Quick Facts

Quick facts

• Originally from south-eastern Europe and north-western Asia, Spotted Knapweed (Centaurea stoebe subsp. australis) is a perennial herb which has become a serious pastoral weed in North America.
• It is currently restricted to one known location on a roadside near the Murrumbidgee River outside Canberra in the Australian Capital Territory, where it has been controlled.
• Its seeds are dispersed mainly by wind but also may spread via agricultural products, farm machinery and along watercourses.
• The species can be spread by transport of root fragments.
• An integrated approach should be used to control Spotted Knapweed, as methods used in isolation or in the short term will allow the plant to regenerate.

What Does It Look Like?

What is it?

Spotted Knapweed (Centaurea stoebe subsp. australis) is an upright perennial herb that usually grows to about 1 m tall (but sometimes up to 1.8 m) with a long stout tap-root. In the first year of growth the plant develops a basal rosette (circular cluster) leaves with petiole (leaf stalk) to 150 mm long laying more or less flat on the ground. Rosetted leaves are deeply-divided to lobbed, lobes are linear to lance-shaped on each side of the midrib. In the second or sometimes third year of growth the plant produces one or more slender much-branched vertical shoots (this process is known as 'bolting'). The stem leaves are arranged alternate along the stem, without a leaf stalk, are smaller in size (than rosette leaves) up 75 mm long and become progressively smaller further towards the top of the plant to 10 mm long. The lower leaves are deeply divided to lobed or oval, uppermost leaves being linear (long and narrow), entire (without teeth or lobes or undivided). All leaves are grey-green, somewhat hairy and covered with small translucent dots (Navie 2004)

The flower heads are egg-shaped, 15–25 mm long and 5–15 mm across, consisting of 30–40 pinkish to purple or rarely white florets (small flowers). The heads are surrounded by several rows of greenish to brown bracts (modified leaves) that have black or dark purple tips with a comb-like fringe of blackish hairs. From a distance the dark-coloured bract tips give the heads a distinctive spotted appearance. Each flower produces a seed with many seeds produced on a flower-head.

The fruits or 'seeds' (cypselas) are pale brown to blackish oval or oblong seeds are topped with a ring of whitish feathery bristles (pappus) that are 1-2 mm long. The flowering shoots die back to the rosette after seed dispersal (Navie 2004).

The correct scientific name for Spotted Knapweed has been the subject of confusion. It appears that the widely-used name Centaurea maculosa is a synonym of an earlier name, C. stoebe, which should take precedence (Ochsmann 2001). The name C. stoebe subsp. stoebe applies only to plants from western, central and eastern Europe which grow for two seasons, flower once and then die. These plants do not appear to become weedy. The weedy taxon in Europe, North America and Australia which is described here grows and flowers over several years and has double the number of chromosomes as C. stoebe subsp. stoebe. These plants are given the name C. stoebe subsp. australis. Some jurisdictions in the United States of America and Canada have listed the plant as a noxious weed under the name C. biebersteinii, but this probably refers to a third taxon (Ochsmann 2001). The confusion is increased because these different taxa (groups) cannot be readily distinguished on appearance alone.

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

Flower colour

Purple, Pink, White

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Spotted Knapweed occupies a wide variety of habitats in its native and naturalised ranges in Europe and in the infestations in North America (Zouhar 2001). It is a potential weed of disturbed sites, pastures, rangelands and crops in temperate and sub-tropical regions (Navie 2004).

Are there similar species?

Spotted Knapweed (Centaurea stoebe subsp. micranthos) is very similar to Black Knapweed (Centaurea nigra) and Greater Knapweed (Centaurea scabiosa). It is also relatively similar to Creeping Knapweed (Rhaponticum repens) [as Acroptilon repens] and Star Thistle (Centaurea calcitrapa).

Black Knapweed  (Centaurea nigra) is also a perennial herb, but it differs from Spotted Knapweed in usually having entire (undivided) leaves in the basal rosette. The stem immediately below the floral head is thickened in Black Knapweed but not in Spotted Knapweed (Dellow & Johnson undated). The floral bracts in Black Knapweed are mostly black or dark brown with numerous blackish hairs, whereas in Spotted Knapweed they are green to brownish with black or dark purple only near the tips and with only 4–7 pairs of hairs on the margin. The seeds of Black Knapweed have a few short bristles in a pappus (tuft) at the top which are about 0.5 mm long, while Spotted Knapweed has whitish feathery bristles reaching 1–2 mm long.

Greater Knapweed  (Centaurea scabiosa) is also a perennial herb with lobed or deeply-divided leaves in the basal rosette, but its flower heads are generally larger than those of Spotted Knapweed (15–20 mm across compared to 5-15 mm across, both measurements not including florets). The seeds of Greater Knapweed are pale yellow or brown with a pappus of dark purplish bristles 4-5 mm long, while Spotted Knapweed seeds are pale brown to blackish with a shorter pappus of whitish bristles.

Creeping Knapweed (Rhaponticum repens) is also a perennial herb with toothed or lobed lower leaves. However, it differs from Spotted Knapweed by having greenish to pale yellowish floral bracts with paler, papery tips which lack hairs in contrast to the dark-tipped and fringed bracts in Spotted Knapweed. The seeds of Creeping Knapweed are topped with whitish bristles (pappus) as are those of Spotted Knapweed, but the pappus in Creeping Knapweed seeds detaches easily while it persists in Spotted Knapweed. In addition, Creeping Knapweed seeds are whitish or sometimes mottled with darker streaks or blotches while Spotted Knapweed has brown to blackish seeds.

Star Thistle (Centaurea calcitrapa) is a biennial plant which has lobed or deeply-divided basal leaves and has relatively small flowering heads like Spotted Knapweed. However, in Star Thistle the bracts surrounding the heads are green to brown with a large yellowish or whitish spine at the tip and several smaller spines near the base, whereas in Spotted Knapweed the bracts lack spines. Star Thistle has whitish seeds with brown streaks or blotches and the seeds are not topped with hairs or scales, in contrast to the brown or blackish seeds with whitish feathery bristles found in Spotted Knapweed (Navie 2004).

Why Is It A Weed?

What are its impacts?

Spotted Knapweed (Centaurea stoebe subsp. australis) exudes a toxin from its roots that inhibits the growth and germination of surrounding vegetation (Bais et al. 2002; Bais et al. 2003). This chemical suppression of competitors is known as allelopathy. Although Spotted Knapweed tends to initially invade disturbed sites, there is also evidence that it may then colonise adjacent undisturbed areas (Rutledge & McLendon 1996). The weed tends to form monocultures and the biodiversity of native plant communities can be impacted in areas of heavy infestation (Tyser & Key 1988) which may then have an impact on the food sources available to native fauna (Thompson 1996).

Agriculture: Invasion by Spotted Knapweed can reduce the productivity of agricultural forage plants and thus decrease the amount of feed available for livestock. Despite earlier fears that Spotted Knapweed may be toxic to stock (Leininger 1988), there is evidence that sheep and cattle can safely consume it, although they exhibit a preference for plants in the earlier (rosette and early bolting) stages of their life cycle (Robbins 1990; Launchbaugh 2001). Sheep and goats have been proposed as a potential means of Spotted Knapweed control in some parts of the United States of America (Olson et al. 1997; Williams & Prather 2006), although some seeds remain viable after passing through the animals' digestive tracts and thus could germinate in a new area (Wallander et al. 1995).

Native ecosystems: Spotted Knapweed can also have a negative impact on environmental factors like water infiltration, with higher levels of surface run-off and sediment yield in areas infested by Spotted Knapweed when compared to native bunchgrass vegetation (Lacey et al. 1989). Spotted Knapweed may also change the fire regime of areas it invades, as it doesn't usually burn as well as grasses, which may change the level of fuel availability at a given site (Zouhar 2001).

Urban areas: The exudates of spotted Knapweed can cause skin irritation in some people and it is recommended that workers removing the weed wear gloves and take other suitable precautions to avoid contact with the sap (Colorado Weed Management Association undated).

How does it spread?

Spotted Knapweed reproduces primarily by seed but s also spread by root fragments. Seed production is generally prolific but varies greatly by plant, year and population, with plants from an irrigated site exhibiting a many-fold increase in the number of seeds produced over plants in a rangeland site. Stands in disturbed sites can maintain their density even if only a fraction of one percent of the seeds produced survives (Schirman 1981).

Seed is mainly dispersed by wind, but may also be spread by agricultural and fire-fighting machinery, vehicles, birds and other animals, in water, and as contaminants of agricultural produce (such as hay). The plant can also grow from secondary rosettes produced along lateral roots and via broken root fragments caused by cultivation and earth-moving.(Zouhar 2001).

What is its history in Australia?

The precise means of introduction of Spotted Knapweed into Australia is not known, although it may have been as a garden ornamental. It was first collected near the Murrumbidgee River in the Australian Capital Territory in 1986 and was then re-collected in nearby localities in 1993 and 1999 (Baker 2003; Australian National Herbarium 2008).

How To Manage It?

Best practice management

Spotted Knapweed (Centaurea stoebe subsp. australis) can be controlled and managed with herbicides with young plants being physical removed ensuring perennial root is removed to avoid regeneration.  Cultivation is generally not recommended as these spreads root fragments to form new plants. The seed of Spotted Knapweed are able to survive in the soil for well over five years (Davis et al. 1993), so populations must be revisited to remove new seedlings and resprouts (Bio-Integral Resources Center 2000). Given the persistent nature of the plant it is likely that a strategic integrated approach will be necessary to effectively prevent the spread of Spotted Knapweed (Brown et al. 1999). 

Chemical control:  Herbicide is effective in controlling small populations if applied when plants are actively growing, particularly in the bolt and bud stages (Sheley et al. 2000), Please also check the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au.

Non-chemical control: Physical control: Single plants or small populations can be removed by hand before setting seed, as long as the entire tap root is pulled out.

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

A generalised growth calendar for Spotted Knapweed in Australia is difficult to determine, as all the collections have been made from the one small population known. These collections were all made between January and March (summer to early autumn) when the plants were in bud or flowering (AVH 2021). In the North American populations, the plants generally germinate in autumn or early spring and then overwinter in rosette form. In the second or subsequent years the stems elongate, flower and produce seed over the summer months before dying back to the rosette in autumn (Watson & Renney 1974).

Where Is It Found?

Which states and territories is it found?

ACT

What areas within states and territories is it found?

In Australia Spotted Knapweed is currently known only from one area in the Australian Capital Territory on a roadside near the Murrumbidgee River (Baker 2003; Dellow & Johnson undated).

Where does it originate?

Spotted Knapweed is native to southern central to south-eastern Europe and north-western Asia (Ochsmann 2001), although it has also subsequently been introduced to western and northern Europe, Asia and New Zealand. It was also accidentally introduced to North America in the late 1800s from Europe, probably as a contaminant of alfalfa or clover seed or in soil discarded after use as ships' ballast, and has since become a widespread weed occupying millions of hectares in the United States of America and Canada (Carpinelli 2005).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

NSW

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

Centaurea stoebe subsp. micranthos

Other scientific names (synonyms)?

Centaurea maculosa auct. non Lam. (name incorrectly applied to this species)

Centaurea stoebe subsp. micranthos

Does it have other known common name(s)?

Spotted Starthistle, Star Thistle