The plants known to be related to the cause of hay fever in the United States are generally studied in three groups, namely, Trees, Grasses and Weeds. Due to the fact that the plants of each group tend to pollinate during separate periods (with some overlapping), there is recognized in the United States three hay fever seasons as follows:
SPRING HAY FEVER, occurring during March, April and May, is caused by tree pollenssometimes spoken of as tree hay fever.
EARLY SUMMER HAY FEVER, occurring during May, June and July, is caused by pollens of grassessometimes referred to as grass hay fever.
LATE SUMMER AND EARLY FALL HAY FEVER, occurring during August or September, continues into October or November. This type is often referred to as ragweed hay fever although it is caused by other weeds as well.
Besides these distinct seasons, the whole summer constitutes the season for a few people who are sensitive to English plantain. This weed starts to pollinate in May and continues all through the warm months.