Invasiveness Assessment - Hardheads (Rhaptonticum repens, Acroptilon repens) in Victoria

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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 Creeping knapweed.

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: Creeping knapweed
Scientific name: Acroptilon repens

Question	Comments	Reference	Rating
Establishment
Germination requirements?	Perennial herb. New growth develops from buds on the roots in spring and summer. Requires natural seasonal disturbance to grow.	DNRE (1998)	MH
Establishment requirements?	In the United States, Russian knapweed invades open, disturbed land. It is sensitive to decreased amounts of sunlight, does not compete well under heavy canopy, and is weakened by dense shade. Requires disturbance and direct access to sunlight.	Zouhar (2001)	ML
How much disturbance is required?	In Australia, it has become a weed of both irrigated crops, such as vines, and dryland cereals in the 300 to 600 mm annual rainbelt. In the U.S., it occurs in disturbed western grassland and shrubland communities. Establishes in highly disturbed environments.	Parsons & Cuthbertson (2001)	ML
Growth/Competitive
Life form?	A perennial herb with creeping roots. Geophyte.	Parsons & Cuthbertson (2001)	ML
Allelopathic properties?	“Creeping knapweed has allelopathic properties, …[which] restrict the growth of such species as oats, beans and tomatoes. Extracts also have a stimulatory effect on some species.”	Parsons & Cuthbertson (2001)	MH
Tolerates herb pressure?	“The leaves have a bitter disagreeable taste but, nevertheless, are grazed by sheep.” In the U.S., animals avoid grazing the plant. Consumed, but not preferred.	Parsons & Cuthbertson (2001) Zouhar (2001)	MH
Normal growth rate?	“Hardheads is an extremely vigorous and efficient competitor for water and nutrients and releases allelopathic chemicals which can suppress the growth of other plants.” Ability to compete strongly with most other species of similar life form.	DNRE (1998)	H
Stress tolerance to frost, drought, w/logg, sal. etc?	Survives through winter as a perennating root system, which may extend to 5 metres in depth. In both Australia and the U.S. it occurs in dryland areas (300 to 600 mm, and 245 mm respectively). Also, in the U.S., it is believed that Russian knapweed tolerates some flooding, but not severe drought.	DNRE (1998) Zouhar (2001)	MH
Reproduction
Reproductive system	The plant reproduces both vegetatively and sexually.	Parsons & Cuthbertson (2001)	H
Number of propagules produced?	“A single Russian knapweed plant can produce about 1,200 seeds per year.” 1,000–2,000 seeds.	Zouhar (2001)	MH
Propagule longevity?	“Seeds are known to survive in soil for at least 5 years, however, the seedlings of creeping knapweed are not often found and it may be that seeds germinate, or seedlings survive, only under uncommon conditions.” Low viability.	Parsons & Cuthbertson (2001)	ML
Reproductive period?	The U.S. experience is that, “Once established in open areas, however, it is highly competitive and spreads aggressively, often forming dense, single-species stands that can persist indefinitely.” Similarly in Australia, “In recent years it has established in the eastern Mallee of Victoria, forming dense patches to the virtual exclusion of other vegetation.” Self-sustaining colony.	Zouhar (2001)Parsons & Cuthbertson (2001)	H
Time to reproductive maturity?	Produces seed annually, though reproduces annually from adventitious buds on horizontal roots.	Zouhar (2001)	MH
Dispersal
Number of mechanisms?	Most seeds fall within about a metre of the parent plant, but greater dispersal can result from water (in channels), contaminated seed and machinery, and animals (both internally and externally). Vegetative material, which is capable of producing new plants, is also spread by cultivation equipment. Propagules spread by humans and animals and occasionally water.	Parsons & Cuthbertson (2001)	MH
How far do they disperse?	The most common method of spread is by vegetative means, which can be exacerbated by cultivation and poor hygiene. Seedlings are not often found.	Parsons & Cuthbertson (2001)	MH

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