Invasiveness Assessment - Siam weed (Chromolaena odorata) in Victoria
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Plant invasiveness is determined by evaluating a plant’s biological and ecological characteristics against criteria that encompass establishment requirements, growth rate and competitive ability, methods of reproduction, and dispersal mechanisms.
| Each characteristic, or criterion, is assessed against a list of intensity ratings. Depending upon information found, a rating of Low, Medium Low, Medium High or High is assigned to that criterion. Where no data is available to answer a criterion, a rating of medium (M) is applied. A description of the invasiveness criteria and intensity ratings used in this process can be viewed here. |
The following table provides information on the invasiveness 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)
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Common Name: Siam weed
Scientific name: Chromolaena odorata
| Question | Comments | Rating | Confidence |
| Establishment | |||
| Germination requirements? | Germination begins at the start of rainy season (Parsons & Cuthbertson 2001). | MH | MH |
| Establishment requirements? | Thrives in open or partly shaded, well drained sites with annual rainfall greater than 1,200 mm. ‘Can grow under dense rainforest canopies but less vigorously [than in open sun]’ (Parsons & Cuthbertson 2001). Light promotes germination but does not become limiting factor (DNRM 2005). Requires natural seasonal disturbances. | MH | MH |
| How much disturbance is required? | ‘Thrives in disturbed areas such as pastures, plantations, clearings, roadsides and riverbanks, especially those in well-lit sites’ (CRC for Weed Management 2003). Also occurs in forests and forest gaps, grassland woodland (Weber 2003). Can occur in areas of minor disturbance. | MH | MH |
| Growth/Competitive | |||
| Life form? | Herbaceous perennial (Weber 2003). Life form – other. | L | MH |
| Allelopathic properties? | Known to produce allelopathic compounds (Parsons & Cuthbertson 2001). ‘..the impoverished growth of the young teak and other timber plants in the nurseries and young plantations could be at least partly due to the allelopathic properties’ (Ambika & Jayachandra 2001). As only ‘partly due’ extent not clear so scored as M. | M | MH |
| Tolerates herb pressure? | Toxic to stock. In Indonesia a leaf-feeding moth and gall fly showed some success in control. No biological control programs in Australia currently but being investigated. (CRC for Weed Management 2003). Not favoured and not under control program | H | M |
| Normal growth rate? | Very competitive. ‘Can out compete and smother crops and native vegetation because of its phenomenal growth rate (20 mm/day or 5 m per year) (DNRM 2005). | H | M |
| Stress tolerance to frost, drought, w/logg, sal. etc? | Tolerant to at least 2 (severe dry period and fire) but susceptible to at least 2 (doesn’t grow well on waterlogged or saline soil) (Parsons & Cuthbertson 2001, CRC for Weed Management 2003). | ML | MH |
| Reproduction | |||
| Reproductive system | Reproduces by seed (through both self and cross-pollination). Not known to vegetatively reproduce (Parsons & Cuthbertson 2001). | ML | MH |
| Number of propagules produced? | Prolific seed-producer. Up to 87,000 seeds per plant have been recorded in India (Parsons & Cuthbertson 2001). | H | MH |
| Propagule longevity? | Seed viability is at least eight years but research is on-going as suspected it could be longer (CRC for Weed Management 2003). | ML | M |
| Reproductive period? | Minimum life span of ten years, still producing viable propagules (CRC for Weed Management 2003). Reproductive period greater than 10 years. | H | M |
| Time to reproductive maturity? | Can reach maturity in less than 12 months (CRC for Weed Management 2003). | H | M |
| Dispersal | |||
| Number of mechanisms? | Significant wind dispersal (fine pappus covering), human and equipment contamination (Parsons & Cuthbertson 2001). Also carried by water (DNRM 2005). | H | MH |
| How far do they disperse? | Spread ‘large distances’, and very likely that some propagules will disperse greater than 1 km as they easily adhere to clothing and equipment with their weightlessness aiding long-distant dispersal (CRC for Weed Management 2003). | H | M |
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