Impact Assessment - Onion weed (Asphodelus fistulosus) in Victoria (Nox)

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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.

Question	Comments	Reference	Rating
Recreation
1. Restrict human access?	An erect annual, biennial or perennial herb to 75 cm high. Its presence would have minimal impact on human access.	P & C (2001)	L
2. Reduce tourism?	Commonly occurring on roadsides, disturbed areas and overgrazed pastures. In flower, the plant would be noticeable and would have a minor negative impact on the visual aesthetic.	P & C (2001)	ML
3. Injurious to people?	“Onion weed allegedly causes dermatitis in some people.” Minor physiological effect	P & C (2001)	ML
4. Damage to cultural sites?	During flowering, dense patches would create a major effect to aesthetics.		MH
Abiotic
5. Impact flow?	Terrestrial species.	P & C (2001)	L
6. Impact water quality?	Terrestrial species.	P & C (2001)	L
7. Increase soil erosion?	“Onion weed grows so thickly that other vegetation is reduced. [Although] the flower stem dies back, most leaves remain alive through summer.” It would not contribute to soil erosion.	P & C (2001)	L
8. Reduce biomass?	“…an invasive weed occupying roadsides, disturbed areas and overgrazed pastures.” Weed replaces biomass	P & C (2001)	ML
9. Change fire regime?	No great change in biomass; not likely to change fire risk.		L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassy woodland (E); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. “Onion weed grows so thickly that other vegetation, particularly grasses, is reduced.” Major effect on annual grasses.	P & C (2001)	MH
(b) medium value EVC	EVC=Herb-rich heathy woodland (D); CMA=Glenelg Hopkins; Bioreg=Dundas Tablelands; VH CLIMATE potential. “Onion weed grows so thickly that other vegetation, particularly grasses, is reduced.” Does not establish when having to compete for moisture with other perennials. Minor effect on species within the lower stratum.	P & C (2001)	ML
(c) low value EVC	EVC=Heathy woodland (D); CMA=Glenelg Hopkins; Bioreg=Dundas Tablelands; VH CLIMATE potential. Impact as in 10(b) above.	P & C (2001)	ML
11. Impact on structure?	“Onion weed grows so thickly that other vegetation, particularly grasses, is reduced. In dense patches, lowered soil nitrogen prevents other plants establishing and competing with the weed.” Likely to have a minor effect on the lower strata. Occurs on roadsides, disturbed areas and overgrazed pasture..	P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“Farmers’ organisations claim that the original carrying capacity of much grazing land in the drier parts of southern Australia has been reduced by 75% because of onion weed.” The plant, ”…is not eaten by stock.” Likely to have at least a minor impact on the source of food for native fauna.	P & C (2001)	ML
15. Benefits fauna?	No benefits.		H
16. Injurious to fauna?	No	P & C (2001)	L
Pest Animal
17. Food source to pests?	Not known as a food source to pests.		L
18. Provides harbour?	Does not provide harbour.		L
Agriculture
19. Impact yield?	“Onion weed grows so thickly that other vegetation, particularly grasses, is reduced, and because it is not eaten by stock, carrying capacity is also reduced. Farmers’ organisations claim that the original carrying capacity of much grazing land in the drier parts of southern Australia has been reduced by 75% because of onion weed.” Likely to have a serious impact on yield.	P & C (2001)	MH
20. Impact quality?	Not a weed of cropping.		L
21. Affect land value?	Its impact on yield and the cost associated with control may reduce the value of land.		M
22. Change land use?	The plant can be readily controlled by cultivation and sewing lucerne or a cereal crop undersown with lucerne. If sown to lucerne, it is recommended that the area is not grazed in the first year. This makes the area temporarily unavailable, which would lead to some economic loss.	P & C (2001)	M
23. Increase harvest costs?	No a weed of cropping		L
24. Disease host/vector?	None evident		L