Lacy ragweed (Ambrosia tenuifolia)

Profile

Impact

Lacy ragweed is a highly competitive perennial plant that is difficult to control. Lacy ragweed grows in thick patches and excludes desirable plant species in native environments, agricultural situations and orchards. Its burrs can contaminate wool and are not easily removed during scouring, in orchards the burrs and pollen cause discomfort to pickers. Like other ragweed species, lacy ragweed produces large amounts of air-borne pollen, causing severe hay fever in susceptible people.  

Distribution

Lacy ragweed is native to the South American countries of Brazil, Bolivia, Paraguay, Uruguay and Argentina. 

It is not known how lacy ragweed first entered Australia. It was first recorded in metropolitan Sydney during the 1930s and is now located as scattered infestations on the central and northern coastal districts, Tablelands and western slopes of New South Wales.

Isolated occurrences have also been recorded in Melbourne and Adelaide. Lacy ragweed is not widespread, but has the potential to spread well beyond its current distribution.

Spread

Lacy ragweed reproduces by seed and by shoots that develop along its creeping underground roots (runners). Infestations can quickly increase in size and density from new shoots that emerge from the extensive root network. Cultivation equipment contaminated with root fragments can cause spread.

The seed of lacy ragweed can be spread large distances by attaching to the fur and wool of grazing stock and native animals. Seed may also be spread by flood waters, roadside slashers, attachment to clothing and contaminated mud stuck to vehicles and earth moving equipment. 

Lifecycle

Seeds germinate in autumn. Seedlings develop a mass of creeping roots throughout winter and early spring. Flowering stems first appear in late spring, followed by flowering in mid-summer and continuing into autumn if favourable conditions persist. Plants die back in autumn, with new growth emerging from the root network.   

Description

Lacy Ragweed is an erect perennial herb growing 50–75 cm high.

Stems

  • covered in bristly hairs
  • woody at the base
  • usually unbranched

Leaves

  • greyish-green in colour
  • covered in short soft hairs
  • divided into narrow segments
  • lacy fern-like appearance
  • 6–8  cm long and 4–5 cm wide
  • lower leaves opposite
  • upper leaves alternate

Flowers

  • male and female flowers
  • twelve greenish-yellow male flowers occur on a flower head 3–7 cm long
  • a single female flower is located at the base of the leaf

Seed

  • 3-4 cm long
  • woody
  • shaped like a blunt cone
  • has a pointed beak surrounded by 4–5 short projections

Habitat

Lacy Ragweed grows in subhumid temperate regions, in lighter soils in open areas, occurring as a weed of roadsides, railway reserves, sand dunes, cultivated fields, degraded pastures and waste areas.

References

Department of the Environment (2011) Weeds in Australia: Ambrosia tenuifolia. Australian Government. www.environment.gov.au/biodiversity/invasive/weeds/identification/index.html. Accessed August 2014. 

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

Parsons, WT and Cuthbertson, EG (2001) Noxious weeds of Australia, CSIRO Publishing, Collingwood.

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Control

Cultivation and mechanical control are not viable control options and could potentially make an infestation worse. Always use property hygiene practices when operating in areas infested with lacy ragweed.

High volume foliar (boom and spot spray) application of registered herbicides can be applied at the budding stage of growth and when plants are actively growing. Repeat applications will be required. Regularly monitor control efforts and follow-up as necessary.

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.


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: Use a minimum of 1500 L /ha of water. Add a surfactant.
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


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Legal requirements

The content provided here is for information purposes only and is taken from the Noxious Weeds (Weed Control) Order 2014 published in the NSW Government Gazette, detailing weeds declared noxious in New South Wales, Australia, under the Noxious Weeds Act 1993. The Order lists the weed names, the control class and the control requirements for each species declared in a Local Control Authority area.

Area Class Legal requirements
Mid-Western Regional 4 Locally Controlled Weed
The growth of the plant must be managed in a manner that continuously inhibits the ability of the plant to spread

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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW Invasive Plants and Animals Enquiry Line on 1800 680 244 or send an email to weeds@dpi.nsw.gov.au

Reviewed 2014