Plants in This Family Other Than Ragweed Which Produce Allergenic Pollen
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama
I have written previously about the leading cause of allergic rhinitis [AR] in the U.S. caused by plant pollen, AR resulting from exposure to ragweed pollen, scientific name Ambrosia sp. Ragweed belongs to the family of plants called Asteraceae. There are several other genera in this plant family which produce airborne pollen and which can trigger AR in sensitive individuals. One of these genera which is reported to the National Allergy Bureau as part of our daily pollen counts is Artemesia. I will write a blog in the near future on members of this genus, plants with common names of sagebrush, mugwort, and wormwood. Today’s blog will focus on a few of the Asteraceae species besides ragweed and sagebrush which show up occasionally in our daily pollen collections and which can potentially trigger AR in some individuals.
The family Asteraceae (sunflower family) is the largest plant family with estimates of 1100-1200 genera and 20,000-33,000 species. Only two other plant families come close to the size of Asteraceae, the orchid family (Orchidaceae), and the legume family (Fabaceae). The scientific name for the sunflower family comes from the type genus Aster, which is Greek for star, and refers to the star-like appearance of the inflorescence in many species. Another name for this plant family is Compositae (an older but still valid name) which refers to the fact that the family has composite flowers. This means what appears as a flower is in actuality an inflorescence called a head or capitulum. A head is composed of numerous smaller flowers. The individual flowers making up a head have five fused petals but lack sepals. Instead of sepals these flowers have structures that are called pappus which surround the fruit (an achene) and serve to aid in seed dispersal by sticking to the fur of animals or being lifted by the wind. The pappus is threadlike, hairy, or bristly in nature. The fluffy head formed from a dandelion inflorescence is a familiar example of pappus that aids in seed dispersal. The size of Asteraceae inflorescences varies greatly among members of this plant family. Some are as small as an eighth of an inch in diameter, while others are upwards to perhaps twelve or more inches in diameter (heads of the giant sunflower Helianthus annus). Leaves are alternate, opposite, and occasionally whorled, and may be simple or pinnately or palmately compound. Plants are most commonly herbaceous but some members of the family grow as shrubs, vines, and even trees.
The pollen grains of Asteraceae plants typically have spines as part of the outer wall (the exine). These spines may be short (as seen in ragweed), or long (as seen in sunflower), or somewhere in between (see figures of pollen grains below). Pollen grains of Asteraceae are tricolporate, meaning they have both pores and furrows. Ragweed pollen grains have short spines and very short furrows and indistinct pores. Iva (marsh elder) produces pollen that has a slightly longer furrow than Ambrosia, but is very similar when viewed under a light microscope. Xanthium (cocklebur) has larger grains with very tiny spines and the grains are slightly larger than those of ragweed and marsh elder. Other members of Asteraceae that produce pollen occasionally seen in our pollen collections have moderately long spines which are spaced further apart than those of ragweed or marsh elder. Examples of these species include Eupatorium (boneset, dog fennel), Baccharis (groundsel bush), and Parthenium (feverfew or carrot grass). Pollen from these plants occasionally shows up in our surveys near the end of ragweed season. These Asteraceae plant pollens possess strong allergens, perhaps shared with ragweed, and should not be ignored as possible causes of allergic rhinitis, particularly if you are sensitive to ragweed pollen.
Some species in the Asteraceae family have longer spines and are larger in diameter than ragweed or dog fennel and could potentially cause allergic rhinitis in sensitive individuals. These types of pollen grains are seen less frequently in surveys because plants producing them are more likely to be insect pollinated species. For example, goldenrod plants produce long-spine pollen grains and flower in the fall during the time that ragweed plants are in flower. Because goldenrod is readily visible along roadsides and in fields, it is often blamed as the culprit causing fall hay fever rather than ragweed. Goldenrod pollen rarely becomes airborne and is thus not likely to be a cause of allergic rhinitis.
If you suffer from AR triggered by pollen during the fall of the year, it is most likely a ragweed allergy or one of the other members of the Asteraceae family of plants. (Note: there are other possible triggers of AR in the fall such as plants in the Urticaceae family, or elm.) Fortunately, the flowering season of these Asteraceae plants typically lasts only a few months so limit outdoor exposure when possible and follow your allergist’s advice regarding antihistamine use and desensitization protocols.