Annual ragweed (Ambrosia artemisiifolia)

Annual ragweed is a herb, 1–2 m tall with fern-like leaves. It causes hay fever, asthma and skin allergies.

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How does this weed affect you?

Annual ragweed:

• produces large amounts of pollen which causes health problems in people
• can cause skin irritation
• competes with crops including vegetable crops
• can be a host for crop pests
• is unpalatable to grazing animals especially horses which leads to overgrazing of desirable pasture plants and dominance of annual ragweed.

Human health

Ragweed produces up to a billion pollen grains per plant. The pollen can cause hay fever, itchy eyes, running nose and asthma. Contact with the plant can cause skin reactions including eczema.

What does it look like?

Annual ragweed is a shallow rooted annual herb that usually grows 0.5–2 m tall but may be up to 3.5 m tall. Flowering is from summer to early winter.

Leaves are:

• green to grey green and slightly paler underneath
• deeply lobed giving a fern-like appearance
• 1–16 cm long and 1–7 cm wide
• hairy on both sides but more so on the underside
• in a rosette at the plant’s base in young plants
• on 2–3 cm long stalks on older stems
• opposite on the lower stems becoming alternate higher up the stem.

Flowerheads:

There are separate male and female flowerheads.

Male flowerheads are:

• cream, yellow or green (though appear yellow when mature and covered in pollen)
• cup-shaped and droop downwards
• 2–5 mm in diameter
• a cluster of 10–100 tiny flowers
• assembled along spike-like clusters up to 20 cm long usually with 10–90 flowerheads per spike
• found at the tips of the stems.

 Female flowerheads are:

• upright
• whitish-green
• a single tiny flower with no petals and 2 red threadlike stigmas
• surrounded by hairy leaf-like bracts
• found in the leaf fork either singular or in small groups of the upper-most leaves below the male flowers.

Seeds are:

• black or brown
• 3–5 mm long and 2–3 mm wide
• beaked with a ring of 4–7 spines each about 1 mm long.

Stems are:

• brownish, green, pinkish red or dark red
• round
• smooth or roughly hairy.

Similar looking plants

Annual ragweed looks like:

• Parthenium weed (Parthenium hysterophorus) which has white star-shaped flowers that are clustered at the tips of stems, rather than on a spike.
• Burr ragweed (Ambrosia confertiflora), which has many clusters of female flowers and hooked spines on the fruit.
• Lacy ragweed (Ambrosia tenuifolia), which is a perennial plant with smaller grey, lacy looking leaves and hairy fruit.

Where is it found?

Annual ragweed grows along the coast of NSW, mainly north of Sydney. Plants have also been found in the Northern Tablelands.

It is native to North America.

What type of environment does it grow in?

Annual ragweed grows in subtropical and temperate climates. It grows best in warm moist areas and can tolerate a wide range of soil types.

It invades disturbed areas such as roadsides, waste areas, and poor or overgrazed pasture. Horse paddocks in coastal areas are often infested. It also invades floodplains and along the edges of waterways.

How does it spread?

Plants produce an average of 3000 seeds per year and seeds can stay dormant for up to 40 years. Seeds can be spread:

• in contaminated soil by machinery
• in floodwaters
• by sticking to fur or wool on animals
• on boots or clothing
• as a contaminate in other seed produce.

References

Bass, D. J., Delpech, V., Beard, J., Bass, P., & Walls, R. S. (2000). Ragweed in Australia. Aerobiologia, 16(1), 107-111.

Business Queensland (2016). Queensland Government. Retrieved 19 June 2020 fromhttps://www.business.qld.gov.au/industries/farms-fishing-forestry/agriculture/land-management/health-pests-weeds-diseases/weeds-diseases/invasive-plants/restricted/annual-ragweed

Department of Agriculture and Fisheries, Queensland (2020). Restricted Invasive Plant: Annual ragweed Fact Sheet. Retrieved 12 April from: https://www.daf.qld.gov.au/__data/assets/pdf_file/0006/60666/annual-ragweed.pdf

Parsons, W.T., & Cuthbertson, E. G. (2001). Noxious weeds of Australia. CSIRO publishing.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 19 June 2020 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Ambrosia

Šauliene, I., Veriankaite, L., & Šaulys, A. (2012). Biometrical assessment of ragweed (Ambrosia artemisiifolia L.). Žemdirbyste (Agriculture), 99(3), 319-326.

Weryszko-Chmielewska, E., & Piotrowska, K. (2008). Ecological features of Ambrosia artemisiifolia L. flowers and characteristics of Ambrosia L. pollen seasons in the condition of Lublin (Poland) in the years 2001-2008. Acta Agrobotanica, 61(2).

More information

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Control

Successful weed control relies on follow up after the initial efforts. This means looking for and killing regrowth or new seedlings. Using a combination of control methods is usually more successful.

Wear appropriate personal protective clothing such as gloves, long sleeves and facemask. Anyone prone to allergies should avoid contact with flowering plants and pollen.

To manage annual ragweed:

• control plants before they set seed
• check for and control seedlings especially from spring to early autumn.

Pasture management

Minimise infestations by maintaining healthy, competitive pastures.

Physical removal

By hand

Dig out or hand-pull young plants.

By machine

Slash or mow young plants before they flower.

Biological control

Two biological control agents are present in New South Wales. Both of these species were released to control Parthenium weed in Queensland. The stem-galling moth Epiblema strenuana has a reasonable impact on annual ragweed. The leaf feeding beetle Zygogramma bicolorata is more widespread and can have a significant impact at some sites.

Chemical control

Spray plants before they flower. Cover all foliage with herbicide.

Herbicide options

See Using herbicides for more information.

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Bromacil 800 g/kg (Various products)
Rate: 3.5 to 6.5 kg per ha
Comments: Industrial weed control: lowest rate suited to lower rainfall areas
Withholding period: Not required when used as directed.
Herbicide group: C, Inhibitors of photosynthesis at photosystem II (PS II inhibitors)
Resistance risk: Moderate

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Dicamba 750 g/L (Kamba® 750)
Rate: 87 mL per 15L of water. Add a surfactant.
Comments: Spot spray prior to flowering. For non-crop situations.
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 750 g/L (Kamba® 750)
Rate: 5.9 L/ ha. Use a minimum of 1500 L of solution per ha. Add a surfactant.
Comments: Boom spray for non-crop situations. Spray prior to flowering.
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 750 g/L (Kamba® 750)
Rate: 400 mL per 100L of water. Add a surfactant.
Comments: Spray prior to flowering. For non crop situations.
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 2021