread from plant fragments, and no environmentally acceptable systemic herbicide is known.
  • Any outbreaks should be reported to local councils or state or territory weed management agencies. Do not attempt control on your own.

What Does It Look Like?

What is it?

Lagarosiphon (Lagarosiphon major) is a perennial, submerged aquatic herb. It reaches its maximum growth in clear water up to a depth of 7 m, but may only grow to 1 m depth in murky water. It has numerous thread-like roots arising from stem nodes that, along with rhizomes (horizontal stems in the sediment), anchor it to the bottom. Fixed or free-floating stems are up to 5 m or sometimes more long and can reach the water surface. They are brittle and sparsely branched, 3–5 mm in diameter.

Leaves are 5–20 mm long and 2–3 mm wide, somewhat stiff and arranged in alternate spirals along the stems, more crowded near the tips of branches than towards the base where they are usually well spaced. They generally have minutely toothed margins, and tapered tips strongly curving downwards towards the stem, except in low alkalinity water where they are straight.

Male and female flowers are on different plants. The solitary, three-petalled female flowers are very small (about 3 mm across), transparent to white or pinkish and float on the surface while remaining attached to the plant by a very thin white to translucent filament- or thread-like stalk. The solitary male flowers break off and float to the surface; neither they nor fruit nor seeds have been recorded outside its native range (Parsons & Cuthbertson 2001; CRC 2003; Sainty & Jacobs 1981 and 2003; Freeman & Faithfull 1998; Sainty & Associates 2002; Land Protection 2006).

For further information and assistance with identification of Lagarosiphon contact the herbarium in your state or territory.

Flower colour


Growth form (weed type/habit)

Aquatic, Herb

Where it currently grows? Preferred habitat

Lagarosiphon grows best in clear, still or slow-moving freshwater with silty or sandy bottoms. This can be in ponds, lakes, dams and streams. It prefers the cooler waters of the temperate zone, with optimum temperatures of 20–23 °C and a maximum temperature of about 25 °C. It can live in high and low nutrient levels and grows best under conditions of high light intensity, but can tolerate shaded conditions. It also tolerates relatively high pH (i.e., alkaline conditions). Growth of Lagarosiphon is greatest in sheltered areas protected from wind, waves and currents (Parsons & Cuthbertson 2001; CRC 2003).

Are there similar species?

There are several water weeds similar to Lagarosiphon that are already present in Australia, especially in the south-east. These include Canadian Pondweed (Elodea canadensis), Hydrilla (Hydrilla verticillata) and Egeria densa. They are all submerged aquatic plants that reproduce vegetatively and inhabit still or slow-moving fresh water. Lagarosiphon can be distinguished from these species because its leaves grow in alternate spirals along the length of the stem; the leaves of all the other species grow in whorls, or circles, around the stem (Ramey 2001; Sainty & Associates 2002; CRC 2003).

Why Is It A Weed?

What are its impacts?

Native ecosystems: Lagarosiphon is an aquatic plant that can dominate freshwater lakes, dams and slow-moving streams. It grows extremely quickly from the bottom of a water body to the surface and forms dense mats several metres thick at or just below the water surface. These mats stop light from penetrating the water and displace native vegetation. Dense infestations can also deplete oxygen in the water (CRC 2003). Native fish, waterbirds and other fauna could be affected where the growth becomes dense and restrictive (Land Protection 2006).

As a submerged aquatic weed, Lagarosiphon is particularly difficult to control and spreads very readily. If it became naturalised in Australia, there is little doubt that it would cause similar detrimental impacts and be as hard to control as it is in New Zealand (CRC 2003).

Human impacts: Infestations of Lagarosiphon choke waterways and reduce the potential for recreational use (e.g. fishing, boating, swimming, diving) and commercial use (e.g. by blocking water intakes for hydro-electricity) (CRC 2003). The long stems could block inlets to water pumping equipment and affect propellers and cooling systems of recreation equipment (Land Protection 2006).

An impact assessment of Lagarosiphon is provided by Victorian Resources online at http://www.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/impact_Lagarosiphon 

How does it spread?

Lagarosiphon is not known to produce seed outside its native range (only female plants are naturalised) and its spread is only by vegetative reproduction. Colonies of Lagarosiphon expand in waterbodies as rhizomes take root and give rise to new growth. It can spread large distances downstream when fragments break off naturally, especially during summer, or are dislodged by fast-flowing water or other disturbance. Roots grow from the nodes (the joints between the segments on the stem) on these fragments, and new plants are formed. In New Zealand, small fragments are frequently transported on boats and trailers, and infestations are often first recorded at boat ramps (CRC 2003). See also Parsons & Cuthbertson (2001) and Redekop et al. (2016).

What is its history in Australia?

Lagarosiphon has been spread throughout the world as an aquarium plant and is often called an 'oxygen plant' or 'oxygenating plant'. However, dense infestations can actually consume more oxygen than they produce, reducing water quality and available oxygen (CRC 2003).

It is a major weed on both the North and South Islands of New Zealand. It is also a naturalised weed in England, the Channel Islands, northern France and Italy (Parsons & Cuthbertson 2001).

It has occasionally been brought into Australia probably as an ornamental aquarium or pond plant, but has been eradicated when found (CRC 2003).

How To Manage It?

Best practice management

Non-chemical control: Preventing the establishment of Lagarosiphon is the best and most cost-effective method of control. It is illegal to grow Lagarosiphon in Australia. The aquarium and garden plant industries have a major role to play in abiding by the relevant legislation. The sale or dumping of Lagarosiphon should be reported to the authorities and people who have unknowingly cultivated it should contact local councils or state or territory weed management agencies for information on how to dispose of it properly. Burial is the quickest means of disposal. Drying in the sun and/or burning are also suitable, providing there is no risk of accidental spread into a waterway (CRC 2003). Coughlan et al. (2020) found that exposure of the plants to steam for about 30 s following removal from the water was extremely effective in killing the plants.

Early detection and eradication are also important to prevent infestations of Lagarosiphon. Small infestations can be easily eradicated if they are detected early but an ongoing commitment is needed to stop new infestations. Any outbreaks should be reported immediately to your state or territory weed management agency or local council. Do not try to control Lagarosiphon without their expert assistance. Control effort that is poorly performed or not followed up can actually help spread the weed and worsen the problem (CRC 2003).

No importation of Lagarosiphon into Australia is permitted because of the risk of further spread, and the potential introduction of male plants and extra genetic diversity that could make future control more difficult (CRC 2003).

The New Zealand experience has shown the difficulty of eradicating Lagarosiphon. Only a few very small infestations have been eradicated and it has continued to spread into new areas. In most cases control is ongoing because, unless all the Lagarosiphon is collected, it can grow back from any remaining fragments. (CRC 2003). Manual removal of Lagarosiphon by hand or mechanically is not effective, as it is impossible to collect all the vegetation, enabling re-infestation from remaining fragments.

Lagarosiphon has also been mechanically harvested from accessible areas, using a variety of methods such as hand cutting and suction dredging. However, it is virtually impossible to mechanically collect all vegetation, even from small areas, and infestations are able to regenerate from remaining fragments (CRC 2003).

Other methods that have been used include covering the plant with black sheeting and lowering water levels to dry out the weed. Biological control is not currently an option (CRC 2003).

Chemical control: At present the only herbicide that is considered to be environmentally acceptable in New Zealand does not kill the rhizomes, so regrowth is inevitable. This herbicide can be applied by helicopter, providing cost-effective control at recreational sites, albeit on an ongoing basis. An environmentally friendly systemic herbicide that targets the rhizomes is being sought (CRC 2003).

See the Australian Pesticides and Veterinary Medicines Authority (APVMA) for chemical information at http://www.apvma.gov.au 

Does it have a biological control agent?


When does it grow? (lifecycle/growth calendar)

Lagarosiphon is a perennial plant which can live for at least several years. It is well adapted to cooler climates and becomes dormant in the northern hemisphere during winter. In spring it breaks its dormancy and growth commences from rhizomes (horizontal stems from the plant base sunk into the sediment) and shoots. It grows fastest during the warmer months of summer when dense, interwoven mats of stems can completely take over the water body. The female flowers appear from summer until early autumn. Growth decreases as light intensity and day length decrease, and is thought to stop below 10 °C (CRC 2003).

Where Is It Found?

Which states and territories is it found?

Not found in any Australian states or territories.

What areas within states and territories is it found?

There are currently no naturalised infestations of Lagarosiphon known in Australia. Small infestations near Melbourne in Victoria and Newcastle in New South Wales were eradicated in the late 1970s and were believed to have originated from ornamental plants in aquariums or ponds. It has also been found in a Sydney aquarium and was intercepted entering Tasmania. A cultivated specimen was recorded in Queensland in 1990 (CRC 2003).

Where does it originate?

There are about 16 species of Lagarosiphon from southern Africa and Madagascar, where they grow in high mountain streams and ponds. No species of Lagarosiphon are native to Australia (CRC 2003).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?


Where is it a declared weed?

Declared in all Australian states and territories.

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?


Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?


Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Lagarosiphon major

Other scientific names (synonyms)?

Lagarosiphon muscoides var. major Ridl. (misapplied by Ridley, H.N. 1886, Journal of the Linnean Society, Botany. 22: 233.)

Does it have other known common name(s)?

Oxygen Weed

Other Management Resources

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