Californian burr (Xanthium orientale)

Californian burr is an annual plant that produces a woody burr. It is very similar to other Xanthium burr species.

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How does this weed affect you?

Californian burr is an invasive weed that:

• contaminates and devalues wool
• competes with pastures, reducing productivity
• can poison livestock if they eat too many seedlings
• competes with and reduces yields of summer crops such as cotton, corn and soybeans
• contaminates and downgrades grain
• has spiky burrs that can injure animals, or people handling them
• can create barriers for livestock and people.

Human health

Californian burr can cause skin irritation and the pollen can cause hay fever.

Livestock poisoning

California burr plants contain carboxyatractyloside (a diterpenoid glycoside), but only in their first two leaves and in the burrs, This toxin causes acute liver damage in cattle, sheep, goats, pigs and birds.

What does it look like?

Californian burr is a robust annual herb that grows up to 2 m tall.

Leaves are:

• green
• rough
• 5–14 cm long and 5–15 cm wide
• oval to roughly triangular shaped with 3 shallow lobes
• similar to grapevine leaves.

Flowerheads are:

• difficult to see
• in clusters of both male and female flowers
• in the leaf axils. 

Burrs are:

• hard and woody with two beaks on the end, 4-6 mm long that curve inwards at the tips
• green when young and brown when mature
• 15–20 mm long
• roughly egg-shaped
• densely covered with hooked spines 2–4  mm long.

Seeds are:

• brown or black
• 4–15 mm long and 5–7 mm wide
• flattened on one side with one seed slightly larger than the other.

Stems are:

• green to reddish-brown
• rough
• branched and spreading on isolated plants but unbranched in dense thickets.

Similar looking plants

Californian burr looks like other weedy Xanthium species in NSW. The best way to tell them apart is by the burrs.

Only Italian cockleburr (Xanthium italicum) also has inward curving beaks at the end of the burr. Italian cockleburr only grows to 1 m and has smaller leaves (up to 9 cm long).

Where is it found?

Californian burr mostly grows western NSW particularly the south-western areas.

It is native to North America.

What type of environment does it grow in?

Californian burr grows in pastures and is more common in wet areas, including irrigation areas.

How does it spread?

By seed

Burrs containing the seeds are usually dispersed from the plant in autumn and winter but they may remain attached on undisturbed plants until the following spring. 

 The burrs are spread:

• by sticking to wool and fur of animals
• by floating in waterways
• in contaminated grain
• in contaminated gravel
• on machinery.

Seeds germinate when the soil is moist in late winter to summer. Flushes of germination can occur after summer storms or irrigation events. On flood prone areas large germination events are common after floods.

References

Cotton CRC (2002) WEEDpak a guide for integrated management of weeds in cotton. Eds Stephen Johnson, Graham Charles, Ian Taylor and Grant Roberts. Cotton CRC, Narrabri.

Hocking, P. J., & Liddle, M. J. (1995). Xanthium occidentale Bertol. Complex and X. spinosum L. In Groves R.H., Shepherd R.C.H. and Richardson R.G. (eds), The biology of Australian weeds. RG and FJ Richardson Publishers. Melbourne, pp. 241-302.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 29 January 2021 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Xanthium~orientale

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious Weeds of Australia, 2nd ed. CSIRO publishing.

More information

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Control

Prevention of seed set of Californian burr is the most important part of managing burr plants and eliminating infestations. Trials have shown that six years prevention of seed set leads to a decline in populations to 1% of previous populations. Repeated control is often needed as many germination events can occur from late winter to summer. Early control can prevent competition with crops and pastures, and later control can prevent seed set of plants that may have been missed by earlier control efforts. Large populations can be treated with herbicides, cultivation or slashing; follow-up control of smaller populations may include spot spraying, chipping (or hand-hoeing) or inter-row cultivation in crops.

Cultivation

Cultivation is an effective method of controlling the seedlings of these weeds. Successive flushes of seedlings during the summer may require follow up control. Inter-row cultivation is commonly used in row cropping such as sorghum, corn and cotton to control seedlings that have germinated after irrigation events.

Slashing

Slashing or mowing are useful in clean-up operations after spraying with herbicide or if infestations are small and scattered. Any burrs from the plant should be removed from the equipment to prevent spread.

Chipping

Chipping or hand hoeing is only economical for small areas, individual plants or isolated populations. It is an effective follow up control method for plants not controlled by other methods to prevent seed set.

Surveillance

After controlling burr plants it is important to monitor these sites for further germination events. High risk areas for new infestations include flood prone areas or areas where stock from burr infested areas have been.

Pasture management

Maintaining ground cover in pastures is vital. Pasture gaps result in an increase in burr germination and seedling survival. It is important to ensure that pastures are not overgrazed in spring and summer to reduce the potential for the establishment of burrs and other weeds.

Grazing

Adult plants are not easily eaten by livestock, due to the roughness of the leaves and stems. Care needs to be taken when grazing to ensure there are no seedling plants, which are toxic to animals and could result in death.

Herbicides

These plants are susceptible to a range of foliar and residual herbicides. 

Foliar herbicides are most effective if the plants are young and actively growing. Plants suffering from moisture stress are difficult to kill. Older plants may require repeat applications. Late control with some herbicides, when the burrs are green, can result in seed sterility – however this is not recommended as the primary form of control.

In crops, residual pre-emergent herbicides are useful, although there are sometimes problems as the burr can germinate and emerge from relatively deep in the soil, below the residual herbicide band. Follow-up control in the form of chipping or inter-row cultivation may be required. Spot spraying is an important tool for areas that are difficult to access and for scattered plants.

Herbicide options

See Using herbicides for more information.

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2,4-D amine 625 g/L (Various products)
Rate: 0.8 – 1.1 L/ha
Comments: Boom spray for seedlings only.
Withholding period: 7 days withholding for grazing
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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2,4-D LV ester 680g/L (Estercide® Xtra)
Rate: 800 mL/ha
Comments: Boom spray application, from seedlings to pre-flowering.
Withholding period: Do not 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 NSW 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.

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Reviewed 2024