Invasiveness Assessment - Pencil willow, Chilean willow (Salix humboldtiana) (syn. S. chinensis) 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 Pencil willow, chilean willow.
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: Pencil willow, chilean willow
Scientific name: Salix humboldtiana (syn. S. chinensis)
| Question | Comments | Rating | Confidence |
| Establishment | |||
| Germination requirements? | S. humboldtiana reproduce vegetatively (Newsholme, 1992). They take root where they lodge in water or on wet ground (Ladson et al, 1997). i.e. opportunistic species. | H | MH |
| Establishment requirements? | Willows are described as heliophyllic (intolerant of shade) (Haines, 2004) and most species …must not be overhumg by larger plants or trees (Newsholme, 1992). | ML | MH |
| How much disturbance is required? | S. humboldtiana establish vegetatively (Newsholme, 1992), and as such, tending to “invade riverbanks, lakesides, drainage channels, swaps, almost all wet places (Webb, Sykes & Garnock-Jones, 1988) that are, by their nature, disturbed ecosystems. | MH | MH |
| Growth/Competitive | |||
| Life form? | Semi-aquatic- All willows are described as growing on the banks of water courses or in moist locations. S. humboltiana grows on banks of watercourses or in moist locations (Howard, 1988). | H | MH |
| Allelopathic properties? | Of the naturalised willows in Australia, allelopathic compounds have been demonstrated in S. rubra (Bokarev, Kefeli & Kapeljusnikova, 1966), S. babylonica (Koul et al, 1991), S. fragilis (Davison, 1965), S. viminalis (Bowen & Hoad, 1968), S. alba (Banzhaf et al, 1983) and S. purpurea (Gilliver, 1947). For other willow species, their “dense shade and mat-forming willow roots suppress and kill indigenous understorey” (Purtle et al, 2001b), suggesting that there might be an allelopathic basis for this lack of understorey beneath willow infestations, however no specific reference to these species was found. | M | L |
| Tolerates herb pressure? | No information | M | L |
| Normal growth rate? | “Salix are fast growing trees or shrubs, rarely prostrate” (Spencer, 1997). Where growth rates are described, tree and shrub forms of Salix are recorded as vigorous or fast growing, e.g. S. alba (Spencer, 1997), S. glaucophylloides (Newsholme, 1992), S. nigra (Cremer, 1995), S. purpurea (Webb, Sykes &Garnock-Jones, 1988) and S. x rubens (Meikle). | H | MH |
| Stress tolerance to frost, drought, w/logg, sal. etc? | FIRE “If a tree is completely girdled by fire at ground level it may die without coppicing… however young trees tend to sprout from the buried portion of their stem” (Cremer, 1999), indicating some fire tolerance in tree and shrub willows in general. FROST All tree and shrub species (except S. humboldtiana) in this study are recorded as tolerating USDA hardiness zones of between 1 and 6 (average minimum temperatures between –46oC and –17oC) (Bailey & Bailey, 1976, Griffiths, 1992). S. humboldtiana is susceptible to frost (Newsholme, 1992). DROUGHT All willows are described as growing on the banks or in the riverbeds of water courses or in moist locations (Carr, 1996; Carr et. al, 1992; Cody, 1996; Davis, 1982; Howard, 1988; Ladson, et al, 1997; Maloney et al, 1999; Munz, 1963; Voss, 1972; Webb, Sykes & Garnock-Jones, 1988) suggesting a low drought tolerance; and only few species (S. cinerea and S. exigua) can also establish on drier soil (but are not necessarily drought tolerant), with S. purpurea alone tolerating extremely dry, sandy soils (Newsholme, 1992). WATERLOGGING Those species able to grow in riverbeds must have high tolerance of waterlogging (S. exigua, Cody, 1996; S. fragilis, Maloney et al, 1999; S. alba, S. x rubens, and S. viminalis; Webb, Sykes & Garnock-Jones, 1988) and also S. cinerea (Cremer, 1999). SALINITY S. alba is “the most tolerant of all willows to brackish water” (Zallar). S. cinerea “tolerates salt-laden air” (Newsholme, 1992), S. fragilis, S. matsudana & S. seringeana tolerate high levels of salinity (Crouch & Honeyman, 1986). S. nigra has moderately high tolerance of salt (Swift, 1997) and S. purpurea shows “considerable tolerance for salt water” (Newsholme, 1992). S. alba are highly tolerant of waterlogging, salinity and frost, have some tolerance of fire, but not of drought. | MH | MH |
| Reproduction | |||
| Reproductive system | Salicacea are dioecious (Tutin, 1993). S. aegyptica may produce male and female catkins (Tutin, 1993). There are mostly only male S. humboltiana in Australia (ARMCANZ, 2001), so whilst this species can cross pollinate other willows, they produce no seed of their own and only propagate vegetatively (Newsholme, 1992). | MH | MH |
| Number of propagules produced? | See Q. 9. No seed produced. | L | MH |
| Propagule longevity? | See Q. 9. No seed produced. | L | MH |
| Reproductive period? | Following reproductive maturity “flowering tends to be ample and to occur every year” (Cremer, 1999). S. cinerea “stands are mostly monoculture excluding 97% of sunlight and most other species” (Cremer, 1999). The oldest stem S. exigua in this study was 31 years (Ottenbreit & Staniforth, 1992). An old S. fragilis tree was observed to 130 years (Meikle, 1984). | H | MH |
| Time to reproductive maturity? | See Q. 9. No seed produced. | M | MH |
| Dispersal | |||
| Number of mechanisms? | For male S. humboldtiana, propagules are only spread by water (Cremer, 1999). | MH | MH |
| How far do they disperse? | For species such as S. humboltiana, which only reproduce vegetatively (Newsholme, 1992), S. x rubens provides evidence that “very few individuals observed long distances from existing mature trees and these were downstream of cities, towns and homesteads” (Shafroth et al, 1994). | MH | MH |
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