Hymenachne (Hymenachne amplexicaulis and hybrids)

Also known as: olive hymenachne

Hymenachne is a semi-aquatic perennial grass that has become a major weed of northern Australia invading freshwater wetlands, flood plains and stream banks.

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How does this weed affect you?

Hymenachne is a semi-aquatic perennial grass that forms dense infestations in freshwater systems. It has become a major weed of northern Australia where it was originally introduced (to Queensland and the Northern Territory) as fodder for cattle in ponded pasture systems. It has since escaped cultivation and become an unwanted pest of wetlands, flood plains, irrigation systems, water storage facilities and sugar cane crops.

Hymenachne infestations displace native plants, reduce biodiversity and threate native fish populations and wetland habitats.

Native hymenachne (H. acutigluma) is a tropical species that grows in northern Australia and is not considered a problem.

Where is it found?

Native to the tropics of South and Central America, hymenachne has been used as a source of dry season fodder for cattle in Venezuela, Mexico and Cuba. It is considered a serious weed in Indonesia, Suriname, Trinidad and the Florida wetlands of USA.

Many infestations occur throughout northern and central Queensland and the Northern Territory, including conservation areas of Kakadu National Park.

In New South Wales (NSW) small, isolated infestations have been found in various locations in the Richmond, Tweed and Clarence River catchments on the North Coast. All known infestations are subject to control programs, with the aim of eradication.

If left undetected, hymenachne has the potential to spread further in northern NSW and become a major weed of wetlands and waterways.

Distribution map

• NSW (image)

How does it spread?

Hymenachne reproduces by seed and broken stem fragments.

Each flower head can produce over 4000 viable seeds, generally in the autumn months as day length decreases. On land, seeds require contact with waterlogged or moist soil for at least 48 hours before germinating. Germination can occur at any time of the year but more commonly from November to March. Seeds can survive in water and are spread during annual flooding events and in mud attached to the fur or hooves of animals. Waterbirds may also be responsible for spreading seed.

Broken stem fragments can be carried to new locations by floodwaters, and then take root in moist soil.

What does it look like?

Hymenachne is a perennial grass that prefers swampy or seasonally flooded areas, growing in water up to 2 m deep.

Key identification features

• Stems are up to 2.5 m tall, contain white pith and are hairless. On land, erect stems can stand up to 1.5 m tall, rising from stems that run along the ground. New plants are produced from horizontal stems that form roots at their lower nodes.
• Leaf blades are 20–35 cm long and 2–3 cm wide, bright green, with light-coloured veins and hairy edges. A key characteristic is that the base of the leaf blade is slightly heart-shaped and clasped around the stem.
• Each flower-head is spike-like and cylindrical, about 8 mm wide and up to 40 cm long.

What type of environment does it grow in?

Hymenachne will invade freshwater wetlands, flood plains and stream banks. Hymenachne thrives in nutrient-rich water. 

Acknowledgements

2006 edition prepared by Rachele Osmond; 2013 edition reviewed by Rod Ensbey and edited by Elissa van Oosterhout.

References

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

Hosking JR, Sainty GR, Jacobs SWL & Dellow LL (in prep) The Australian WeedBOOK.

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Control

Hymenachne is difficult to control and is capable of spreading from plant fragments, requiring strict hygiene procedures during its removal. If you suspect you have found hymenachne, immediately 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 30/06/2022
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 comments.
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
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 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 dealings Must not be imported into the State or sold
Central West	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
Greater Sydney	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
Hunter	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
North Coast	Regional Recommended Measure* Land managers should mitigate the risk of the plant being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant.
North West	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfill the general biosecurity duty for this weed click here

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Reviewed 2018