Tree pollen Is the primary cause of early-spring hay fever. Although this type of hay fever is generally mild and of short duration, some individuals suffer rather violent reactions. In selecting tree species for urban environments, it may be wise to consider the allergenic qualities of their pollen. Large populations of trees that produce allergenic pollen should be avoided.
The fact that tree pollen is responsible for only a small proportion of the total number of hay-fever and asthma cases is of no comfort to those who are sensitive to such pollen. For example, Balyeat and Rinkel (1932) report that in two cases they were able to control sensitivity to many pollens and animal epithetelium (cellular tissues) but were unable to control sensitivity to paper-mulberry pollen. The severity of asthmatic symptoms during the paper-mulberry season was so marked in these two cases that both individuals had to move to areas where this species does not grow.
The hay-fever season is usually divided into five periods: early spring, late spring, early summer, late summer, and winter. The dates and duration of these periods vary among different geographic regions. Plants responsible for allergenic toxicity also vary in different regions. Wodehouse (1986, 1971) describes seasonal pollen production by various plants in different regions of the United States.