Ambrosia Pollen

The most important cause of allergenic rhinitis and pollen asthma in North America is pollen from ragweed, Ambrosia species, a widespread genus in the Asteraceae (sunflower family). Ragweeds are annual or perennial herbs with lobed or divided leaves ranging from small plants such as Ambrosia artemisiifolia (short or common ragweed) 30 cm to 1.5 m in height to A. trida, giant ragweed, which can reach 4 to 5 m tall . Although 7 species of ragweed occur in Oklahoma and 21 species in North America, most allergy problems are caused by A. artemisiifolia and A. trifida, two species that account for more hay fever than all other plants together. Both are pioneer plants that are well adapted to invading disturbed soils. In natural habitats these species are restricted; however, in areas where human intervention has cleared existing vegetation, ragweed quickly becomes established.

Ragweed Flowers and Flowering

Ragweed is monoecious with male and female flowers produced on the same plant. The small male flowers (each with five stamens) are grouped into inflorescences with 10 to 20 florets borne together in a cupule or involucre of bracts. Approximately 50 to 100 involucres occur on each of the many terminal flowering spikes. the thousands of male flowers on each plant result in the release of approximately one billion pollen grains. It is estimated that 1 million tons of ragweed pollen are produced each year in North America.  Although pollen is released from the stamens in early morning, the pollen does not become airborne until midmorning as dew dries and humidity decreases. The pollen grains are tricolporate with an echinate exine.

Ragweed Pollen Release

In northern areas of the U.S. and in Canada, ragweed pollen release begins in early August, peaks by early September, and is complete in early October. An early killing frost in late September often shortens the ragweed season in northern areas. In southern states of the U.S., including Oklahoma, the ragweed season begins later in August and continues until later in the fall with significant pollen levels occurring through October. In Florida as well as other areas of the Gulf Coast and the southwest, the ragweed season may be different with some species of ragweed flowering in spring, others during the winter and others potentially flowering year-round. 

Factors Affecting Pollen Release

It is well known that weather conditions influence the day to day variability of atmospheric pollen concentrations. Warm dry windy conditions are optimal for pollen dispersal, while high humidity and rainfall, conversely, retard pollen dispersal and may cause washout. Seasonal abundance of pollen, however, depends on growing conditions that influence plant growth, flower formation, and pollen production earlier in the year.

Tulsa Ragweed Season

Ambrosia pollen is the dominant pollen type in the Tulsa atmosphere for two and one-half months, from mid-August until late October. It is present in the atmosphere on 96% of days during this period. Although occasional ragweed pollen occurs in the atmosphere before August 15 and in late November and December, it is believed that these result from long distance transport and not local sources. Field studies indicate that pollen release of most local ragweed plants begins during the third week of August. Ragweed concentrations generally show a yearly peak during the second week of September on or about Sept 10. The graph below represents the average daily ragweed pollen season in Tulsa over 25 years; however, day-to-day meteorological events will influence the daily concentration. Rainy weather will wash pollen from the atmosphere and also temporarily inhibit the release on new pollen. So a graph of pollen release for one season will typically shows major spikes and valleys representing the influence of daily weather.