Lacy ragweed (Ambrosia tenuifolia)

Lacy ragweed is an invasive plant that restricts the growth of desirable pasture species and causes respiratory allergies in people.

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How does this weed affect you?

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.  

What does it look like?

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
• greenish-yellow male flowerheads are cupshaped, 2.5 mm in diameter and hang down from spike-like clusters 3-7 cm long
• a single female flower is located at the base of the leaf

Seed

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

Where is it found?

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.

How does it 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.   

What type of environment does it grow in?

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.

More information

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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

See Using herbicides for more information.

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Dicamba 500 g/L (Various products)
Rate: 600 mL per 100 L of water
Comments: High volume spot spray.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Dicamba 500 g/L (Various products)
Rate: 8.8 L/ha
Comments: Use a minimum of 1500 L /ha of water. Add a surfactant.
Withholding period: Do not harvest, graze or cut for stock food for 7 days after application.
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 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 2014