9.2.1 What Are the Common Types of Allergenic Weeds in North America?

North American weeds of allergenic importance can be divided into two groups: composites and chenopod-amaranths. Other weeds may be of local importance as well.

The composite weeds (Asteraceae family) include three large tribes of common, highly allergenic weeds: Heliantheae, Anthemideae, and Astereae tribes. The Heliantheae tribe includes cocklebur, marsh elder, and ragweed. Ragweed remains the primary allergenic weed in most of North America (▶Fig. 9.1). Multiple species of ragweed exist including false, giant, short, and western; these have been shown to strongly cross-react with one another. Short ragweed is often used as the representative for this family for testing and treatment unless the specific climate generates a different locally abundant weed.

The Anthemideae (sage) tribe contains mugwort, sagebrush, wormwood, sage, and chrysanthemum; the latter two are edible members. Mugwort is the most important weed along the Pacific coast of North America as well as in Europe. Mugwort cross-reacts approximately 80% with ragweed.

Baccharis species, aster (daisy), and goldenrod are members of the Astereae (daisy) tribe. Goldenrod is predominantly entomophilous for pollination, but sensitivities to the airborne pollen might still exist. This tribe is most important along the coastal and western United States.

Additional composite weeds may not be major sources of pollen, but rather are potent contact sensitizers. Some examples include the Cynareae tribe (e.g., thistles, artichoke) and the Cichorieae tribe (e.g., dandelion, chicory). Lettuce and stevia are additional edible members of the Asteraceae family.

The Amaranthaceae family is closely related to the Chenopodiaceae family, hence often termed the Amaranth-Chenopods. These weeds are very common and are major pollinators. Amaranth examples include cockscomb, careless weed, cottonweed, pigweed, water hemp, and the edible quinoa. The Chenopod family is represented by Kochia (e.g., burning bush), goosefoot, lamb’s quarters, Mexican tea, and the edibles: beet, spinach, and sugar beet. The Amaranths strongly cross-react, while the Chenopods cross-react only mildly; a single Amaranth can be used for allergy testing and treatment within the family, while Chenopods usually need to be treated individually.