Quick Facts

Quick facts

• 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 listed as a Weed of National Significance (WoNS).
• 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.

What Does It Look Like?

What is it?

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. They are mostly hairless, but often have some hairs along the margins near their bases. Where the leaf blade meets the leaf sheath there is a small membranous flap (ligule) 1 to 2.5 mm long (Navie 2007).

Hymenachne can hybridise with the native Hymenachne species (Hymenachne acutigluma) and the hybrid, known as Hymenachne × calamitosa, is morphologically intermediate between both parental species. As a weed the hybrid is as problematic as Hymenachne itself and should therefore be controlled accordingly (Clarkson et al. 2011).

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) (Navie 2007).

For further information and assistance with identification of Hymenachne contact the herbarium in your state or territory.

Flower colour

Green

Growth form (weed type/habit)

Grass

Where it currently grows? Preferred habitat

Hymenachne thrives in wetter tropical and sub-tropical environments, where 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 (Navie 2007).

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 (CRC 2003).

Are there similar species?

Native Hymenachne (Hymenachne acutigluma) is easy to confuse with the introduced, weedy Hymenachne (Hymenachne amplexicaulis). However, the introduced Hymenachne has distinctive stem-clasping leaf bases, whereas the Native Hymenachne does not (Land Protection 2006). The weedy hybrid between both species, Hymenachne × calamitosa, is morphologically intermediate, without stem-clasping leaf-bases but with wider leaves than Native Hymenachne (Clarkson et al. 2011).

Why Is It A Weed?

What are its impacts?

Hymenachne is a Weed of National Significance (WoNS) 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 blocks waterways, potentially causing flooding, threatening drinking water and also blocks drainage and irrigation channels in cropping areas (CRC 2003).

The environmental threats posed to wetlands are severe, as Hymenachne forms dense stands that displace native water plant species. By forming floating mats, these stands also reduce available habitat for native animals (Wearne et al. 2010). Compared to native vegetation, Hymenachne stands exhibit lower species diversity of plants and of many groups of insects, whereas introduced fish are more abundant (Houston & Duivemvoorden 2002).

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 (CRC 2003).

How does it spread?

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 (CRC 2003).

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 (CRC 2003). Seeds can also be dispersed in contaminated agricultural produce (Navie 2007).

Stem fragments are most commonly spread in floodwaters, but in the past they were also been deliberately introduced into new regions by graziers (Navie 2007). 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 (CRC 2007).

What is its history in Australia?

Hymenachne was deliberately imported into Australia in the 1970s, with the intention of using it in ponded pastures in northern Queensland that were too deep for Para Grass (Urochloa mutica). Following trials it was approved for release in Queensland in August 1988, and was quickly taken up by landholders and used to further stimulate ponded pasture development throughout coastal northern Queensland. Hymenachne quickly escaped cultivation within a few years of being released (CRC 2003; Wearne et al. 2010).

By 1997 dozens of Hymenachne infestations were reported in sugar cane growing country, particularly in low-lying cane plantations and drainage ditches, throughout northern Queensland. In 2000 the total area of infestation in Queensland was estimated to be at least 1000 hectares (CRC 2003).

How To Manage It?

Best practice management

Chemical control: Several herbicides currently used in Queensland to control Hymenachne have been approved for use under 'off-label' minor use permits. A variety of application methods can be used, including spraying by hand, boom or helicopter. Spraying must be repeated about every three months to control regrowth (CRC 2003).

Non-chemical control: Mechanical or physical removal is difficult because of Hymenachne's ability to reproduce vegetatively from very small pieces. Mechanical harvesters have been used to remove Hymenachne and other aquatic weeds from deeper waters in the Burdekin Shire in Queensland, but this requires ongoing control efforts on a monthly basis. In drier regions, constant heavy grazing has been used to remove Hymenachne from ponded pasture systems (Land Protection 2006). Flooding can offer some control, particularly when above-ground Hymenachne vegetation has recently been removed, making it impossible for the plant to keep up with rising water levels (CRC 2003).

Biological control: There is ongoing research into a biological control agent that will attack the introduced Hymenachne, but not other species such as the closely related native Hymenachne (Hymenachne acutigluma). Most insects found on Hymenachne in Australia are also found on native Hymenachne (Bell et al. 2011). However, a blissid bug (Ischnodemus variegatus) found in invasive Hymenachne populations in Florida seems to be highly specific for this species and hardly affects native Hymenachne (Diaz et al. 2009).

Check with your local council or state/territory government agency about its requirements for Hymenachne control. Also see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au .

For further information see the Hymenachne Control Methods and Case Studies (available at https://www.weeds.org.au/WoNS/Hymenachne/

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Germination of Hymenachne seeds can take place all year round. The seeds require contact with waterlogged, or at least moist, soil for 48 hours before germination can take place (CRC2003). Furthermore, high daytime temperatures, marked differences between day and night temperature and the presence of light induce germination (Wearne et al. 2010). In northern Australia most rainfall occurs between November and March and this is the most likely time for germination. However, seed also survives in water and germinates when water levels recede during the dry season (CRC 2003). Hymenachne seeds also form a long-lived soil seed bank, with about 8-24% of seeds still viable after eight years (Wearne et al. 2010).

Hymenachne populations flower mostly between April and July, with a peak of flowering for 1-2 weeks during April and May (Wearne et al. 2008). In southern Queensland and northern New South Wales, flowering is delayed by at least one month. Flowering is normally triggered when day length decreases to less than 12 hours. If enough moisture is available, Hymenachne populations continue to flower at a lower rate throughout the year.  Seed is mostly set from May to July (Wearne et al. 2008; Wearne et al. 2010). As Hymenachne is a perennial, the shoots can brown off if conditions become dry and regrow during the wet season (CRC 2003).

Where Is It Found?

Which states and territories is it found?

NT, QLD

What areas within states and territories is it found?

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 (Navie 2007). In northern Queensland it is mostly found in the wet tropics region, as far south as Ayr (CRC 2003). However, it is also present in a few isolated locations in central and south-eastern Queensland (Bostock and Holland 2007). In the Northern Territory it is present in the Adelaide, Daly, Finniss and Mary rivers floodplains, as well as Arafura Swamp in northern central Arnhem Land (CRC 2003). Hymenachne is also found in northern New South Wales. The species is estimated to cover about 8,000 ha in Queensland, about 3,000 ha in the Northern Territory and about 55 ha in New South Wales (Wearne et al. 2010).

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 (CRC 2003).

Where does it originate?

Hymenachne is native to Mexico, Central America, the Caribbean and tropical South America (GRIN 2007).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

YES

Where is it a declared weed?

Declared in all states and territories

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Hymenachne amplexicaulis

Other scientific names (synonyms)?

• Agrostis monostachya Poir.
• Hymenachne myosurus (Rich.) Nees
• Panicum amplexicaule Rudge
• Panicum amplexicaule var. "a" erecta Doll (incorrect spelling)
• Panicum amplexicaule var. "b" deflexa Doll (incorrect spelling)
• Panicum amplexicaule var. deflexum Doll
• Panicum amplexicaule var. erectum Doll
• Panicum hymenachne Desv.
• Panicum myosuros Rich. (incorrect spelling)
• Panicum myosurus Rich.

Does it have other known common name(s)?

Olive Hymenachne, Water Stargrass, West Indian Grass, West Indian Marsh Grass