Quick Facts

Quick facts

• Creeping Knapweed (Rhaponticum repens) is an introduced, upright, non-spiny perennial herb with an extensive root system, leafy much-branched stems, and pinkish to purple thistle-like flower heads.
• It is spread by seed but mostly by its extensive, creeping root system. New plants are generated from root sections that are broken off and can be transported by farm equipment, road-making machinery or animals.
• Creeping Knapweed is potentially poisonous to livestock and a troublesome weed of horticultural and cereal crops (mainly in semi-arid areas), pasture, wasteland, roadsides and disturbed sites.
• It has the potential to further invade cropping and grazing areas.
• Once established it is difficult to eradicate.

What Does It Look Like?

What is it?

Creeping Knapweed (Rhaponticum repens) is in the daisy family. It is an upright or ascending perennial herb to 800 mm to 1 m high. It has many woody underground creeping stems (rhizomes) in the top 300 mm of the soil extending for up to 2 metres, and capable of producing new plants. The below ground roots grow down to a depth of 5 to 7 metres. Plants produce two types of leaves, initially a basal rosette (radiating cluster), and then a stem is produced from and stem leaves are produced. Leaves can be variable in shape, and leaves can be hairy to smooth without hairs. The basal rosette leaves (cluster of leaves at base) are more or less lanceolate (lance-shaped,  about 4 times as long as broad, broadest in the lower half and tapering to the tip) to oblanceolate (lance shaped, about 4 times as long as broad, broadest in the upper half, tapering to a narrow base) in outline, irregularly lobed and toothed, without spines, up to 150 mm long and to 50 mm wide. Rosette leaves soon wither are shed from the plant as if they did not exist at all after the an upright stem elongates. Stems the centre of the rosette, elongates (grows upright) and are slender, ribbed and rather woody, branching freely in the upper part. The stem leaves alternate along the stem. The lower stem leaves are like the rosette leaves and soon wither. The upper leaves become smaller and less toothed towards the top. These upper stem leaves are oblong (length a few times greater than width, with sides almost parallel and ends rounded ) or elliptic (oval) to lanceolate (lance-shaped, about 4 times as long as broad, broadest in the lower half and tapering to the tip), are 10-50 mm long by 2-10 mm wide, margins entire (without teeth or lobes) or slightly toothed and tip pointed.

The flower-heads are borne individually at the end of branches in urn-shaped, thistle-like, flower-heads, 5-9 mm in diameter (at base). The flower-heads are composed of numerous pinkish or purple or rarely white, deeply 5-lobed, 10-15 mm long, tubular flowers (called florets), emerging, and much exceeding the small rows involucre bracts (leaf-like structures) that surround the florets. The flowers fan out up to about 25 mm across when exceeding the bracts that are several rows, green to white, spineless, hairy overlapping bracts. The outer bracts obovate (egg-shaped with widest end at the tip) to orbicular (more or less circular), with a broad entire appendage; inner bracts lanceolate with an acuminate (pointed tapering gradually to a point) plumose (feather-like) appendage.

The ripe fruits or 'seeds' (cypselas) are borne in tightly closed seed heads. Seeds are pale, longitudinally striate (fine parallel), 3-4 mm long, topped by a loosely attached bristly pappus (tuft) of numerous stiff, unequal, minutely barbed, white hairs up to 8 mm long (Kleinschmidt & Johnson 1977; Friend 1983; Wilding et al. 1986; Lamp & Collet 1989; Murray 1992; van Rangelrooy 1993; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001; VicFlora 2016).

For further information and assistance with identification of Creeping Knapweed, contact the herbarium in your state or territory.

Flower colour

Purple, Pink

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Creeping Knapweed (Rhaponticum repens) grows in semi-arid to sub-humid temperate regions, usually in clay or clay loam soils but not restricted to any particular soil type. It is an aggressive weed of cultivation, especially irrigated horticultural crops (such as vines and orchards) and dryland cereals (mainly in the 300-600 mm annual rainfall belt), as well as occurring in pastures, often in moist sites, along roadsides, channel banks in irrigated pastures, waste areas or other disturbed sites (Lamp & Collet 1989; Murray 1992; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001; Richardson et al. 2006; Victorian Department of Primary Industries 2007). They can tolerate all soil types and drought conditions for many years because of the longevity of their extensive root system (Agriculture Victoria 2021).

Are there similar species?

Creeping Knapweed (Rhaponticum repens)  is sometimes referred to as a thistle or being similar to a thistle but without prickles.

For example, the flower-heads of Creeping Knapweed are like small Spear Thistles (Cirsium vulgare) in shape and colour but without the prickles (Kleinschmidt & Johnson 1977).

Perennial Thistle or Californian Thistle (Cirsium arvense) has spiny leaves while thistle species of Carduus and Onopordum have spiny stems and leaves.

Creeping Knapweed is closer to several species of Centaurea, including Spotted Knapweed (Centaurea maculosa), which has flower head bracts with brown or black tips. 

Black Knapweed (Centaurea nigra), which has dark brown to blackish bracts fringed with long, fine hair-like teeth (Richardson et al. 2006).

The bracts in Creeping Knapweed are green to white with pale yellow membranous or papery margins and densely hairy tips (Parsons & Cuthbertson 2001).

Why Is It A Weed?

What are its impacts?

Creeping Knapweed (Rhaponticum repens) has an extensive, fast-growing root system enabling rapid spread, plant hardiness and the ability of dense patches of Creeping Knapweed to smother and exclude other species, thus making it a very competitive agricultural weed (Pritchard 1992; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001). Prolonged contact with Creeping Knapweed may cause skin sensitisation and dermatitis in humans (McIlroy & Faithfull 1998).

Agriculture: Creeping Knapweed spreads aggressively and can render agricultural land and rangeland useless in a relatively short space of time (van Rangelrooy 1993).  In Victoria, Creeping Knapweed is recognised as one of the most competitive of all weeds (Agriculture Victoria 2021). However, in South Australia it does not normally dominate pastures, spreading slowly within a paddock and taking up to 20 years to become dominant after its introduction (Government of South Australia 2021). Once dominant they can virtually exclude all other vegetation and reduce yield in dry land cropping by as much as 80 per cent. Initially a common weed of irrigated vines, fruit and other crops (e.g. along the Murray River from Swan Hill to Merbein), it has become a serious competitor of cereal crops and dryland pastures in north-western Victoria, e.g. in dryland cropping in the Swan Hill-Kerang area of the Victorian Mallee, where it can form dense patches to the virtual exclusion of other vegetation and reduce the yield of cereal crops by as much as 75% or more (Pritchard 1992; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001).

Due to its root system which can extend to 5 metres deep, once established it relies on subsoil moisture and can live and dominate in lower winter-rainfall climates. It can take advantage of the lack of competition in summer fallows, as its rate of growth is proportional to light intensity. Due to its deeper and more efficient root system, in dry years a heavy infestations can kill the cereal crop by water stress (Government of South Australia 2021) and heavily infested paddocks are sometimes no longer able to be cropped (Parsons & Cuthbertson 2001).

As a major competitive weed of cereal crops, it can cause heavy yield losses and resulting economic impacts (Pritchard 1992). In summer its active growth can completely suppress the growth of other crops such as Sudan Grass and Sorghum (Kleinschmidt & Johnson 1977). It forms dense patches shading out crops and pastures, as well as being a vigorous and efficient competitor for water and nutrients (McIlroy & Faithfull 1998; Lloyd 2006). However, it's growth rate declines under shading and it does not compete well under a heavy canopy, and is weakened by dense shade (Government of South Australia 2021).

It impacts on the productivity of pastures, and has little feed value. The leaves, although having a bitter disagreeable taste, are still grazed by sheep (Parsons & Cuthbertson 2001), but may be poisonous and kill sheep if eaten in large amounts (McIlroy & Faithfull 1998; Lloyd 2006). Overseas claims of Creeping Knapweed being poisonous to livestock and causing a neurological disorder in horses (which can result in death by starvation) have not been reported in Australia (van Rangelrooy 1993; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001).

It has allelopathic properties (releases chemicals) that restrict growth of other species such as oats, beans and tomatoes, which might contribute to it occurring in almost pure Creeping Knapweed stands (McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001). Seed is occasionally found in oats, linseed and canary seed (Friend 1983). The seeds are bitter and impart an unpleasant taste to flour made from contaminated grain (Parsons & Cuthbertson 2001).

Native ecosystems: Primarily an agricultural weed, grows on the edge of open or degraded, disturbed native vegetation.

Urban areas: Mainly a weed of agriculture and disturbance e.g. by cultivation, erosion or earth movement, it may grow by roadsides and sheds and yards, abandoned areas where it may have been spread by machinery and vehicles.

How does it spread?

Creeping Knapweed's (Rhaponticum repens) extensive perennial root system is its' main means of reproduction and propagation (Pritchard 1992; van Rangelrooy 1993). Plants spread by creeping roots, often forming large colonies (Thorp and Wilson 1998 -). Root fragments as short as 2.5 cm can produce shoots if planted at shallow depths (Pritchard 1992). New plants and infestations can start from pieces of root dragged to new areas by cultivation equipment or dropped by grazing animals after they pull plants from the ground (Pritchard 1992; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001). Small discrete patches increase in size and number, to the point where they often join up to form almost continuous cover (Pritchard 1992).

Creeping knapweed is an obligate outcrosser dependent on bees for pollination. It may also spread as seed, which normally falls within about a metre of the parent plant. The fruiting heads, containing several seeds, and remains closed and are dispersed as a unit by water, contaminated seed and machinery, and animals (both internally and externally). Seed dispersal is of minor importance compared to vegetative spread, as seedlings of creeping knapweed are rarely found; possibly seed viability is low, or the conditions for successful seedling establishment are a rarely fulfilled ( Government of South Australia 2021). Seeds are not well dispersed by wind because most are retained within the flower-heads and those that are released quickly lose their weakly attached pappus (hairy parachute), hence most seeds fall close to the parent plant. There may be greater dispersal by water movement (particularly along channels), in contaminated agricultural seed such as lucerne, on contaminated equipment (such as farm machinery), on sacking, or in mud or fodder (e.g. hay). The weed is also spread by grazing animals, because seeds pass through their digestive systems and remain viable, as well as adhering to their coats (McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001).

Initial introduction of this weed into Australia would have been as seed, and seed is probably the source of new infestations that are remote from existing plants. However, much of the spread of the weed is vegetative due to the vigour and regenerative powers of the root system, which also make it difficult to control (Pritchard 1992).

What is its history in Australia?

The first Australian record of the plant being naturalised was in 1907 from north-eastern Victoria, near Wangaratta.

It subsequently appeared in southern Queensland in 1916; New South Wales 1920; and South Australia in 1929 (AVH 2021; Pritchard 1992).

It was probably introduced into Australia as seed, possibly as a contaminant of an imported product (Parsons & Cuthbertson 2001; Lloyd 2006).

Creeping Knapweed was used in ancient medicine as a blood purifier and to treat fevers and skin disorders (Parsons & Cuthbertson 2001).

How To Manage It?

Best practice management

Creeping Knapweed (Rhaponticum repens) extensive perennial root system resists control efforts (Pritchard 1992). Once established this weed is difficult to control (especially in crops) and eradicate (Kleinschmidt & Johnson 1977; Lloyd 2006). Observing equipment hygiene protocols is extremely important to avoid spread of propagules to new areas or clean properties (McIlroy & Faithfull 1998). Successful weed control requires follow up after the initial efforts. This means looking for and killing regrowth or new seedlings. Using a combination of control methods is usually more successful.

Chemical control: Creeping Knapweed can often be effectively chemically controlled by using various herbicides (Kleinschmidt & Johnson 1977; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001; Laufenberg et al. 2005; Western Australian Department of Agriculture and Food, undated). Early detection and eradication of small patches using herbicides and follow-up treatments is recommended (McIlroy & Faithfull 1998).  Small incursions on a property can be destroyed by prompt spot spraying. Please see DPI NSW (2019); Invasive Species Unit, Biosecurity SA (2018); the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Non-chemical control: Physical control:  Young seedling can be chipped out removing roots, however, only used for new and small infestations.

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Creeping Knapweed (Rhaponticum repens) is a perennial with long-lived roots. Germination occurs during autumn and winter, with seedlings emerging in winter and spring to initially form rosettes, most of which do not produce flowering stems in the first summer. During this stage the plants develop an extensive and deep root system both laterally and vertically (the tap root may extend many metres down). In undisturbed plants, aerial growth usually dies off by autumn. New aerial shoots and growth from horizontal roots and stem bases develop in the following spring and into summer. These plants produce flowering stems by early summer, and flowering occurs during spring to autumn. Plants disturbed in late summer or autumn can continue to produce new shoots into winter. After seeding, the top growth dies off in late summer or autumn. Plants are dormant during winter. Cultivation stimulates new shoot growth from the roots, so active growth can be observed throughout the summer and autumn until the top growth is killed by cooler winter temperatures (Kleinschmidt & Johnson 1977; Wilding et al. 1986; Pritchard 1992; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001).

Where Is It Found?

Which states and territories is it found?

NSW, QLD, SA, VIC

What areas within states and territories is it found?

Creeping Knapweed (Rhaponticum repens) occurs from south-eastern Queensland (Darling Downs and Moreton Districts) south through the Western Slopes of New South Wales into northern Victoria, then west along the New South Wales-Victorian border area to far south-western New South Wales, north-western Victoria and south-eastern South Australia, particularly in the Upper Murray irrigation areas and on the Eyre Peninsula) (Kleinschmidt & Johnson 1977; Murray 1992; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001; Victorian Department of Primary Industries 2007).

It was found once in Western Australia, near Ravensthorpe, and could flourish in this part of the state if it were allowed to become established (Hussey et al. 1997; Lloyd 2006).

Where does it originate?

Creeping Knapweed's (Rhaponticum repens) native distribution extends from eastern Europe to Asia Minor and Mongolia (Pritchard 1992; van Rangelrooy 1993; McIlroy & Faithfull 1998; Parsons & Cuthbertson 2001).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

 SA, VIC, WA.

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Rhaponticum repens

Other scientific names (synonyms)?

• Acroptilon repens (L.) DC.
• Centaurea picris Pall.
• Centaurea picris Willd.
• Centaurea repens L.

Does it have other known common name(s)?

Blue Weed,  Blueweed, Hardhead, Hardhead Thistle, Russian Centaurea, Russian Knapweed, Russian Thistle