Weed Alert: Hymenachne

Hymenachne (Hymenachne amplexicaulis)

World Status | Identification | Spread | Control | Legislation

Hymenachne is a semi-aquatic perennial grass that has become a major weed of wetlands, flood plains and sugar cane crops of northern Australia.

Originally introduced to Queensland and the Northern Territory as a ponded pasture species, it has invaded freshwater wetlands, flood plains and river banks. It forms dense infestations, displaces native plant species, reduces biodiversity and threatens native wetland habitat.

World Status

Native to the tropics of South and Central America, it is a serious weed in Australia, the West Indies, Indonesia, the Florida wetlands of USA and Suriname.

Once considered a source of dry season fodder for cattle, it has been used as a forage crop in Australia, Venezuela, Mexico and Cuba.

Hymenachne is considered one of Australia’s worst weeds, is a Weed of National Significance and a Class 1 noxious weed in NSW. It has the potential to spread further in northern NSW and become a major weed of wetlands and waterways. Small infestations have already been found on the North Coast, in the Richmond River catchment.

A native hymenachne species occurs in northern Australia. It does not grow south of Mackay,  Queensland and is not considered a problem.

Identification

An erect grass that can grow in water up to 2 m deep. It prefers swampy situations such as riverbanks and seasonally flooded areas.

Stems

Hairless, up to 1.6 m tall and containing a white pith. Submerged stems quickly produce roots from nodes.

Leaves

Leaf blades are 20–35 cm long and 2–3 cm wide. A distinctive characteristic of hymenachne is that the base of the leaf blade clasps around the stem. This is a key identifying feature of hymenachne, particularly when identifying it for the first time.

Flowers

The flower-heads are spike-like and cylindrical, about 8 mm wide and up to 40 cm long. The plant flowers in summer and autumn.

Growth and Spread

Hymenachne reproduces by seed, broken stem fragments and stolons. A large number of viable seeds are produced and require contact with waterlogged or moist soil for at least 48 hours before germination can occur. Germination can occur all year round.

Stolons run along the ground and produce roots at each node, forming a new plant. Broken fragments of plant can be carried to new locations by floodwater and will take root in moist soil. It thrives in nutrient- rich water.

Seed is spread during annual flooding events, in mud attached to waterbirds or in contaminated stock feed. Flood waters will also transport fragments into waterways, irrigation storage facilities, sugar cane crops and natural wetlands.

Control

Control of hymenachne is difficult. Most success has been obtained from repeated herbicide treatment to infestations. Burning and heavy grazing prior to flooding have also provided minimal control.

Legislation

Hymenachne amplexicaulis is declared noxious throughout NSW as a Class 1 weed under the NSW Noxious Weeds Act 1993. As a notifiable weed all outbreaks of hymenachne must be reported to the local council within three days.

Acknowledgements

Prepared by Rachele Osmond

References

• CRC for Australian weed management (2004) Weed Management Guide: Hymenachne (Hymenachne amplexicaulis).
• Csurhes, S.M., Mackey, A.P. and Fitzsimmons (1999) Hymenachne (Hymenachne amplexicaulis) in Queensland. Pest Status Review Series – Land Protection. Department of Natural Resources and Mines, Qld.
• Hosking, J.R., Sainty, G., Jacobs, S. and Dellow, J. (in prep.) The Australian WEEDbook.