Impact Assessment - Marijuana (Cannabis sativa) in Victoria

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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?	“A bushy, erect sometimes slightly woody herb 2 – 7 metres high. A sun loving plant, it becomes naturalised in open areas along streams, roadsides, fence lines and in neglected fields.” Dense stands of this plant may impede individual access.	P & C (2001)	ML
2. Reduce tourism?	Infestation densities are not recorded and, “it is not usually competitive with crops.” Not likely to affect recreational activities greatly. Minor visual effect.	P & C (2001)	ML
3. Injurious to people?	The fruit is toxic.	P & C (2001)	H
4. Damage to cultural sites?	Branching taproot to 100 cm with dense laterals extending to about 3 meters. Root system not vigorous. Dense patches would create a moderate negative visual effect.	P & C (2001)	ML
5. Impact flow?	Terrestrial species.	P & C (2001)	L
6. Impact water quality?	Terrestrial species.	P & C (2001)	L
7. Increase soil erosion?	Branching taproot to 100 cm with dense laterals extending to about 3 meters. Competitiveness with other vegetation unknown. The plant itself would not contribute to soil erosion.	P & C (2001)	L
8. Reduce biomass?	“A bushy, erect sometimes slightly woody herb 2 – 7 meters high…it becomes naturalised in open areas along streams, roadsides, fence lines and in neglected fields.” Likely to slightly increase biomass.	P & C (2001)	L
9. Change fire regime?	In dense infestations, woody stems remaining after plant dies may increase the frequency of fire risk.		ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC= (); CMA=; Bioreg=;
(b) medium value EVC
(c) low value EVC
11. Impact on structure?	“It survives on very little water,” possibly because of the extensive root system. Survival may be to the detriment of understorey species. Infestation density not known; minor effect on the flora strata.	P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	Dense patches may reduce food source of native species.		ML
15. Benefits fauna?	No documented benefits.		H
16. Injurious to fauna?	“Mules and horses have been poisoned in Greece.” Potentially toxic to native fauna.	P & C (2001)	H
Pest Animal
17. Food source to pests?	No known as a food source to pests.		L
18. Provides harbour?	“A bushy erect shrub.” In dense patches, it may provide limited harbour for rodents.	P & C (2001)	ML
Agriculture
19. Impact yield?	“It is not usually competitive with crops but, in India, sometimes significantly reduces maize yields.” May be toxic to stock.	P & C (2001)	MH
20. Impact quality?	No known to affect the quality of agricultural produce.		L
21. Affect land value?	Control of this weed is not difficult, nor does it appear to affect agricultural yields. In arable areas, “the most effective and cheapest method of control is to establish strongly competitive pastures.” Not likely to affect land values.	P & C (2001)	L
22. Change land use?	Invasiveness and persistence of the plant is not documented. “Effective control can also be obtained with herbicides in both non-crop and cropping situations.” Not likely to result in change in land use.	P & C (2001)	L
23. Increase harvest costs?	No evidence.		L
24. Disease host/vector?	None evident.		L

Impact Assessment Record - Marijuana (PDF - 30KB)
Impact Assessment Record - Marijuana (DOC - 58KB)
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