Impact Assessment - Horehound (Marrubium vulgare) 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?	A bushy perennial herb, 30 cm tall and 75 cm wide. In dense infestations, its growth habit may present a minor nuisance to pedestrian traffic.	Blood (2001)	L
2. Reduce tourism?	A low growing plant (30 cm); dense patches would have a minor effect on aesthetics, recreational activities not seriously affected.	P & C (2001)	ML
3. Injurious to people?	The plant is not toxic, but the fruit has 10 backward-curved hooked spines; not known to be harmful or cause injury.	P & C (2001)	ML
4. Damage to cultural sites?	Roots or growth habit unlikely to cause structural damage. Dense patches would create a moderate negative visual impact on a cultural site.	Blood (2001)	ML
Abiotic
5. Impact flow?	Terrestrial species.	P & C (2001)	L
6. Impact water quality?	Terrestrial species.	P & C (2001)	L
7. Increase soil erosion?	In one area in Victoria where horehound has flourished, soil erosion has increased markedly.	P & C (2001)	MH
8. Reduce biomass?	Generally invades neglected areas such as fence lines, roadsides, channel banks, around buildings, on sheep camps and neglected urban areas. Biomass would increase.	P & C (2001)	L
9. Change fire regime?	Bushy perennial. Increase in biomass would likely cause an increase in the intensity of fire risk.	Moerkerk (2000)1	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassy woodland (E); CMA=North Central; Bioreg=Goldfields; VH CLIMATE potential Prefers open areas. “…has the potential for dense establishment.” “Seedlings do not establish in dense pastures.” Impact lessened with overstorey cover. Major displacement of grasses/forbs.	Moerkerk (2000) P & C (2001)	MH
(b) medium value EVC	EVC=Riverine grassy woodland (D); CMA=Goulburn Broken; Bioreg=Murray Fans; VH CLIMATE potential Impact as in 10(a) above.	Moerkerk (2000) P & C (2001)	MH
(c) low value EVC	EVC=Rocky outcrop shrubland (LC); CMA=Wimmera; Bioreg=Central Victorian Uplands; VH CLIMATE potential. Impact as in 10(a).	Moerkerk (2000) P & C (2001)	MH
11. Impact on structure?	“Has the potential to establish in pastures and open areas as dense monocultures.” It is present in Victoria in medium to large populations. Monoculture in lower stratum.	Moerkerk (2000) Carr et al (1992)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	In Victoria it is widespread in medium to large populations and invades dry coastal vegetation, mallee shrubland, lowland grassland & grassy woodland, and rock outcrop vegetation. “Because of its bitter taste, horehound is not palatable to stock.” “Horehound is relatively drought tolerant and has the potential for dense establishment in many more areas of Australia.” This plant would reduce the habitat for fauna species.	Carr et al (1992) P & C (2001)	MH
15. Benefits fauna?	No benefits apparent.		H
16. Injurious to fauna?	Burrs are not injurious; no toxic properties.	P & C (2001)	L
Pest Animal
17. Food source to pests?	Not known as a food source to pest animals.		L
18. Provides harbor?	A low growing plant (30 cm tall) it may provide limited harbor for rodents or rabbits in low densities. The plant does occur near rabbit warrens mainly due to the seeds sticking to the animal’s fur.	P & C (2001)	MH
Agriculture
19. Impact yield?	“In early trials with 2,4-D in the Victorian Wimmera, treated plots yielded 20% more wheat than the plots where Horehound was unchecked.” The plant has serious impact on crop yields.	P & C (2001)	H
20. Impact quality?	“…it is seldom eaten by stock. When it is eaten, however, milk and possibly meat are tainted.”	P & C (2001)	ML
21. Affect land value?	Due to the limited control methods available during the 1930s, presence of the weed in cropping situations, “…greatly reduced land value.” In spite of improvements to control methods, its occurrence in paddocks today would most likely reduce the land value.	P & C (2001)	M
22. Change land use?	“Fortnightly cultivations killed all plants after 2 years.” Temporary loss of land for productive activities that would lead to significant loss.	P & C (2001)	H
23. Increase harvest costs?	“…it also interferes with harvesting in some areas.” Interference may increase either time or labour to conduct harvest.	P & C (2001)	M
24. Disease host/vector?	In the US, Cardaria spp. have been found to host viral pathogens that cause beet western yellows and potato leaf roll. Potential threat to Victorian producers?	Jewett (2003)	H