Quick Facts

Quick facts

  • Originally from North America, Lacy Ragweed (Ambrosia tenuifolia) is an erect perennial herb with grey-green, lacy, leaves to 80 cm long, forming large colonies from creeping runner-like roots.
  • It occurs as a weed of roadsides, railway reserves, sand dunes, cultivated fields, degraded pastures and waste areas, in scattered colonies.
  • Most populations occur in New South Wales, but it is also recorded in southeast Queensland, South Australia, Victoria & Western Australia.
  • It is spread by seed attached to animals or in mud. Seed may also be spread by floodwater or arrive as a contaminant in fodder or topsoil from infested areas.
  • It causes problems in agriculture by competing with pasture plants, contaminating wool and interfering with the harvest in orchards.
  • Its pollen is highly allergenic to humans.
  • It can be controlled successfully with herbicides and with slashing and mowing.

What Does It Look Like?

What is it?

Lacy Ragweed (Ambrosia tenuifolia) is an erect perennial herb to 750 mm high, stoloniferous (with slender stems growing horizontal or trailing along the ground, producing roots and erect shoots at its nodes). Stems with bristly hairy, simple or sparsely branched stems, woody towards the base. The leaves are 30–80 mm long and 40–50 mm wide, arranged alternate or opposite on the stem, rhombic in outline, are grey-green and are bipinnatisect (twice-divided into narrow segments), segments mostly less than 1.5 mm wide, giving a lacy fern-like appearance, leaves . Leaf surfaces greyish from a covering of long white hairs, leaf stalk (petiole) to 20 mm long.

Flowers are greenish-white. Two types of flower-heads are borne on the same plant. Male flower-heads are many, comprising 12 tiny male florets, and are grouped into cup-shaped heads, about 2.5 mm in diameter, in spikes at the end of the stems, a ring of fused bracts enclosing the florets. Female flower-heads are fewer and are in 1-flowered heads, solitary or in small clusters, borne between the stems and bases of the upper leaves. Each female flower produces a seed with many seeds produced on a flower-head.

The fruits or 'seeds' (cypselas) are bluntly top-shaped, woody, 3–4 mm long, with a pointed beak, slightly ribbed, surrounded by 4 to 5 short projections (Parsons & Cuthbertson 2001), short spines above, . Flowers spring and summer.

For further information and assistance with identification of Lacy Ragweed contact the herbarium in your state or territory.

Flower colour

White, Green

Growth form (weed type/habit)


Where it currently grows? Preferred 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 (Parsons & Cuthbertson 2001).

Are there similar species?

Lacy Ragweed is not readily mistaken for other species.

Why Is It A Weed?

What are its impacts?

Lacy Ragweed (Ambrosia tenuifolia) is a very competitive plant, often to the detriment of crops and pasture and can result in reduction of the local flora and increased human allergic reactions. A weed of roadsides, waste areas, creek banks, pastures and uncultivated fields. As with the other Ambrosia species, it is noted for the amount of allergenic pollen it produces, causing severe hayfever and contact dermatitis in susceptible people (Parsons & Cuthbertson 2001).

Human health: The pollen contains highly potent allergens which cause respiratory allergies such as hay fever and can aggravate asthma (Parsons & Cuthbertson 2001; Schaffner et al. 2020). This weed along with other species in the genus is a major cause of respiratory allergies in the United States, its native country.

Agriculture: Lacy Ragweed is highly competitive and will invade crops and pastures. Negative impacts include crop losses, decrease in fodder availability. The aerial parts are known to be allelopathic, that is, the plant produces chemicals that inhibit germination and growth of other plant species. It is unpalatable to cattle and is not grazed by stock, so dense infestations can reduce pasture productivity considerably (Agriculture Victoria 2020; Parsons & Cuthbertson 2001). Its burrs contaminate wool and are not easily removed during scouring. This adds to the cost of manufacture, because acid carbonate must be used for removal. The plant is also a problem in orchards, competing for nutrients, interfering with the harvest, and both pollen and burrs cause discomfort to pickers.

Native ecosystems: Lacy Ragweed is mostly known from pastures and roadsides, but could potentially invade grasslands and grassy open vegetation.

Urban areas: Lacy Ragweed spreads along roadsides and could invade townships and urban areas, including waste areas and abandoned land or gardens, impacting human health. As with the other Ambrosia species causes severe hayfever and contact dermatitis in susceptible people (Parsons & Cuthbertson 2001; Hussey et al. 1997).


How does it spread?

As with other Ambrosia species, Lacy Ragweed can be spread long distances because of its seed, which attaches itself to sheep, fur, and clothing. It is also spread by water during flooding, and in mud on animals, people and vehicles and machinery. It is commonly spread in soil and gravel used in road building and gardening. Natural growth of existing colonies occurs because of the creeping roots, and transfer of root fragments severed during cultivation to other areas (Parsons & Cuthbertson 2001).

What is its history in Australia?

It is not known when or how Lacy Ragweed arrived in Australia, but it was first recorded as naturalised in New South Wales at Ashfield in the Sydney metropolitan area in 1932 and near Newcastle in 1937 (Parsons & Cuthbertson 2001).

How To Manage It?

Best practice management

Lacy Ragweed (Ambrosia tenuifolia) can be managed similar to other perennial ragweed. Lacy Ragweed survives and spreads by spreading root-stocks and also by seeds.  Herbicides, however, give good control in pastures, suppressing the Lacy Ragweed and allowing pasture species to compete effectively. Slashing and mowing can be used, but is not effectively controlled by cultivation. In fact, it usually makes the infestation worse by spreading pieces of the perennial root and stimulating development from root buds. . 

Chemical control: Spraying is usually performed at the budding stage of growth (Parsons & Cuthbertson 2001). Herbicides, allow control but such treatments may need to be repeated over several years to prevent re-colonisation from dormant seeds in the soil (Parsons & Cuthbertson 2001). In South Australia  Ragweeds are difficult to control requiring repeated application of herbicides such as triclopyr (Government of South Australia 2014). Foliar spray is used in New South Wales and South Australia, with information available at DPI NSW (2019) & Invasive Species Unit, Biosecurity SA (2018).

Please also see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au.

Non-chemical control: Physical control: Where feasible, plants can be pulled by hand. However, contact with flowering plants and pollen should be avoided by those prone to allergies.

Mechanical control: Lacy Ragweed survives mowing, trampling, grazing and cultivation due to the longevity and dormancy of its seed. Plants may be slashed or mown prior to setting seed (i.e. at the early flowering stage or immediately prior to flowering). Checks should be carried out to ensure flowering is prevented in any re-growth that occurs. Regrowth may occur from soil seed banks and these plants must also be controlled, normally with herbicides. 

Caution If plants have set seed , mechanical methods may make infestation worse, spreading seeds. Always use property hygiene practices when operating in areas infested with Lacy Ragweed.

Cultural: Lacy Ragweed is not effectively controlled by cultivation. In fact, it usually makes the infestation worse by spreading pieces of the perennial root and stimulating development from root buds. Young plant that have not developed a root system could be hand pulled.

Fire: With heavy infestations, opportunistic burning can be a useful tool in controlling Annual Ragweed if paddocks have not been overgrazed. Burning needs to be done when adequate soil moisture will allow good grass cover to grow back.

Biological control: The Ragweeds has also been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. Zygogramma bicolorata, a beetle, and Epiblema strenuana a stem gall moth, have both successfully established on Ragweeds in Queensland, to the north coast of New South Wales.

Does it have a biological control agent?


When does it grow? (lifecycle/growth calendar)

The seeds of Lacy Ragweed germinate in autumn and the rootstock and runners are established during winter. Growth increases in spring and flower stems are produced in early summer. Flowering begins in mid-summer and continues until early autumn. Aerial growth dies in autumn and new growth develops from the rootstock and creeping roots (Parsons & Cuthbertson 2001).

Where Is It Found?

Which states and territories is it found?


What areas within states and territories is it found?

Lacy Ragweed is most common in New South Wales found on the Central and North Coasts, the Central Tablelands and South Western Slopes.

In Queensland it has been found recently in the south-east but only a few records exist.

In South Australia it has been recorded around Adelaide and surrounds but no databased collections exists past 1974 (AVH 2021)

In Victoria , it has been found occasionally in the Melbourne and Adelaide areas (Parsons & Cuthbertson 2001).

In Western Australia, few records found south of Perth.

Where does it originate?

Lacy Ragweed is a native of North America, but does not appear to have spread widely (Parsons & Cuthbertson 2001).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?


Where is it a declared weed?


Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?


Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?


Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Ambrosia tenuifolia

Other scientific names (synonyms)?


Does it have other known common name(s)?

Burr Ragweed, Lacy Ambrosia, Narrow-Leaf Ragweed

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