Quick Facts

Quick facts

  • Originally from North America, Perennial Ragweed (Ambrosia psilostachya), is an erect robust perennial herb with grey-green, deeply lobed leaves and greenish-yellow flowers, growing to 1-2 m tall, forming dense colonies.
  • It grows in grasslands and other open vegetation, usually as a weed along roadsides, in uncultivated fields and waste areas.
  • The seeds have projections that enable them to attach to animals and clothing. The seeds and rhizomes also spread in mud on vehicles and farm machinery and in soil and gravel used in road building and gardening.
  • Perennial Ragweed causes problems in agriculture by strongly competing with crops and pasture. It also inhibits germination and growth of other plants.
  • It is strongly allergenic causing severe hayfever in many people.
  • Effective control can be obtained with herbicides.

What Does It Look Like?

What is it?

Perennial Ragweed (Ambrosia psilostachya) is an erect stout robust perennial herb to 1 metre tall, occasionally 2 metres tall. Young plants form a basal rosette of leaves during the early stages of growth, then elongate producing erect stems. As plants mature, they produce creeping underground stems (rhizomes) or stolons (slender, prostrate or trailing stems, producing roots at its nodes, also known as stem joints). Both stolons and rhizomes produce plants that normally forming dense colonies. The erect stems are hairy, striated (grooved) and woody at the base and branching in the upper half. Leaves are aromatic, 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. Leaves can have no, or a short petiole (leaf stalk), lanceolate (lance-shaped) to ovate (egg-shaped with widest part near the base of leaf) in outline, mostly 40-120 mm long and 20-40 mm wide, are deeply lobed, often with toothed lanceolate segments 2–4 mm wide, surfaces hairy (pubescent), or often somewhat rough to the touch (scabrous), sometimes (hairs pressed to the stem (appressed-hairy).

Flowers are cream to greenish-yellow. The male flowers (comprising several tiny tubular male florets) form shortly stalked hemispherical heads, 3 mm in diameter, which are grouped into branching spikes at the ends of the stems. A small number of female florets, without petals, form 1-flowered heads, clustered between the base of the upper leaves and the stems, and surrounded by a ring of bracts. In the male flowers, the bracts are fused together. Each female flower produces a seed with many seeds produced on a flower-head.

The fruits or 'seeds' (cypselas) are greyish brown, woody, about 3-4 mm long with a short pointed beak surrounded by 4 or 5 blunt projections (Parsons & Cuthbertson 2001).

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

Flower colour

Yellow or green

Growth form (weed type/habit)


Where it currently grows? Preferred habitat

Perennial Ragweed (Ambrosia psilostachya) is mostly found in warmer temperate and sub-tropical environments. It is a common weed of pastures, open woodlands, roadsides, disturbed sites, waste areas, creek banks and riparian vegetation, and is occasionally also found growing in cultivation DEEDI (2020). Perennial Ragweed occurs in grasslands and other open vegetation, usually on lighter soils, often as a weed along roadsides, in uncultivated fields, vacant lots and waste lands (Parsons & Cuthbertson 2001). In Queensland it is recorded as a weed of roadsides, waste areas, creek banks, pastures and uncultivated fields (Brisbane City Council, 2020). 

Are there similar species?

Perennial Ragweed (Ambrosia psilostachya) is commonly mistaken for other Ambrosia species, in particular, Burr Ragweed (Ambrosia confertiflora). However, their seeds are very different, with the seed of Perennial Ragweed having a short beak surrounded by 4-5 short projections, and that of Burr Ragweed being covered with 10-20 short hooked spines (Thorp & Wilson 1998 -; Parsons & Cuthbertson 2001). 

Perennial Ragweed is very similar to annual ragweed (Ambrosia artemisiifolia). However annual ragweed does not produce underground stems (rhizomes), the fruiting involucre (a group of bracts surrounding the base of a flowerhead) is also smooth with out hairs (glabrous) or nearly so; leaves are always petiolate (with a leaf stalk). 

Perennial Ragweed fuiting involucre (a group of bracts surrounding the base of a flowerhead) is always distinctly hairy, and some leaves do not have a leaf stalk VicFlora (2016).

Why Is It A Weed?

What are its impacts?

Perennial Ragweed (Ambrosia psilostachya) is a strongly 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 (Brisbane City Council 2020).

Agriculture: Perennial Ragweed is highly competitive and will invade crops and pastures. Negative impacts include crop losses, decrease in fodder availability (CABI 2020). 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).

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

Urban areas: Perennial 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, Perennial Ragweed is noted for the amount of allergenic pollen it produces, causing severe hayfever and contact dermatitis in susceptible people (Parsons & Cuthbertson 2001; Hussey et al. 1997).

How does it spread?

Perennial Ragweed (Ambrosia psilostachya) is spread by seed and vegetatively

Vegetative shoots are produced along the creeping underground stems (rhizomes) of established plants. Infestations spread locally via these lateral creeping underground stems (rhizomes) and eventually develop into large colonies. Also, seed tends to drop close to the parent plant and germinate locally. Natural growth of existing colonies occurs because of the production of shoots from buds along the creeping roots and transfer of root fragments severed during cultivation to other areas (Parsons & Cuthbertson 2001).

Perennial Ragweed can be spread long distances by movement of seed and creeping underground stems (rhizomes). Seed attaches itself to sheep, fur, clothing and other fibrous material. Seeds are dispersed by animals, vehicles, water and during the movement of soil, while rhizome fragments may be spread by cultivation machinery Brisbane City Council. (2020). Also spread in gravel and soil used in road building and gardening.

There is some data to suggest that many of the coastal colonies of Perennial Ragweed originated from seed dispersed by United States of America Army trucks moving through the area during World War II (Parsons & Cuthbertson 2001). It is commonly spread in soil and gravel used in road building and in soil used for topdressing.

What is its history in Australia?

It is not known how or when Perennial Ragweed (Ambrosia psilostachya) arrived in Australia, but it was first recorded as naturalised at Narromine, New South Wales, in 1922 (Parsons & Cuthbertson 2001). However, earlier collections from Australia do exist from Victoria and Tasmania (AVH 2020), but these may be cultivated records.

How To Manage It?

Best practice management

Perennial Ragweed (Ambrosia psilostachya) survives and spreads by spreading root-stocks and also by seeds. Herbicides can effectively be used to control

Chemical control: 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 Perennial Ragweed is 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 avalible 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: Perennial 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: Perennial 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)

Perennial Ragweed (Ambrosia psilostachya) seeds germinate in autumn and plants develop rapidly in winter and spring, during which an extensive mass of creeping perennial roots is produced in the upper 30 cm of soil, with the occasional branch extending down to 1 m. Flowering begins in mid-summer and continues until early autumn. Aerial growth dies in autumn and new growth develops from the 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?

Perennial Ragweed (Ambrosia psilostachya) is widespread in New South Wales, on the North Western Slopes and Plains and the North Coast. There are scattered outbreaks on the Central Western Slopes, the Central Tablelands, the Central Coast and the Murrumbidgee Irrigation Area.

It is also widespread in south-east Queensland and occurs on the Darling Downs and the lower south coast. 

It has been recorded in the Adelaide metropolitan area and along the Murray near Berri (AVH 2020).

In Victoria it is reported that Perennial Ragweed does not currently occur, after small infestations were eradicated in the 1960s (Agriculture Victoria 2020).

Western Australia, mainly around the Perth metropolitan area.

Where does it originate?

Perennial Ragweed (Ambrosia psilostachya) is native throughout central Canada and the western half of the United States, with the exception of northern California, Oregon and Washington State but is prevalent throughout. Since World War II, it has become a serious weed in parts of eastern Europe, and western Asia. It also occurs in South America, Japan, Mauritius (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 psilostachya

Other scientific names (synonyms)?

Ambrosia artemisiifolia L. (misapplied by Black, J.M. 1929, Bignoniaceae – Compositae. Flora of South Australia. 4: 602.)

Does it have other known common name(s)?

Dunbible Weed, Ragweed, Roman Wormwood, Western Ragweed, Herbe Solferino.

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