Invasiveness Assessment - Alligator weed (Alternanthera philoxeroides) 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 Alligator 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 - 1026 KB)
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Common Name: Alligator weed
Scientific name: Alternanthera philoxeroides

Question	Comments	Reference	Rating
Establishment
Germination requirements?	“Growth (vegetative) starts with development of node buds on old stems in early spring”.	P & C (1992)	MH
Establishment requirements?	“Establishes readily wherever there is an adequate water supply…it has a low light requirement”.	P & C (1992)	H
How much disturbance is required?	Established in rice crops and inundated pastures.	P & C (1992)	MH
Growth/Competitive
Life form?	Aquatic (emergent or free flowing) as well as terrestrial.	P & C (1992)	H
Allelopathic properties?	None described.		L
Tolerates herb pressure?			M
Normal growth rate?	“Strong competition in rice fields and pastures prone to water logging, often crowding out desirable species”. “Alternanthera philoxeroides competes successfully with other plants with the exception of water hyacinth. (Spenser & Coulson 1976); (Coulson 1977); (Julien & Broadbent 1980 p. 154).	P & C (1992)	H
Stress tolerance to frost, drought, w/logg, sal. etc?	Tolerant of frosts, water logging and salinity. “It requires a warm growing season but will tolerate cold winters including severe frosts which may kill emergent stem material. (Coulson 1977)”. (Julien & Broadbent 1980 p. 152).	P & C (1992)	MH
Reproduction
Reproductive system	“Reproduction in the field is entirely vegetative”.	P & C (1992)	MH
Number of propagules produced?	“Reproduction in the field is entirely vegetative”.		L
Propagule longevity?			L
Reproductive period?	Produces thick mats. (See picture P & C 1992 p. 157). “Floating mats of alligator weed are generally monocultures”. (Julien & Broadbent 1980 p. 152).	P & C (1992)	H
Time to reproductive maturity?	Nodes on old stems in early spring stems and flowers in mid summer - flowering until March. Some seeds produced but rarely ever viable in Australia.	P & C (1992)	H
Dispersal
Number of mechanisms?	Water currents and cultivation.	P & C (1992)	MH
How far do they disperse?			M

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