hymenachne

Hymenachne amplexicaulis
habit (Photo: Land Protection, QDNRW)
infestation (Photo: Sheldon Navie)
infestation (Photo: Land Protection, QDNRW)
habit in flower (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)
Grass
Alternate
Simple
Green
Green

A large spreading grass with thick pithy stems. These stems are often trailing or creeping (sometimes floating on water) and regularly produce roots at their joints. Its leaves have large, relatively broad, stem-clasping leaf blades (10-45 cm long and 2-6 cm wide). Its elongated seed-heads are spike-like in appearance (10-50 cm long and 0.8-2 cm wide). They contain numerous individual flower spikelets that are 3-4 mm long.

Common names 
Also known as: hymenachne, olive hymenachne, water stargrass, West Indian grass, West Indian marsh grass,
Family 
Poaceae
Deciduous 
No
Flowering time 
Spring - Autumn
Native/Exotic 
Exotic
Origin 
Native to the Mexico, Central America, the Caribbean and tropical South America.
Notifiable 
No
State declaration 
Category 3 - Must not be distributed or disposed. This means it must not be released into the environment unless the distribution or disposal is authorised in a regulation or under a permit.
Council declaration 
As per State Declaration
Known 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.

Impact and control methods 

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.

Stem 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 fruits 

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.

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: hymenachne (Hymenachne amplexicaulis) can reach up to 3.5 m in height and its leaf bases are quite broad and conspicuously clasped around the stem. Its flower spikelets are also relatively small (3-4 mm long).Hymenachne acutigluma only grows to about 2 m tall and its leaf blades do not clasp the stem to the same degree. Its flower spikelets are also relatively large (4.5-5.5 mm long).