Hymenachne (Hymenachne amplexicaulis and hybrids)

Also known as: olive hymenachne

Hymenachne is a tall perennial semi-aquatic grass. It can outcompete native plants and reduce numbers of fish and other aquatic animals.

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How does this weed affect you?

Hymenachne forms dense infestations which:

• compete with native plants
• reduce food and habitat for fish and other aquatic animals
• restrict native fish movement and breeding
• reduce water quality
• block drains and irrigation channels
• can damage infrastructure when large rafts of hymenachne float away in floods
• can invade and reduce yields in sugar cane.

What does it look like?

Hymenachne is a perennial grass that can be up to 3.5 m tall. It can grow upright or semi-upright on land or float in water.

Leaves are:

• bright green with light coloured veins
• up to 50 cm long and 3 cm wide
• hairy on the edges
• slightly heart-shaped where they join around the stem
• alternate along the stem.

 Seedheads are:

• cylindrical
• 10–40 cm long and 1–2 cm wide
• green
• made up of densely packed spikes each up to 5.5 mm long.

Seeds are:

• 1–2 mm long and 0.6 mm wide
• oval-shaped.

Stems are:

• hairless
• filled with white pith which helps them float.

Roots:

Roots are produced from the nodes on the horizontal stems.

Where is it found?

There are small, isolated infestations in the Richmond, Tweed and Clarence River catchments on the North Coast of NSW. All known infestations have control programs to eradicate them.

Hymenachne is native to the South and Central American tropics. Hymenachne was introduced to QLD and the NT as fodder for cattle in ponded pastures. It has since escaped cultivation and become a weed. If left undetected, hymenachne has the potential to spread further in northern NSW and become a major weed of wetlands and waterways.

What type of environment does it grow in?

Hymenachne grows in tropical and subtropical climates. It prefers nutrient-rich freshwater in swampy or seasonally flooded areas. It grows in wetlands, floodplains, irrigation channels, sugarcane crops and on stream banks. Hymenachne can grow in water up to 2 meters deep.

The cultivar named 'Olive' can grow in the tropics and warm temperate areas.

Maps and records

• Recorded presence of Hymenachne during property inspections (Map: Biosecurity Information System - Weeds, 2017-2024)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

How does it spread?

Hymenachne reproduces by seed and broken stem fragments.

By Seed

Each flower head can produce over 4000 seeds. Seeds can remain viable in the seed bank for 8 years. They need contact with damp soil for at least 48 hours before germinating.  Seeds spread:

• downstream in flood waters
• in mud that has attached to animals, people and vehicles
• by birds, either stuck to their bodies or via droppings
• in contaminated agricultural produce.

By plant parts

Plants can grow from broken stem fragments. Floodwaters move stem fragments to new locations. The fragments take root in damp soil.

References

Charleston, K. (2006). Hymenachne (Hymenachne amplexicaulis) Control methods and case studies, Queensland Department of Natural Resources, Mines and Water.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved July 2021 from https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Hymenachne~amplexicaulis

Sainty, G. R., & Jacobs, S. W. (2003). Waterplants in Australia (No. Ed. 4). Sainty and Associates Pty Ltd.

Wearne, L. J., Clarkson, J., Grice, A. C., van Klinken, R. D., & Vitelli, J. S. (2010). The Biology of Australian Weeds 56.'Hymenachne amplexicaulis'(Rudge) Nees. Plant Protection Quarterly, 25(4), 146-161.

More information

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Control

Hymenachne is difficult to control and requires hygiene procedures during its removal.

If you suspect you have found hymenachne contact a local council weeds officer who will assist with identification, removal and eradication.

Herbicide options

See Using herbicides for more information.

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PERMIT 13921 Expires 31/03/2027
Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: 14 L / hectare
Comments: Apply by boom, handgun or knapsack, a maximum of 4 times a year. Refer to permit for further critical comments.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW	Prohibition on certain dealings Must not be imported into the state, sold, bartered, exchanged or offered for sale.
Greater Sydney	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
Hunter	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
North Coast	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
North West	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfil the general biosecurity duty for this weed click here

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Reviewed 2024