Creeping knapweed (Rhaponticum repens)

Also known as: hardhead thistle, Russian thistle, Russian knapweed

Creeping knapweed is an erect herb up to 1 m tall with purple flowers. It invades horticultural and cereal crops and can reduce yields by up to 80%.

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How does this weed affect you?

Creeping knapweed:

• secretes chemicals that limit the growth of nearby plants
• competes with cereal crops and can reduce yields by 80%
• invades vineyards and horticultural crops
• has bitter-tasting seeds that can taint flour (even in small quantities)
• invades irrigation drains and channels
• is toxic to horses.

Livestock poisoning

Creeping knapweed is toxic to horses. Eating the weed can cause chewing disease (Nigropallidal encephalomalacia). The disease causes irreversible brain damage and there are at least two known cases of death in Australia from horses eating creeping knapweed. Symptoms include: constant chewing, lethargy, drooping head, aimless walking, lack of muscle control, and partial paralysis in the tongue.

What does it look like?

Creeping knapweed is an upright, branched perennial herb that grows to about 1 m tall. The leaves die-off in autumn and plants are dormant through winter.

Leaves

There are two types of leaves: some in a clump (rosette) at the base of the plant and others along the stem.

Rosette leaves at the base of the plant are:

• oval shaped
• up to 15 cm long
• sometimes have lobed or toothed edges
• can die off during winter.

Leaves along the flower stem are:

• 1–5 cm long
• less than 1 cm wide
• sometimes have toothed edges
• silvery green when young, becoming greyish green with age.

Flowers are:

• thistle-like with 1–1.5 cm long purple petals, which are sometimes pink or white
• up to 2.5 cm wide
• single on the end of the stem
• surrounded by scales (bracts) below the petals which are:
  • green
  • papery thin
  • with pale yellow, slightly bristly tips. 

Seeds are:

• creamy white and sometimes mottled
• oval-shaped with a tuft of stiff barbed hairs up to 8 mm long 
• 3–4 mm long and 2–3 mm wide.

Stems are:

• branched
• covered in soft grey hairs when young
• less hairy and slightly grooved when older.

Roots are:

• scaly and dark brown to black when older
• either vertical or horizontal.

Horizontal roots:

• grow in the top 30 cm of soil
• extend several metres out from the plant
• contain many buds that develop into new plants. 

Vertical roots can grow up to 7 metres deep. 

Similar looking plants

Creeping knapweed looks like thistles. Unlike thistles, knapweeds don’t have sharp spines on the leaves.

There are also other knapweeds that look similar:

• Spotted knapweed (Centaurea stoebe subsp. micranthos), which usually has deeply lobed leaves and scales (bracts) with a dark-brown or black tip.
• Black knapweed (Centaurea x moncktonii), which usually has oval-shaped leaves and golden-brown scales (bracts).

Larkdaisy (Centratherum punctatum) also looks like knapweeds, but its leaves have serrated edges.

Where is it found?

In NSW, some isolated patches grow throughout the western slopes. Larger infestations occur in southern inland NSW where it is a weed of irrigated crops, pastures and roadsides of the Riverina district.  Well-established infestations are growing along the NSW-Victoria border, particularly between Shepparton and Mildura.

Creeping knapweed is native to central Asia from the Caspian Sea to Mongolia. 

What type of environment does it grow in?

Creeping knapweed grows best in:

• semi-arid to sub-humid temperate climates
• clay or clay-loam soils
• areas with an annual rainfall of 300–600 mm.  

Its extensive root system helps it tolerate all soil types and drought.

Maps and records

• Recorded presence of Creeping knapweed during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

How does it spread?

By seed

The seeds are heavy and not easily spread by wind. Most seeds fall within one metre of the parent plant. Seeds can move in hay, seed, grain and machinery. Water spreads the seeds along streams and irrigation channels. Grazing animals can eat the seeds which survive to pass through the gut. Seeds can stick to fur and fibres.

By plant parts

New plants grow from buds on the horizontal roots. Root fragments as small as 2.5 cm can become new plants. Cultivation equipment can spread root fragments. Livestock can pull plants from the ground and move the weed to new areas. 

References

Department of the Environment (2011). Weeds in Australia: Rhaponticum repens, Australian Government. Retrieved from: www.environment.gov.au

Elliott, C. R. B., & McCowan, C. I. (2012). Nigropallidal encephalomalacia in horses grazing Rhaponticum repens (creeping knapweed). Australian veterinary journal, 90(4), 151-154.

Hosking, J.R., Sainty, G.R., Jacobs, S.W.L. & Dellow, L.L. (in prep) The Australian WeedBOOK.

McKenzie, R. (2020). Australia's poisonous plants, fungi and cyanobacteria: A guide to species of medical and veterinary importance. CSIRO PUBLISHING.

Parsons, W.T. & Cuthbertson, E.C. (2001). Noxious weeds in Australia, CSIRO Publishing, Collingwood.

Victorian Department of Environment and Primary Industries (2014). Hardheads. Retrieved from: www.depi.vic.gov.au/agriculture-and-food/pests-diseases-and-weeds/weeds

More information

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Control

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.

Creeping knapweed’s extensive root system makes it difficult to control. 

To tackle creeping knapweed:

• prioritise treating of small, isolated patches in otherwise healthy sites
• for large infestations, treat the perimeter first
• control regrowth in spring before it flowers
• control new seedlings at the same time (it won’t flower in the first year, but it is easier to kill before the root system becomes well developed)
• keep checking for regrowth and new seedlings for many years (because roots are long-lived and seed can lie dormant for many years)
• avoid cultivation as this can increase the infestation and spread the weed to new areas
• maintain ground cover by promoting the re-establishment of desired plants.

Chemical control

Spraying

When: Check labels because some herbicides are best applied before flowering and some when flowering.

Follow up: Each spring for many years.

Apply herbicides when the plant is actively growing. Treatment while the plant is dormant (during colder months) may not be effective. Post-emergent herbicides should be sprayed an extra 3–4 metres around the infestation to control roots and seedlings

Apply as a spot spray or boom spray.

Herbicide options

See Using herbicides for more information.

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2,4-D 300 g/L + Picloram 75 g/L (Tordon® 75-D)
Rate: 1.3–2.0 L/100 L of water
Comments: Spot spray late spring to summer in non crop areas.
Withholding period: Do not graze or cut crops (except sugar cane 8 weeks) or pastures for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Amitrole 250 g/L + Ammonium thiocyanate 220 g/L (Various products)
Rate: 1.1 L per 100 L of water
Comments: Actively growing plants before flowering.
Withholding period: Nil
Herbicide group: 34 (previously group Q), Inhibition of lycopene cyclase
Resistance risk: Moderate

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Dicamba 750 g/L (Kamba® 750)
Rate: 87mL per 15 L of water. Add a surfactant.
Comments: Spot spray at flowering. For non-crop situations.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Dicamba 750 g/L (Kamba® 750)
Rate: 400 mL per 100 L of water. Add a surfactant.
Comments: Spray prior at flowering. For non crop situations.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Dicamba 750 g/L (Kamba® 750)
Rate: 5.9 L/ha Use a minimum of 1500 L of solution per ha. Add a surfactant.
Comments: Boom spray for non-crop situations. Spray at flowering.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
Murray	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfil the general biosecurity duty for this weed click here

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Reviewed 2023