Invasiveness Assessment - Prickly acacia (Acacia nilotica ssp. indica) in Victoria (Nox)
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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 Prickly acacia.
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: Prickly acacia
Scientific name: Acacia nilotica ssp. indica
| Question | Comments | Rating | Confidence |
| Establishment | |||
| Germination requirements? | ‘Seeds germinate after significant rainfall in late spring and summer’ (DNRME 2004) ‘germination does not occur until triggered by water penetrating the hard seed coat. Germination occurs between 14° and 37°C. (Mackey 1998). Requires natural seasonal disturbance of rainfall and summer temperatures. | MH | MH |
| Establishment requirements? | ‘..but seedlings are shade intolerant. Although seedling establishment can occur in drier times, survival is low. If rainfall following germination is below average, the majority of seedlings on black soil will die within two years’ (Mackey 1998). Seedlings require open conditions and regular rainfall. | ML | MH |
| How much disturbance is required? | Dense infestations occur where there is a plentiful supply of water, for example, along watercourses and bore water drains. With an abundant supply of water and little shading by other plants, growth is rapid, but, ‘on open downs where it is drier, a good grass cover may reduce growth’ (Mackey 1998). More likely to establish in disturbed situation where there is little competition. | ML | MH |
| Growth/Competitive | |||
| Life form? | Tree, commonly to 6 metres, but can grow to 15 metres in optimal situation (Mackey 1998). Other life form. | L | MH |
| Allelopathic properties? | None described. Growth of perennial grasses under the canopy is greatly inhibited particularly during dry periods, however, annual and ephemeral species appear to be unaffected. Effect due to competition perennials for moisture and nutrients ( Mackey 1998). | L | MH |
| Tolerates herb pressure? | Seedlings are spiny, but sheep are known to browse on the lower branches of established plants. Domesticated animals consume the abundant seedpods in preference to the tree (Mackey 1998). Tolerates herbivory pressure. | MH | MH |
| Normal growth rate? | Given access to water, seedling growth is rapid (Mackey 1998). | H | MH |
| Stress tolerance to frost, drought, w/logg, sal. etc? | It is reported that frost kills young plants and prevents seed set in older, established trees (Mackey 1998). However, in some frost prone areas in Queensland infestations of prickly acacia are established and spreading. Prickly acacia tolerates salinity if fresh water is present. It has a deep taproot, which enables it to survive dry periods. In Africa it is known to tolerate seasonal waterlogging (Duke 1983). Tolerates two or more stressors. | H | MH |
| Reproduction | |||
| Reproductive system | Reproduces sexually; self and cross pollination. Most flowers are functionally male while others are hermaphroditic (Mackey 1998). Prickly acacia does not reproduce vegetatively. | ML | MH |
| Number of propagules produced? | Annually each tree produces an average of about 30000 seeds (Mackey 1998). | H | MH |
| Propagule longevity? | In bore drains (i.e. where water is plentiful) seed half-life was found to be 10 months, but some seed is known to survive up to seven years (Mackey 1998). | L | MH |
| Reproductive period? | Prickly acacia is known to live for up to 60 years (DNRME 2004). | H | M |
| Time to reproductive maturity? | The plant reaches sexual maturity between 2 to 5 years (DNRME 2004). | ML | M |
| Dispersal | |||
| Number of mechanisms? | Domestic animals, particular cattle are the most important vectors of seed. Ingested seed can remain in cattle for up to six days leading to viable seed being dispersed over large distances. Other vectors are floodwaters and wind (<25 m) (Mackey 1998). | MH | MH |
| How far do they disperse? | Depending upon cattle movement, dispersal would be at least one kilometre, possibly greater (Mackey 1998). | H | MH |
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