Creeping knapweed (Rhaponticum repens)

Also known as: hardhead thistle, Russian thistle

Creeping knapweed is an erect, deep-rooted and long-lived perennial herb growing to around 1 m high. It is an invasive weed of horticultural and cereal crops, where it forms dense patches and excludes all other vegetation.

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How does this weed affect you?

Creeping knapweed is an erect, deep-rooted and long-lived perennial herb growing to around 1 m high. It invades irrigation drains, channels and along roadsides and is a highly competitive weeds of cereal crops, reported to reduce yields by up to 80%. Having allelopathic properties, it prevents other plants from successfully growing in the same area. This allows creeping knapweed to exclude all other vegetation from the area and form a monoculture. The seeds of creeping knapweed are extremely bitter and small quantities can taint flour milled from contaminated grain.

Where is it found?

Native to central Asia from the Caspian Sea to Mongolia. It has spread to most temperate parts of the world and considered an important weed of crops, orchards and vineyards in the USA, Canada, Argentina and South Africa. It has also become weedy in parts of its native range, including Iran, Turkey and southern Russia.

Most infestations occur throughout the semi-arid inland parts of southern Australia. Isolated infestations occur throughout south eastern South Australia, with larger infestations in the Murray River and Mallee districts.

In Victoria, creeping knapweed is scattered throughout the north-western and northern areas of the state. It is a weed of concern in the Mallee dryland cropping areas and is a prominent weed along the Murray River, particularly of irrigated vineyards.

Creeping knapweed is prominent in the Darling Downs region of southeast Queensland where it is a problematic weed of winter and summer crops in both dryland and irrigated situations.

In NSW, isolated patches are present in various locations 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. 

How does it spread?

Creeping knapweed reproduces by root fragments and by seed. Buds are present along the lateral roots of creeping knapweed, from which new plants arise. Lateral roots can advance about 1 meter per year. Broken root fragments as small as 2.5 cm in length are capable of growing and developing new plants. Roots fragments can be spread to new areas by cultivation equipment.

The seeds are heavy and not well adapted to wind dispersal, with most seed remaining in the seed head or falling within 1 metre of the parent plant. The main method of seed spread is through contaminated hay, seed, grain and machinery. Seed can also be spread by water movement, particularly along streams and irrigation channels.

Animals can also assist in seed and root dispersal. Viable seeds are able to move through the digestive tract of grazing animals. Livestock can also pull plants from the ground with roots intact, moving the weed from one location to another.

Lifecycle

Seedlings emerge in winter and spring and form a rosette. They do not flower in the first year— using their resources to develop a deep and extensive root system. Plants are dormant throughout winter, with foliage dying off by autumn. New growth develops from the roots the following spring. The roots are very long lived. Flowering stems are produced from late spring and into summer. Seeds can remain viable in the soil for a number of years in dry conditions.

What does it look like?

Thistle-like in appearance with no spines.

Stems

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

Leaves

  • Variable—consisting of rosette and stem leaves.

Rosette leaves

  • silvery green when young, becoming greyish green
  • lobed
  • lance-shaped
  • up to 15 cm long and 2–5 cm wide
  • with toothed edges

Stem leaves

  • alternately arranged
  • 1–5 cm long and 0.2–1 cm wide
  • sparsely covered in fine hairs
  • upper leaves smaller
  • edges either smooth or slightly toothed

Flowers

  • a single urn-shaped flowerhead occurs at the end of each stem
  • up to 2.5 cm in  diameter
  • flowerhead consist of many pinkish or purple tubular 1–1.5 cm long florets
  • many bracts (leaf-like structures) surround the flowerhead
  • bracts are green with papery thin, pale yellow hairy tips

Seeds

  • creamy white and sometimes mottled
  • oval in shape
  • 3–4 mm long and 2–3 mm wide
  • has a tuft of stiff barbed hairs up to 8 mm long

Roots

  • consists of vertical and horizontal roots
  • horizontal roots are located in the top 30 cm of soil, extend several metres across and contain many buds that eventually develop into new plants
  • vertical roots can reach to depths of 5–7 m
  • older roots are scaly and dark brown to black in colour

Habitat

Creeping knapweed prefers semi-arid to subhumid temperate climates with an annual rainfall of 300–600 mm.  It tolerates all soil types and periods of drought due to its extensive root system.

Acknowledgements

Written by Rachele Osmond.

Technical review by Philip Blackmore.

References

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

Ensbey, R (2011) Noxious and environmental weed control handbook. NSW Department of Primary Industries, Orange. Available at http://www.dpi.nsw.gov.au/agriculture/pests-weeds/weeds/publications/noxious-enviro-weed-control

Hosking JR, Sainty GR, Jacobs SWL & Dellow LL (in prep) The Australian WeedBOOK.

Murray L (2014) PlantNET – New South Wales Flora Online: Rhaponticum repens (L.) Hidalgo. Available at http://plantnet.rbgsyd.nsw.gov.au

Parsons WT and Cuthbertson EC (2001) Noxious weeds in Australia, CSIRO Publishing, Collingwood.

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

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Control

Once established, creeping knapweed is difficult to control due to its extensive root system. Avoid using cultivation as broken root pieces can increase infestations or spread the weed to new locations.

Herbicides

Herbicides should be applied when the plant is actively growing and before flowering. This is usually from late spring (October) to summer (February).

The herbicide should be applied as a spot spray or boom spray depending on the chosen herbicide.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.


2,4-D 300 g/L + Picloram 75 g/L (Tordon® 75-D)
Rate: 1.3–2.0 L per 100 L of water
Comments: Spot spray.
Withholding period: 1-8 weeks (see label).
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


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: Q, Bleachers: Inhibitors of carotenoid biosynthesis unknown target
Resistance risk: Moderate


Dicamba 500 g/L (Kamba® 500)
Rate: 130 mL per 15 L of water
Comments: Knapsack spray.
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


Dicamba 500 g/L (Kamba® 500)
Rate: 600 mL per 100 L of water
Comments: High volume spot spray.
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


Dicamba 500 g/L (Kamba® 500)
Rate: 8.8 L/ha
Comments: Boom spray. Spray at flowering, using a minimum water rate of 1500 L/ha.
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
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 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*
Land managers should mitigate the risk of new weeds being introduced to their land. Land managers should mitigate spread from their land. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfill the general biosecurity duty for this weed click here

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Reviewed 2017