Impact Assessment - Hymenachne, olive hymenachne (Hymenachne amplexicaulis) in Victoria (Nox)
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Assessment of plant invasiveness is done by evaluating biological and ecological characteristics such as germination requirements, growth rate, competitive ability, reproduction methods and dispersal mechanisms. Assessment of plant impacts, however, is determined by the extent to which a plant affects a land manager’s environmental, economic and social resources.
The relative importance of these resources varies depending upon the value people place on them and, as such, the assessment process is subjective. For example, a farmer is likely to place a higher emphasis on the impact of a plant on production (economic resource) than its impact on areas of natural vegetation occurring on the farm. Conversely, a Landcare or Friends group would value environmental or social resources more than economic resources.
Recognising that the value of resources vary between different land tenures, plant impact assessments allow a prioritisation of resources by land managers. Assessments can apply at a local, regional or state level, and the relative values of each resource identified may differ at each level.
The impact assessment method used in the Victorian Pest Plant Prioritisation Process uses three broad resource categories: social, environmental and agricultural, each with a number of related attributes. For example, social resources include such attributes as how the plant affects human access for recreation, or if it creates a health risk due to toxicity or by producing allergens.
| Each resource attribute, or criterion, is assessed relative to a list of intensity ratings. Depending upon information found in relation to each criterion, a rating of Low, Medium Low, Medium High or High is assigned. Descriptions of the impact criteria and intensity ratings used in this process can be viewed here. |
The following table provides information on the impact of Hymenachne, olive hymenachne
A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:
Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1 MB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).
Common Name: Hymenachne, olive hymenachne
Scientific name: Hymenachne amplexicaulis
| Question | Comments | Rating | Confidence |
| Recreation | |||
| 1. Restrict human access? | Perennial grass to 2.5 metres high. It invades waterways including drains, lagoons, creeks and rivers (Diaz et al 2003). Heavy infestations may be a major impediment to access waterways. | H | MH |
| 2. Reduce tourism? | “Overseas, hymenachne forms floating mats and grass islands” (NRM 2001). Serious impact on water-based recreation. | H | M |
| 3. Injurious to people? | The plant has no physical or toxic properties that may injure humans. However ‘…the mats that the plants form create a haven for mosquitoes that are vectors of Ross River Fever and encephalitis’ (NRM 2001). Therefore score as medium. | M | M |
| 4. Damage to cultural sites? | Plant not likely to cause damage to indigenous or cultural European sites. | L | MH |
| Abiotic | |||
| 5. Impact flow? | “Hymenachne can choke drains and small watercourses, increasing flooding by reducing the flow capacity of the drainage networks” (Diaz et al 2003). Major impact on either surface or sub-surface flow. | MH | MH |
| 6. Impact water quality? | “Heavy infestations reduce the infiltration of sunlight…[and] prevents the exchange of air, which normally occurs on an open water surface. As plant material decomposes it uses oxygen causing water pollution and stagnation” (Diaz et al 2003). High effects in dissolved O2. | H | MH |
| 7. Increase soil erosion? | “Hymenachne can choke drains and small watercourses, increasing flooding” (Diaz et al 2003). High potential for erosion with offsite implications | MH | MH |
| 8. Reduce biomass? | Density of infestations suggests biomass would significantly increase. (Cruz & Salazar 1989). In aquatic situations, increase in biomass is a negative attribute. | H | H |
| 9. Change fire regime? | Occurs in wetland situations (NRM 2001). Unlikely to contribute to fire risk. | L | MH |
| Community Habitat | |||
| 10. Impact on composition (a) high value EVC | EVC=Swamp scrub (E); CMA=Corangamite; Bioregion=Victorian Volcanic Plain; CLIMATE=M. “Hymenachne can form pure stands that replace native wetland plants.” High CLIMATE potential would restrict impact. Unlikely to establish as monoculture, but would still significantly displace grass species. | MH | MH |
| (b) medium value EVC | EVC=Damp heathland (D); CMA=Glenelg Hopkins; Bioregion=Glenelg Plain; CLIMATE=M. | MH | MH |
| (c) low value EVC | EVC=Wet heathland (LC); CMA=Glenelg Hopkins; Bioregion=Victorian Volcanic Plain; CLIMATE=M. Impact as in 10(a) above. | MH | MH |
| 11. Impact on structure? | “Hymenachne can form pure stands that replace native wetland plants” (ARMCANZ 2000) | H | MH |
| 12. Effect on threatened flora? | This species is not documented as posing an additional risk to threatened flora in Victoria. | MH | L |
| Fauna | |||
| 13. Effect on threatened fauna? | This species is not documented as posing an additional risk to threatened fauna in Victoria | MH | L |
| 14. Effect on non-threatened fauna? | “A large infestation of hymenachne is a physical barrier for aquatic and semi-aquatic animals, restricting their territorial movements and breeding activities” (NRM 2001). Can ‘reduce resources available for feeding, breeding and shelter of native fauna’ (Diaz et al 2003). Reduction in habitat for fauna. | MH | MH |
| 15. Benefits fauna? | No known benefits to native fauna. | H | MH |
| 16. Injurious to fauna? | Not known to be harmful to fauna. It was grown in Queensland for a high-quality ponded pasture grass (NRM 2001) and in Florida (Hill 1996). | L | MH |
| Pest Animal | |||
| 17. Food source to pests? | Not known as a food source to pest animals. | L | MH |
| 18. Provides harbour? | Not known to provide harbour for pest animals. | L | MH |
| Agriculture | |||
| 19. Impact yield? | Originally grown in Queensland for a high-quality ponded pasture grass, the species would have no negative impact on yield in grazing situations. It may restrict access to watering points, and its capacity to occur in irrigation channels may impact on the yield of irrigated crops (NRM 2001). However, there is no data to support the level of impact. | M | L |
| 20. Impact quality? | No data on effect on quality of produce. | M | L |
| 21. Affect land value? | No data available to determine impact on land value. | M | L |
| 22. Change land use? | No data available to suggest change in land use. | M | L |
| 23. Increase harvest costs? | Does not affect harvesting activities. | L | MH |
| 24. Disease host/vector? | None described | L | MH |
This table can also be viewed as a PDF document (printer friendly).
Impact Assessment Record - Hymenachne (PDF - 69KB)
Impact Assessment Record - Hymenachne (DOC - 63KB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).
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