APC (2020). The Australian Plant Census, Council of Heads of Australasian Herbaria, Available at: https://biodiversity.org.au/nsl/services/apc . [accessed 27/11/2020].
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Annual Ragweed (Ambrosia artemisiifolia) is a shallow-rooted, erect herb, growing 1-2 m high (rarely higher) with fern-like, slightly rough, grey foliage. Seedlings and young plants form a basal rosette of leaves during the early stages of growth, subsequently elongates and produces a rounded erect stem, bearing leaves. Leaves are aromatic or not, oppositely arranged (borne at the same point on opposite sides of the stem) at the base of the plant, but are alternately arranged (borne singly at different points along a stem) further up the stems. Lower leaves always have petiole (leaf stalk) to 20 mm long, with upper leaves become almost entire (undivided) and with a much shorter a leaf stalk. Leaves are lanceolate (lance-shaped) to ovate (egg-shaped with widest part near the base of leaf) in outline, mostly 10-40 mm long and 10-50 mm wide, are deeply lobed, often with toothed lanceolate segments 2–4 mm wide, with upper leaves become almost entire (undivided). Leaf surfaces are finely hairy (pubescent), or smooth without hairs (glabrous), with hairy undersides.
Two types of flower-heads are borne on the same plant. Male flower-heads are born in cone-shaped aggregations in spikes on the stem from 30-100 mm long reported up to 200 mm long. Each flower head is 2–3 mm diameter, composed of 10-100 tiny florets (individual flowers), that are greenish, appearing yellow when mature because of the abundant pollen. Florets slightly exceeding involucral bracts (a group of bracts (modified leaves) surrounding the base of a flower-head). Female flower-heads (each with a single floret) are born singly or in clusters and hidden by the bases of the upper leaves. The fruiting flowering head is about 3 mm long,
The fruits or 'seeds' (cypselas) 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.
For further information and assistance with identification of Annual Ragweed contact the herbarium in your state or territory.
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).
Annual Ragweed is most likely to be confused with other Ambrosia species, most obviously Perennial Ragweed (Ambrosia psilostachya), from which it can be distinguished by its annual habit, distinctly stalked leaves and more prominently spiny fruiting clusters.
Parthenium Weed (Parthenium hysterophorus) may also be confused with Annual Ragweed. However, it has ribbed rather than rounded stems and masses of small, white, bisexual flower-heads borne at the tips of the branches (Navie 2004).
Annual Ragweed (Ambrosia artemisiifolia) is a weed of agriculture, and disturbed sites, railway reserves, wasteland, vacant lots, waterways and floodplains (Parsons & Cuthbertson 2001). Annual Ragweed is also a serious human health hazard due to the effects of allergenic pollen and can cause hayfever and asthma in humans ( Business Queensland 2016; Parsons & Cuthbertson 2001).
Human health: The pollen contains highly potent allergens which cause respiratory allergies such as hay fever and can aggravate asthma (CABI 2020; Palmer & McFadyen 2012; Parsons & Cuthbertson 2001; Schaffner et al. 2020). This weed is a major cause of respiratory allergies in the United States, its native country (Bass et al. 2000). Annual Ragweed is also considered invasive in more than 30 countries in Europe and prior to 2013, 13.5 million people suffered from Ambrosia-induced allergies, causing costs of Euro 7.4 billion annually (Schaffner et al. 2020). In 1997, 30-40% of people sampled in the coastal city of Casino NSW, most of whom suffered allergenic symptoms during the Ragweed season, showed a positive skin test reaction to Ragweed pollen (Bass et al. 2000 In: Palmer and McFadyen 2012).
Agriculture: 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).
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).
Native ecosystems: Known as a weeds of waterways and floodplains, (Palmer & McFadyen 2012).
Urban areas: A weed of roadsides, waste places, vacant lots, and gardens.
Annual Ragweed is spread by seed and a therophyte (an annual plant characterised by long-term survival in seed bank) (Buttenschøn, et al. 2009). Some seeds may remain dormant but viable for more than 40 years, (Parsons & Cuthbertson 2001).
The seeds can attach to animals, or be transported in mud attached to animals or vehicles and machinery. The spines on the seeds allow them to attach to sheep, furred animals, wool-packs, bags and clothing, aiding dispersal over long distances. Seed are also spread by flood-water, or arrive as a contaminant in fodder or topsoil from infested areas. Movement of contaminated gravel and soil used in road-making or garden topdressing is a common method of spread (Parsons & Cuthbertson 2001).
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 2001; Bass et al. 2000).
Annual Ragweed (Ambrosia artemisiifolia) can be managed with herbicides and physical and cultural methods. Annual Ragweed survives mowing, trampling, grazing and cultivation due to the longevity and dormancy of its seed.
Chemical control: A number of herbicides are registered for control of Annual Ragweed. As it has developed resistance to at least one herbicide in Europe, care should be taken to rotate the herbicides used to avoid this problem (Parsons & Cuthbertson 1992). Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au .
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. Annual 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.
Cultural: Infestations can be minimised by maintaining healthy, dense pastures (DPI, NSW 2019). Stock will only occasionally eat ragweed when it has set seed and will subsequently pass the seed. Most improved pasture grasses will suppress Annual Ragweed, provided a dense, healthy ground cover is maintained.
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: Annual Ragweed 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 Annual Ragweed, and are known to be widespread and exerting a degree of control in most effected areas from Brisbane in Queensland, to the north coast of New South Wales ( Palmer and McFadyen 2012).
As the name suggests Annual Ragweed establishes each year, normally germinating from spring through to summer. Germination can occur at other times of the year if conditions are suitable. Flowering usually occurs from mid to late March, after which plants die. Late-germinating plants may over-winter in mild areas and survive until the following autumn (Land Protection 2007).
NSW, QLD, SA, TAS, VIC, WA.
Annual Ragweed is now widely 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 localised, in Victoria. The related species perennial ragweed (Ambrosia psilostachya) and Annual Ragweed are naturalised to a limited degree in South Australia and Western Australia.
Also recorded from Tasmania with a single plant growing under a bird feeder. The plant is presumed to have arisen from spilt birdseed (AVH 2020).
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).
Common Ragweed, Ambrosia, Bitterweed, Hay-fever Weed, Hogbrake, Hogweed, Horseweed, Asthma Plant.