Impact Assessment - Siam weed (Chromolaena odorata) in Victoria

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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 Siam weed

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: Siam weed
Scientific name: Chromolaena odorata

Question	Comments	Rating	Confidence
Recreation
1. Restrict human access?	Forms dense impenetrable thickets between 1.5 to 5 m high. Much-branched shrub (Parsons & Cuthbertson 2001). ‘Engulfs native vegetation especially along river banks and on forest fringes’ (Bilowol 2005). High nuisance value with access to areas difficult.	MH	MH
2. Reduce tourism?	Found in North Queensland and concern that the weed may interfere with tourism activities including white water rafting, bushwalking, camping and fishing (DNRM 2005b ). Some recreational uses affected.	MH	M
3. Injurious to people?	‘May cause skin complaints and asthma in allergy-prone people’ (DNRM 2005a). ‘Hand weeding of Chromolaena reportedly caused skin allergy in a number of plantation workers … and the dry stumps left after weeding operation caused poisonous wounds in the feet of the workers’ (Ambika & Jayachandra 2001).	MH	MH
4. Damage to cultural sites?	Weeds occur in the Wet Tropics which is considered part of a “living cultural landscape” by Rainforest Aboriginal peoples to whom it is has spiritual, social, economic and historic significance (Wet Tropics Management Authority 2002). Not likely to cause structural damage but may have moderate visual effect.	ML	M
Abiotic
5. Impact flow?	Terrestrial species	L	MH
6. Impact water quality?	Terrestrial species	L	MH
7. Increase soil erosion?	Fibrous root system extending up to 30 cm. Although the above-ground foliage dies off, the dense mass of leaf stems would provide soil cover (Parsons & Cuthbertson 2001). Unlikely that the weed would contribute to large-scale soil movement.	L	MH
8. Reduce biomass?	‘Forms dense stands which prevent the establishment of other species’ (ISSG 2000). Where the plant occurs on pastures, clearings, riverine or wetland areas, biomass would increase significantly.	L	MH
9. Change fire regime?	‘Cause more frequent and intense bushfires as dry Siam weed stalks burn hotter and flames go higher into trees than a pure grass fire’ (DNRM 2005a).	H	M
Community Habitat
10. Impact on composition (a) high value EVC	The potential distribution of Chromolaena odorata excludes Victoria (Kriticos, Yonow & McFayden 2005). No impact on EVCs in Victoria.	L	H
(b) medium value EVC	The potential distribution of Chromolaena odorata excludes Victoria (Kriticos, Yonow & McFayden 2005). No impact on EVCs in Victoria.	L	H
(c) low value EVC	The potential distribution of Chromolaena odorata excludes Victoria (Kriticos, Yonow & McFayden 2005). No impact on EVCs in Victoria.	L	H
11. Impact on structure?	‘In the establishment phase can choke out (plantation) crops as well as young forest trees’ (Parsons & Cuthbertson 2001). ‘Forms dense thickets that persist and prevent the establishment of all other species’ (Weber 2003). The plant would have a major effect on all layers.	H	MH
12. Effect on threatened flora?	No information available.	MH	L
Fauna
13. Effect on threatened fauna?	The plant affects the Nile crocodile nesting areas by shading and cooling riverbanks skewing the sex balance toward females (Leslie 2001). Potential to do same in Australia but not relevant in Victoria as establishment of weed shown to be highly unlikely and fauna species not in Victoria.	L	MH
14. Effect on non-threatened fauna?	‘Could completely replace pasture grasses, leaving native wildlife with nothing to eat’ (Bilowol 2005). As the plant replaces all other species, has the potential to reduce habitat for fauna.	MH	MH
15. Benefits fauna?	No known benefits to fauna.	H	MH
16. Injurious to fauna?	Toxic to stock ‘killing more than 3000 cattle annually in the Philippines. Also causes abortions’ (Parsons & Cuthbertson 2001). Potential to affect non-threatened native fauna.	H	MH
Pest Animal
17. Food source to pests?	Not a known source of food to pests.	L	MH
18. Provides harbour?	‘In some countries dense thickets harbour wild pigs and rodents’ (Parsons & Cuthbertson 2001). Has the potential to provide harbour for minor pest species.	ML	MH
Agriculture
19. Impact yield?	A major pest of crops in Asia and Africa and ‘some agricultural areas in South-East Asia have been abandoned because Siam weed has taken over pasture and crops’ (CRC for Weed Management 2003).	H	M
20. Impact quality?	‘..carry a number of seed borne fungi, like Fusarium culmorum, F. moniliforme, F. semisecturm and F. solani have been reported as pathogens of food crops’ (Ambika & Jayachandra 2001). Fusarian Head Blight (FHB) is caused by several species of Fusaria. “FHB infected grain may be downgraded at the market’ (McMullen & Stack 1999). Has the potential to have a minor impact on quality of produce.	ML	MH
21. Affect land value?	‘ ..it was estimated the weed would cost more than $14.5 million annually to manage within the agriculture sector if allowed to take hold in Australia’ (Bilowol 2005). No documented evidence to show that the weed affects land value but with the high costs associated with management in places where it has affected plantations it has the potential to decrease land value.	M	MH
22. Change land use?	A major pest of crops in Asia and Africa and ‘some agricultural areas in South-East Asia have been abandoned because Siam weed has taken over pasture and crops’ (CRC for Weed Management 2003). Risk that the presence of this weed could change land use.	H	M
23. Increase harvest costs?	No documented evidence but as the plant ‘forms dense tangled thickets’ (Ambika & Jayachandra 2001) it is assumed that there would be some increase in the cost of harvesting in both time and money. Potential for thickets to interfere with hand harvesting of crops.	M	MH
24. Disease host/vector?	‘Alternative host for fungal diseases’ (Parsons & Cuthbertson 2001).	M	MH

Impact Assessment Record - Siam weed (PDF - 75KB)
Impact Assessment Record - Siam weed (DOC - 75KB)
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