Weeds in Australia


Ambrosia artemisiifolia


Annual Ragweed (Ambrosia artemisiifolia) is a shallow-rooted, erect herb, growing 1-2 m high (rarely higher) with fern-like, slightly rough, grey foliage. The leaves are deeply divided with hairy undersides and have a distinct leaf-stalk; upper leaves become almost entire (undivided) and stalkless. Two types of flowerheads are borne on each plant. Male flower heads are born in cone-shaped aggregations up to 20 cm long, with each head (made up of 10-100 tiny florets) small and greenish, appearing yellow when mature because of the abundant pollen. Female flower-heads (each with a single floret) are born singly or in clusters and hidden by the bases of the upper leaves. The 'seeds' (actually 1-seeded fruits or achenes) are small, black, top-shaped, rough and surrounded by a hard ring of bracts (modified leaves) the tips of which form 4-7 spines each about 1 mm long. Flowers late summer and autumn (Land Protection 2007; Weeds Australia undated).

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


Annual Ragweed is now naturalised in south-eastern Queensland and northern New South Wales, in the North Coast and the Central Coast areas. In Queensland serious infestations occur near Stanthorpe, Inglewood, Gympie, Gin Gin and Atherton. It is recorded from, but uncommon and localized, in Victoria. The related species perennial ragweed (Ambrosia psilostachya) is naturalized in South Australia and Western Australia, Queensland, Victoria and New South Wales (Land Protection 2007).

Key points:
  • Annual Ragweed (Ambrosia artemisiifolia) is a fast-growing plant native to North America.
  • It can become very abundant on disturbed sites such as along creek banks (particularly on sandbanks deposited by floods) and on poorly managed, overgrazed pastures and wasteland.
  • It is a significant human health hazard, as the pollen contains highly potent allergens that can aggravate asthma and cause respiratory allergies such as hay fever.
  • Heavy infestations can reduce pasture productivity.
How it spreads:

Annual Ragweed is spread by seed attached to animals or in mud. The spines on the seeds allow them to attach to sheep, furred animals, woolpacks, bags and clothing, aiding dispersal over long distances. Seed may also be spread by floodwater or arrive as a contaminant in fodder or topsoil from infested areas. Movement of contaminated gravel and soil used in roadmaking or garden topdressing is a common method of spread (Parsons & Cuthbertson 1992; Weeds Australia undated).

Where it grows:

Annual Ragweed is found in subhumid temperate to subtropical regions on a wide range of soils. It often colonises bare areas on roadsides and banks of watercourses (particularly on sandbanks deposited by floods), disturbed sites and waste areas. It may invade pasture from these areas. Horse paddocks are often infested in coastal areas (Parsons & Cuthbertson 1992; Navie 2004; Land Protection 2007).

Flower colour:Green
Distribution map:



Annual Ragweed can invade and develop into dense infestations in degraded and overgrazed pastures, reducing productivity. It is a fast-growing plant which can invade and suppress poorly managed pastures. Although cattle will eat Annual Ragweed to a small extent, other pasture species will be grazed in preference. Overgrazing will result in the loss of grass cover and a population explosion of Annual Ragweed and other weeds. It is unpalatable to horses and overgrazed horse paddocks often host large infestations (Parsons & Cuthbertson 1992; Land Protection 2007).

In the United States it is a serious weed of cereal, maize, vegetable, sunflower, soybean and tobacco crops and can significantly reduce yields (Parsons & Cuthbertson 1992).

It is also potentially a serious human health hazard. The pollen contains highly potent allergens which cause respiratory allergies such as hay fever and can aggravate asthma. This weed is a major cause of respiratory allergies in the United States, its native country (Bass et al. 2000).


Annual Ragweed is a native of North America, where it is most common in the eastern and north central parts of the United States and the central and eastern provinces of Canada (Parsons & Cuthbertson 1992).


It is not known exactly where or when Annual Ragweed was introduced to Australia. It was recorded as naturalised at Murwillumbah, New South Wales in 1908 and first recorded in Queensland at Currumbin in 1915. Ragweed hay fever was noted in 1959 and dermatitis documented in 1963 (Parsons & Cuthbertson 1992; Bass et al. 2000).


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