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

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:

"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.

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.

John Ready (Specialist Weed Control, Burdekin pers. com.): Especially inspect and spray before main seeding in April/May. Younger stems may delay seed production until early spring, so probably time the second visit before this. Each flower spike can produce 4,000 highly viable seed well suited to germinate in our conditions. Only about 10% of seeds remain viable after 16 months (laboratory or field conditions?).Occasional plants produce seed throughout the year.
DNR web based information advised: "…Based on these results a submission to the National Registration Authority (NRA) requesting an "Emergency-Use permit (PER3492) was granted. The application permits the use of Roundup Biactive to be used at the rate of 12 litres per hectare (4320 g active ingredient per ha) for the control of Hymenachne in irrigation and drainage areas."
John Ready (Specialist Weed Control, Burdekin pers. com.) has killed Hymenachne reliably by spraying for decades, sometimes very large areas and up to 1.8m tall. He advises using 11 litres/ha "Roundup Bioactive" but finds a concentrated mixture better with much reduced water contamination since run-off is less. Application strength 1litre (360gm/litre glyphosphate) to 1-2 litres of water. The kill is good, but not 100%. Experienced applicators can reduce the rate (and water contamination) for soft younger plants. The largest stemmed most fibrous plants will have a reduced kill rate. Where possible he burns dead tops to stimulate the new growth that is easily killed. Dead stems will burn or smoulder adequately over water. On cleared ground after removal massed seedlings then germinate. A light spray kills these easily.
John Ready (Specialist Weed Control, Burdekin pers. com.) advises the only way is eradication. After the initial spray, treat missed patches after 1 month, treat regrowth 3 months later and treat seedlings three months later.
John Ready (Specialist Weed Control, Burdekin pers. com.) especially uses quad bikes that do minimal damage, also Argo 8 wheel tracked all terrain vehicles, canoes are very effective, and boats.
Paul Horrocks (Department of Natural Resources), Jan 2001, advises to use 14 litre/ha, glyphosphate at 360g/l active ingredient as Roundup Bioactive for boom sprays. Spot spray at around 2.0 litres/100 litres concentration at 360g/l (i.e. normal strength) active ingredient. Actually the ideal kill concentration is probably a good deal higher, rather this figure represents the maximum concentration of Roundup Bioactive that is permitted for large scale application in wetlands. No other herbicide is permitted for use in wetlands whether seasonally dry or while flooded. Above this application rate ecological testing is required to verify suitability. This application rate gives around 50% kill but does kill 90-100% of above water stems and leaves. The recommendation is to burn these and a repeat application of herbicide works very well on the new regrowth. A different herbicide is more effective but is not permitted in wetlands. This other herbicide seems to have reduced effectiveness when the plant is seeding heavily (late April/May) and this difficulty may also occur with Roundup Bioactive. Otherwise the plant should generally be actively growing. Water may deoxygenate with dead matter only in large area spraying exercises. To avoid deoxygenation, the recommendation is to spray say 10m wide strips, leave 10-20m wide strips in between to do later. In this manner aquatic life can leave deoxygenated areas and survive adequately.
In Burdekin, Hymenachne is P2 obligatory removal where no grazing, P3 can remain when in an area that is suitable for grazing.
The situation is so serious that an aquatic $0.25 million muncher is proposed as part of 3 million dollars project for Burdekin natural wetlands to remove Water Hyacinth, Paragrass, and other aquatic weeds where rampant (Les Searle pers com.). Hopefully the same control measures will not be necessary throughout the region.

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

Effectively Paragrass seems to propagate vegetatively from floating mats and stolons rather than seed. References indicate that some viable seed may be produced. The twice annual wetlands Hymenachne inspection will ensure adequate inspection for outbreaks.
Paul Horrocks (Department of Natural Resources); advises, Jan 2001: A very high percentage of Paragrass is killed using one application of Roundup Bioactive at 9 l/ha with a boom spray, or spot spray at 1.3 litres/100 litres concentration assuming 360g/l (i.e. normal strength) active ingredient. Water may deoxygenate with dead matter only in large area spraying exercises. To avoid deoxygenation, spray say 10m wide strips, leave 10-20m wide strips in between to do later. In this manner the fish can happily leave deoxygenated areas and survive quite adequately.

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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