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Scientific Name
Synonyms
Family
Common Names
Origin
Cultivation
Naturalised Distribution
Habitat
Habit
Distinguishing Features
Stems and Leaves
Flowers and Fruit
Reproduction and Dispersal
Environmental Impact
Other Impacts
Legislation
Management
Similar Species
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Click on images to enlarge

infestation (Photo: Sheldon Navie)

infestation (Photo: Land Protection, QDNRW)

habit in flower (Photo: Land Protection, QDNRW)

habit (Photo: Land Protection, QDNRW)

stems with roots (Photo: Land Protection, QDNRW)

stems and large leaves (Photo: Land Protection, QDNRW)

stem-clasping bases of leaf blades (Photo: Land Protection, QDNRW)

close-up of young leaves (Photo: Sheldon Navie)

seed-heads (Photo: Land Protection, QDNRW)

young seed-head in flower (Photo: Chris Gardiner)

seedlings (Photo: Land Protection, QDNRW)

close-up of adventitious roots at the stem joints (Photo: Sheldon Navie)

close-up of base of leaf blade (Photo: Sheldon Navie)

close-up of leaf underside (Photo: Sheldon Navie)

Hymenachne amplexicaulis

Scientific Name

Hymenachne amplexicaulis (Rudge) Nees

Synonyms

Agrostis monostachya Poir.
Hymenachne myosurus (Rich.) Nees
Panicum amplexicaule Rudge
Panicum amplexicaule Rudge var. deflexa Döll
Panicum amplexicaule Rudge var. erecta Döll
Panicum hymenachne Desv.

Family

Gramineae (South Australia)
Poaceae (Queensland, New South Wales, the ACT, Victoria, Tasmania, Western Australia and the Northern Territory)

Common Names

hymenachne, olive, olive hymenachne, trompetilla, water stargrass, West Indian grass, West Indian marsh grass

Origin

Native to the Mexico, Central America, the Caribbean and tropical South America.

Cultivation

Hymenachne (Hymenachne amplexicaulis) has been cultivated in ponded pastures in northern Australia. The cultivar that has been widely grown in Australia is known as olive hymenachne ( Hymenachne amplexicaulis 'Olive').

Naturalised Distribution

This species is relatively widespread but has a somewhat scattered distribution at present. It is spreading throughout the tropical wetlands of northern Australia and is most common in the coastal districts of northern Queensland and the Northern Territory. It has also been recorded in central and south-eastern Queensland.

Also naturalised in tropical Asia and in south-eastern USA (i.e. Florida).

Habitat

This species thrives in wetter tropical and sub-tropical environments. It is a weed of swamps, wetlands, seasonally flooded areas, waterways, riverbanks and other water bodies. It is also quite common in sugar-growing areas, where it is occasionally known to invade plantations.

Habit

A large long-lived (i.e. perennial) grass that is either upright (i.e. erect) or semi-upright (i.e. ascending) from a creeping (i.e. prostrate) base, or floating on shallow water. It commonly grows 1 to 2.5 m tall, but can occasionally reach up to 3.5 m in height.

Distinguishing Features

Stems and Leaves

Its aboveground stems are relatively robust (i.e. up to 12 mm or more thick) and are often trailing or creeping in nature (i.e. they are often stoloniferous). The bases of the stems regularly produce roots (i.e. adventitious roots) at their joints (i.e. nodes). This species sometimes also produces short underground stems (i.e. rhizomes). The upright (i.e. erect or ascending) flowering stems (i.e. culms) are hairless (i.e. glabrous) and filled with a lightweight, white pith which aids in floatation.

The leaves consist of a hairless (i.e. glabrous) or hairy (i.e. pubescent) leaf sheath, which partially encloses the stem, and a leaf blade. The relatively large, elongated (i.e. lanceolate), leaf blades (10-45 cm long and 2-6 cm wide) have broad bases, that are conspicuously clasped around the stem, and pointed tips (i.e. acute apices). They are mostly hairless (i.e. glabrous), but often have hairy (i.e. pubescent) margins near their bases. Where the leaf blade meets the leaf sheath there is a small membranous flap (i.e. membranous ligule) 1-2.5 mm long.

Flowers and Fruit

The elongated seed-head (10-50 cm long and 8-20 mm wide) is dense and spike-like in appearance (i.e. a spiciform panicle). It is actually made up of numerous short branches that are held closely to the main flowering stalk (i.e. rachis). These seed-heads contain large numbers small elongated (i.e. lanceolate) flower spikelets (3-5.5 mm long). Each of these flower spikelets consists of two bracts (i.e. glumes) and two tiny flowers (i.e. florets), only one of which produces a seed. Flowering occurs from late spring through to early autumn, but is most common during early autumn.

The flower spikelets break off and fall from the seed-head entire when they are mature. The small 'seed' (i.e. caryopsis or grain) is oval (i.e. ellipsoid) in shape (1-2 mm long and 0.6 mm wide) and enclosed in several floral bracts (i.e. two glumes, a palea and a lemma).

Reproduction and Dispersal

This plant reproduces by seed and also vegetatively via stem fragments. The seeds are dispersed by birds, in floodwaters, in mud and in contaminated agricultural produce. Stem fragments are most commonly spread in floodwaters, but in the past they were also been deliberately introduced into new regions as a ponded pasture grass.

Environmental Impact

Hymenachne (Hymenachne amplexicaulis) is a Weed of National Significance and is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, economic and environmental impacts. It has escaped from cultivation as a ponded pasture grass in recent years and now seriously threatens the wetlands of northern Australia.

Hymenachne (Hymenachne amplexicaulis) primarily invades permanent water bodies and seasonally inundated wetlands. The environmental threats posed to wetlands are severe, as it forms dense stands that reduce plant diversity and available habitat for native animals. In the Northern Territory, infestations have been found in important conservation areas including the Mary River and Kakadu National Parks, and at Murganella on the Cobourg Peninsula.

Other Impacts

Hymenachne (Hymenachne amplexicaulis) also blocks waterways, potentially causing flooding and threatening drinking water, and blocks drainage and irrigation channels in cropping areas. It has the potential to cause severe impacts on the sugar cane industry, by reducing the quality of crops and infesting irrigation channels. In one reported case, an infestation in a cane crop caused a 50% reduction in the price of the sugar.

The potential also exists for hymenachne (Hymenachne amplexicaulis) to severely detract from the high tourism value of natural wetland systems in northern Australia (e.g. Kakadu National Park). The potential loss of natural wildlife resources (e.g. fish and waterfowl) to indigenous people is also an impact that has both environmental and economic consequences.

Legislation

This species is declared under legislation in the following states and territories:

Management

For information on the management of this species see the following resources:

Similar Species

Hymenachne (Hymenachne amplexicaulis) may be confused with the closely related native species Hymenachne acutigluma. However, these two species can be distinguished by the following differences: