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Wetland Weed Identification/Extent Facts: Hymenachne (Hymenachne amplexicaulis), Paragrass (Brachiaria mutica) and Alemangrass (Echinachloa polystachya)

  1. Hymenachne (Hymenachne amplexicaulis)

Is now widespread. Most wetlands and permanently damp watercourses are likely to contain this weed. Many occurrences will be small but rapidly expanding. Large occurrences will probably also be rapidly expanding.

We adopt: "Weeds of National Significance" Hymenachne strategy
Queensland NRM facts sheet and locally compiled information as follows

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Image: shorter Paragrass alongside the similarly tall Alemangrass and Hymenachne.
Compared with Paragrass, Alemangrass appears slightly bluer and Hymenachne slightly yellowish.
Plantation/Ocean Creeks, Ayr, assistance by Les Searle (Burdekin Landcare).

 

It would seem that most seeding takes place in April/May, with occasional plants producing seed from September. Each fruiting spike can produce 4,000 seed.

There is a native species of Hymenachne that is rarely found naturally in the Townsville region: Hymenachne acutigluma. Joe Vitelli (Department of Natural Resources, Tropical Weeds Research Centre, pers. com.) advises firstly to forget about the native species, 99% of the time it will be the exotic. He says both species are easy to distinguish, even as seedlings, by examining the nature of the many fine longitudinal lines in the leaves. The 2001 draft Qld. Herbarium identification key needed updating; it should read: H amplexicaulis has strongly auriculate leaf orifice (leaf blade base or leaf sheath top) whereas H. acutigluma is not strongly auriculate (Jeanette Kemp, BK Simon, Qld Herbarium pers. com.). Previous draft advice has been shown to be inadequate here and by others elsewhere, it was: spikelets 4.5-5.5 mm long = H. acutigluma (native sp.); H. amplexicaulis = spikelets 3-4mm.

 

Paragrass (Brachiaria mutica) is so widespread in the region that it is likely to be present everywhere.

Alemangrass (Echinochloa polystachya) is established in the Burdekin region in at least one location where it was planted. (Les Searle, Burdekin Landcare pers. com.). However it is also found in the Townsville Town Common (RJ Cumming pers. com.). Perhaps neither of these outbreaks has spread very far in the years they have been established, but that is no reason for complacency. Large wetlands near Proserpine are densely covered (J Kemp, Qld. Herbarium, pers. com.). It is very similar to Paragrass, but stems are usually much larger, leaves and leaf stems are without Paragrasses fine hairs and the foliage is bluer. It also grows in deeper water and stands higher. (Mr Les Searle, Lower Burdekin Landcare Group Coordinator pers. com.)

 

Alemangrass, (Echinochloa polystachya)

Tall wetland grass that may be blue-ish in colour. Few if any hairs on leaves and robust thick stems. Leaf blade base, at the junction of the leaf blade and sheath (collar), is not with ears like Hymenachne. The margin of the leaf blade above the collar may have several small teeth (visible on the far left?). Inside the collar (not shown) may be an eyelash like fringe. All three species have a similar straggly wetland Paragrass like habit: long stems float on fresh water, root freely at the nodes and leafy stem ends rise up above the water.

Centre image: two typically very hairy Paragrass (Brachiaria mutica) leaf sheath contrasted with two almost hairless and much larger diameter Alemangrass (Echinochloa polystachya) stems. Guineagrass (Panicum maximum) stems are similar to Paragrass but nowhere near as hairy. Alemangrass may be slightly bluer than Paragrass. Hymenachne's stem diameter is similar to Alemangrass.

Hymenachne (Hymenachne amplexicaulis) is easily identified: firstly by the wetland habitat, looks like Alemangrass with tallish erect leafy robust stems. Hymenachne seems to have a yellowish green leaf colour, but most distinctive are the ear like (auriculate) lobes on the leaf base at the top of the leaf sheath. Flower stems seem much smaller in diameter. Habit: long stems float on fresh water, root freely at the nodes and leafy stem ends rise up above the water.

 

  

Local Wetland Weed Control Facts compiled 2001 before the Hymenachne strategy was published: 

  1. "Roundup Bioactive" herbicide is completely legal to use on Hymenachne in wetlands at rates per ha. as approved for other species in wetlands (e.g. Paragrass, Cumbungie) and because "Roundup Bioactive" is effective. No other herbicide is approved for use in wetlands even for small areas, so no other herbicides are permitted legally.
  2. Hymenachne (Hymenachne amplexicaulis) (John Ready Specialist Weed Control, Burdekin pers. com.) It's rapidly expanding extent within the region using both vegetative and seed dispersion by both floods and birds means that a twice yearly inspection will be required for permanent exclusion. From scratch to an infested area the size of a house-block would be much quicker than 2-3 years. The worst case: an inspected and clear lagoon was chocked in 6 months. Miss an inspection and it can be necessary to restart the eradication process all over again.

Spray outbreaks immediately. Spraying three more times should get close to complete eradication.

 

  1. Paragrass (Brachiaria mutica) is the most widespread of the three weeds regionally.

 

  1. Alemangrass (Echinochloa polystachya) is much easier to kill with "Roundup Bioactive" than Hymenachne (John Ready, (Specialist Weed Control, Burdekin pers. com.)

It may only spread vegetatively (Les Searle, Burdekin Landcare, pers. com.). However since Paragrass stolons can apparently be relocated by birds possibly the stolons of Alemangrass can be spread that way also (Stems for nests? How did it get into the Town Common?).

  

Paragrass control trials were also done by:

Northern Territory University
Darwin, NT 0909
Ph: (08) 89 467 261
Fax: (08) 89 466 847
Email:
michael.douglas@ntu.edu.au)

 

Links

Hymenachne:

Para Grass:

 

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