Hymenachne

Hymenachne has a distinctive leaf.	Close-up of a Hymenachne stem and leaf.	Hymenachne, south of Casino.

Hymenachne (Hymenachne amplexicaulis) 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, floodplains and river banks. It forms dense infestations, displaces native plant species, reduces biodiversity and threatens native wetland habitat.

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 and is a Weed of National Significance. It has the potential to spread further in Northern NSW and become a major weed of wetlands and waterways. Small infestations have been found on the North Coast in the Richmond River catchment. Infestations have been discovered at Cobaki, Georgica, Lillian Rock, Lismore Race Course, Yorklea, Leeville, West Coraki and Oaky Creek. 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 two metres deep, it prefers swampy situations such as river banks and seasonally flooded areas.

• Stems are hairless, up to 1.6m tall and contain a white pith. Submerged stems quickly produce roots from nodes.
• Leaf blades are 20-35cm long and 2-3cm wide. A distinctive characteristic of Hymenachne is that the base of the leaf blade clasps around the stem. This is a key identifying feature, particularly when identifying it for the first time.
• The flower heads are spike-like and cylindrical, about 8mm wide and up to 40cm long. The plant flowers in summer and autumn.

Growth and spread

In NSW, Hymenachne only reproduces by broken stem fragments and stolons. 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. 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 before flooding have also provided minimal control.

If you have seen this plant, please report it to Rous County Council or Industry and Investment NSW for positive identification.

Distribution map as at May 2017

Predictive mapping supplied by Queensland Biosecurity