False Ragweed

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Code: w4
Latin name: Franseria acanthicarpa (Synonym: Ambrosia acanthicarpa)
Source material: Pollen
Family: Asteraceae (Compositae)
Common names: False Ragweed, Bur ragweed, Annual Burweed

Allergen Exposure

False Ragweed grows almost over all the continental United States. Related species are found in Mexico, Hawaii and Australia. False Ragweed is considered to be a major source of pollen allergy in certain areas of the USA where the plant is common.

False Ragweed is an erect, bushy summer annual, similar to the genus Ambrosia, and growing to a height of 1.5m. The leaves are 8cm long and 7cm wide, alternate on the upper stems, grey-green in colour and bipinnately lobed. The foliage is covered with white to grey short, bristly hairs.

The plant flowers from August to November. The flower heads are small, greenish, and composed of staminate (male) or pistillate (female) disc flowers. Staminate and pistillate heads are separate on a single plant (a monoecious structure). The pistillate heads are clustered in the leaf axils below the spikes. False Ragweed is both insect- and wind-pollinated, but the relative rarity of the plant makes its copious pollen clinically less important overall than that of the other Ragweeds. The fruit becomes a bur. Burs are highly variable, but often golden-brown. The bur is 4 to 8mm long, typically with 6 to 30 2 to 5mm-long sharp-pointed, flattened spines that are straight at the tip (not hooked). The seed matures from August to October. Burs disperse by clinging to shoes or clothing, or to the feet, fur or feathers of animals.

False Ragweed may be found on dry slopes, sandy flats, alluvial plains, grasslands, coastal areas, forestry regeneration sites and other disturbed sites, and agricultural fields. Although the plant inhabits many natural plant communities, it can become a pest.

Allergen Exposure

No allergens from this plant have yet been characterised.

Potential Cross-Reactivity

An extensive cross-reactivity among the different individual species of the genus could be expected, as well as to a certain degree among members of the family Asteraceae. (1),  (2) This was confirmed in a study using a serum pool from patients sensitive to Short Ragweed, whereby the cross-reactivity of IgE antibodies to six Ragweeds was studied through the radioallergosorbent test. Extracts were analyzed for their inhibitory activities, with solid-phase allergens prepared from all of the Ragweed pollens. Also, samples of serum were absorbed with the various solid-phase allergens and the reactivity of the remaining IgE antibodies was determined. Two patterns of reactivity were observed. Short, Giant, Western, and False Ragweeds displayed comparable reactivity in both inhibition and absorption experiments. Slender and Southern Ragweed were considerably less active, indicating that they lacked allergenic groupings possessed by the other species. These same patterns of cross-reactivity were found using Ragweed pollens from four commercial sources. (3)

A second study documented close cross-reactivity between False Ragweed and Short Ragweed. A water-insoluble material, extracted from Short Ragweed and False Ragweed pollen, contained at least five proteins. Two (RFA2 and RFB2) were isolated and shown to possess antigenicity as well as allergenicity. Immunodiffusion tests of RFB2, isolated from False Ragweed and Short Ragweed, showed immunological identity. (4)

Clinical Experience

IgE mediated reactions

Asthma, allergic rhinitis and allergic conjunctivitis, similar to sensitisation from other Ragweeds, occur in sensitised individuals. (5) Symptoms may be elicited either due to sensitisation to this species, or due to cross-reactive mechanisms with other members of the Ragweed genus.

Other reactions

Contact dermatitis to False Ragweed has been documented. (6)

Compiled by Dr Harris Steinman, harris@allergyadvisor.com


  1. Yman L. Botanical relations and immunological cross-reactions in pollen allergy. 2nd ed. Pharmacia Diagnostics AB. Uppsala. Sweden. 1982: ISBN 91-970475-09.
  2. Weber RW. Cross-reactivity of pollen allergens: impact on allergen immunotherapy. Ann Allergy Asthma Immunol 2007;99(3):203-11.
  3. Leiferman KM, Gleich GJ, Jones RT. The cross-reactivity of IgE antibodies with pollen allergens. II. Analyses of various species of ragweed and other fall weed pollens. J Allergy Clin Immunol 1976;58(1 PT. 2):140-8.
  4. Su SN, Harris J, Lau GX, Han SH. Aqueous-organic solvent extraction of water-insoluble protein from ragweed pollen. Zhonghua Min Guo Wei Sheng Wu Ji Mian Yi Xue Za Zhi 1987;20(2):104-12.
  5. Mohapatra SS, Lockey RF, Polo F. Weed pollen allergens. Clin Allergy Immunol 2008;21:127-39.
  6. Guin JD, Skidmore G. Compositae dermatitis in childhood. Arch Dermatol 1987;123(4):500-2.


As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.