Description
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.
For further information and assistance with identification of Lagarosiphon contact the herbarium in your state or territory.
Distribution:
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.
Habit:
Aquatic, Herb
Key points:
- 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.
How it spreads:
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.
Where it grows:
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.
Flower colour:
White
Distribution map:
Impacts:
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. Native fish, waterbirds and other fauna could be affected where the growth becomes dense and restrictive.
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 hydroelectricity). The long stems could block inlets to water pumping equipment and affect propellers and cooling systems of recreation equipment.
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.
Origin:
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.
