Hymenachne

• Hymenachne (H. amplexicaulis and H. x calamitosa) are declared weeds under the Tasmanian Biosecurity Act 2019 ​and associated Regulations. The importation, sale and distribution of hymenachne are prohibited in Tasmania.
  

• These species are also Weeds of National Significance (WONS).
  

• The legal responsibilities of landholders and other stakeholders in dealing with hymenachne are laid out in the Statutory Weed Management Plan for Hymenachne.
  

• Hymenachne does not occur in Tasmania.
  

• Hymenachne are perennial, robust, aquatic grasses to 2.5 metres tall. They form dense stands, growing above or below water, with roots in the ground. The stems can form stolons that run along the ground and produce new plants by rooting at the nodes. 
  

• Hymenachne x calamitosa is a naturally occurring hybrid between Hymenachne amplexicaulis and the Australian native species Hymenachne acutigluma.
  

• Hymenachne amplexicaulis has long leaves (100 to 450 mm) and the leaf base may be up to 30 mm wide and covered with long hairs. The upper part of the leaf is narrower and without hairs. The leaf blade is heart-shaped at its base where it clasps around the stem. Flowers occur as a long, cylindrical cluster at the end of a spike. The leaves of H. x calamitosa lack the stem-clasping leaf bases of H. amplexicaulis and are narrower, but broader than the leaves of the native H. acutigluma.
  

• Hymenachne are prolific seeders; spread is via seed and broken stem fragments which are spread by water and in mud on animals and birds.
  

Hymenachne habitat and flower, © Photostream.​

Hymenachne are serious weeds of water bodies and seasonal wetlands. They can invade and dominate waters where emergent and floating attached/ submergent native vegetation occurs. Resulting hymenachne grass beds reduce plant diversity and in deeper river channels it is capable of forming a floating mat over the water surface, shading submerged vegetation.

Changes in vegetation structure as a result of hymenachne invasion have influenced macroinvertebrate and fish species composition. Compared with areas dominated by native vegetation, areas dominated by hymenachne support a higher relative abundance of introduced fish.

Hymenachne has been shown to block drainage and irrigation channels and water storages that supply water to sugarcane farms, increasing costs to the industry. 

Field observations suggest that H. x calamitosa will be at least as invasive under Australian conditions as Hymenachne amplexicaulis, (Clarkson et al, 2011).

Where does Hymenachne occur?

• Hymenachne is native to seasonally flooded lowlands and riverbanks throughout tropical and subtropical areas of South and Central America. The species is a significant weed in Florida (United States), Trinidad (West Indies), and Surinam.
  

• In Australia, Hymenachne amplexicaulis has naturalised in Queensland, the Northern Territory and New South Wales.
  

• Hymenachne x calamitosa has been recorded at two widely separated locations in far north Queensland, (Clarkson et al, 2011).
  

• The single native species, Hymenachne acutigluma occurs in coastal and near coastal areas of Queensland and across the Top End of the Northern Territory.
  

• Hymenachne is considered to be a low climate match for Tasmania.
  

• For further information or help in identifying hymenachne, refer to the Weed Management Guide (Weeds of National Significance) – Hymenachne, published by the CRC for Australian Weed Management and the Commonwealth Gov’t.
  

If you locate hymenachne anywhere in Tasmania, or if you find a plant that you think could be hymenachne, immediately contact Biosecurity Tasmania on 03 6165 3777 to report this weed.