Name:
Hymenachne(Hymenachne amplexicaulis)
Description:
Hymenachne (Hymenachne amplexicaulis) is a large perennial grass with upright or semi-upright
stems from a creeping 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. Its aboveground stems are relatively robust (up to 12 mm or
more thick), often creeping in nature, and regularly produce roots at their joints. The upright
flowering stems are hairless and filled with a light weight pith which aids in flotation. This species
sometimes also produces short creeping underground stems (rhizomes). The leaves consist of a
hairless or hairy leaf sheath, which partially encloses the stem, and a large spreading leaf blade. The
elongated leaf blades (10 to 45 cm long and 2 to 6 cm wide) have broad bases, which
are conspicuously clasped around the stem, and pointed tips.
The elongated inflorescence (10 to 50 cm long and 8 to 20 mm wide) is dense and spike-like in
appearance. It consists of numerous short branches that are held closely to the main flowering
stalk. These inflorescences contain large numbers of elongated flower spikelets 3 to 5.5mm long.
Flowering occurs from late spring through to early autumn, but is most common during early autumn.
The flower spikelets detach and fall from the seed-head entire when they are mature. The small seed
is oval in shape (1 to 2 mm long and about 0.6 mm wide) and remains enclosed inside the floral
bracts (glumes).
For further information and assistance with identification of Hymenachne contact the herbarium in
your state or territory.
Distribution:
Hymenachne is relatively widespread but has a scattered distribution at present, which is largely
based around the areas where it was deliberately planted in the past. 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.
In northern Queensland it is mostly found in the wet tropics region, as far south as Ayr. However, it is also present in a few isolated locations in central and south-eastern Queensland. In the Northern Territory it is present in the Adelaide, Daly, Finniss and Mary rivers floodplains, and at Arafura Swamp in northern central Arnhem Land.
Based on climate, Hymenachne could potentially occur in all seasonally flooded tropical wetlands,
including the Kimberley Ranges and the central coastal region of Western Australia, the Top End of
the Northern Territory and most of eastern coastal and central Queensland.
Habit:
Grass
Key points:
- Hymenachne (Hymenachne amplexicaulis) is a large perennial semi-aquatic grass commonly growing 1 to 2.5 m tall, with upright or semi-upright stems from a creeping base.
- It was previously promoted as a ponded pasture grass, but has escaped cultivation and is now a serious
environmental and crop weed. - Hymenachne forms dense stands that reduce plant diversity and available habitat for native animals.
- It invades tropical wetlands and waterways and threatens large areas of northern Australia, including national parks, sugar cane plantations and water reservoirs.
- Hymenachne’s ability to reproduce vegetatively from relatively small stem segments makes it very difficult to contain and control.
How it spreads:
Hymenachne reproduces by seed and vegetatively from broken stem fragments. It produces large numbers of viable seeds, and graziers have reported good germination of Hymenachne simply by throwing seeds into ponds.
Seed is transported downstream during annual flooding and can also be spread in mud attached to animals and vehicles. It is thought that waterbirds, particularly Magpie Geese (Anseranas semipalmata), either spread the seed in their droppings or transport it on their bodies, as infestations have been discovered at remote Magpie Geese feeding grounds. Seeds can also be dispersed 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 by graziers. Only a small fragment of the grass is required for a new plant to grow, with a minimum of two nodes either planted in mud or simply placed in shallow water. Under natural conditions floods break off segments and can spread them large distances downstream.
Where it grows:
Hymenachne thrives in wetter tropical and sub-tropical environments. It is a weed of swamps, wetlands, seasonally flooded areas, waterways, riverbanks, drainage channels and other water bodies. It is also quite common in sugar-growing areas, where it is known to invade plantations.
As a semi-aquatic grass, Hymenachne thrives best on clay soils that are inundated during the wet season rains but dry out to some extent in the dry season. The subsoil must remain moist during the dry season, as Hymenachne can only withstand short periods of drought. Because it grows so rapidly, it flourishes in wetlands that receive sediment and nutrient-rich water. However it does not tolerate brackish water and does not grow well in shaded areas.
Flower colour:
Green
Distribution map:
Impacts:
Hymenachne 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. Since escaping from cultivation in recent years it has seriously threatened the wetlands of northern Australia. Hymenachne invades permanent water bodies and seasonally inundated wetlands. It also blocks waterways, potentially causing flooding and threatening drinking water, and blocks drainage and irrigation channels in cropping areas.
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. The potential exists for Hymenachne to severely detract from the high conservation and tourism value of such natural wetland systems in northern Australia. The potential loss of natural wildlife resources (such as fish and waterfowl) to indigenous people is an impact that has social, environmental and economic consequences. It also has the potential to cause severe impacts on the sugar cane industry, by reducing the quality of crops and infesting drainage and irrigation channels. In one reported case, an infestation in a cane crop caused a 50% reduction in the price of the sugar.
Origin:
Hymenachne is native to Mexico, Central America, the Caribbean and tropical South America.
