Invasiveness Assessment - Nodding thistle (Carduus nutans) 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 Nodding thistle.

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: Nodding thistle
Scientific name: Carduus nutans

Question	Comments	Reference	Rating
Establishment
Germination requirements?	“Most musk thistle seedlings emerge in the fall, although seedlings can also emerge in large numbers in the spring.” Requires natural seasonal disturbance.	Zouhar (2002)	MH
Establishment requirements?	“… more seeds germinate and establish on bare soils in open pastures and other poorly vegetated sites. Exposure to darkness… inhibited germination.” Requires bare ground with direct access to sunlight.	Zouhar (2002)	ML
How much disturbance is required?	“It is one of the weeds which establishes will on bare ground occurring in pastures at the end of summer or on sheep camps or bare patches caused by rabbits, pasture insects etc.” Establishes in highly disturbed systems.	Parsons & Cuthbertson (2001)	ML
Growth/Competitive
Life form?	Annual or biennial herb. Other category.	Parsons & Cuthbertson (2001)	L
Allelopathic properties?	“there is overseas evidence that the plant produces allelopathic compounds which could contribute to its competitiveness.” Properties may seriously affect some plants.	Parsons & Cuthbertson (2001)	MH
Tolerates herb pressure?	“… it is not readily grazed because of its spiny foliage. Its presence also discourages animals from grazing neighbouring pasture plants.” In the U.S., “… cattle and domestic sheep and goats have been observed consuming flowers and seedheads.” Rarely eaten.	Parsons & Cuthbertson (2001) Zouhar (2002)	MH
Normal growth rate?	“It is one of the weeds which establishes well on bare ground occurring in pastures at the end of summer. In contrast, seedlings do not establish where reasonable pasture cover is maintained. [It] is a poor competitor with other thistle species.” However, once established as rosettes, shoots ‘bolt’ depending upon weather and altitude. In the U.S., it flowers and starts to produce seed 45 to 55 days after it bolts. Moderately rapid growth rate, similar to competitive species.	Parsons & Cuthbertson (2001) Beck (2003)	MH
Stress tolerance to frost, drought, w/logg, sal. etc?	Plants require a period of vernalisation for floral initiation to occur. In the U.S., this period is a minimum of at least 40 days below 10ºC. “Musk thistle flowers and seeds are produced over a long period… continuing as later flowers mature until after fall frost.” Plant tolerates frost. “… musk thistle occupies habitats ranging from saline soils in low altitude valleys.” Salt tolerant.	Zouhar (2003)	ML
Reproduction
Reproductive system	Reproduces sexually. “Each floret bears male and female organs and fertilization is primarily by outcrossing. Musk thistle can also produce a large number of seeds through self-pollination.”	Parsons & Cuthbertson (2001) Zouhar (2003)	ML
Number of propagules produced?	“Each plant produces about 7,000 viable seeds.”	Parsons & Cuthbertson (2001)	H
Propagule longevity?	“Seed longevity in the field depends on depth of burial; those on or near the surface lose viability in 2 to 4 years, whereas seeds buried 5 to 20 cm may survive for at least 10 years.” Some seeds survive for more than 10 years.	Parsons & Cuthbertson (2001)	MH
Reproductive period?	Annual or biennial herb. Mature plant produces seed for only one year.	Parsons & Cuthbertson (2001)	L
Time to reproductive maturity?	Annual or biennial herb. Can produce propagules in under one year.	Parsons & Cuthbertson (2001)	H
Dispersal
Number of mechanisms?	The seed is equipped with a pappus, but it readily detaches, limiting the potential for wind-borne dispersal. Some seed may be dispersed in mud, water machinery, fodder and agricultural seed.	Parsons & Cuthbertson (2001)
How far do they disperse?	“Measurements in New Zealand show that 91% of nodding thistle seed captured fell within 1 to 2 metres of the plant and none was detected more than 10 metres away.” “The majority of seeds are deposited in a dense pattern near the parent plant.” Some dispersal beyond 200 metres may occur, but most will be within 20 metres.	Parsons & Cuthbertson (2001) Zouhar (2002)

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