All plants produce pollen. The pollens that causes allergies in mammals comes from trees, grasses and weeds. Generally speaking, trees blossom and produce pollen in the "spring." Grasses tend to produce pollen during the "summer." Weeds tend to produce pollen during the "fall." This is simplistic. Several factors affect how well plants grow and so the pollen counts: hours of daylight, temperature, rainfall, and humidity. If the ideal conditions are lengthened, plants remain at their peak levels and produce more pollen more longer periods of time. Greenville happens to have all the "right" amounts of these factors, as do many cities in the Southeast. Pollen counts are also affected by the local terrain. For example, suppose the city is on the ocean, like Charleston, then there may be prevailing that blow inland from the ocean, which would bring air without pollen. Even if the wind is outbound, there are a minimal amount of trees nearby to produce pollen. The result is low tree pollen counts. Greenville happens to be inland and surrounded in all directions by large forests. The result is very high tree pollen counts.
My predictions for the tree pollen season will be that the levels of pollen will at best average and probably low. Why? The temperatures have been significantly cooler than average. It has not been a balmy spring. Trees don't grow as well in cooler weather. Locally in Greenville, we have had technically in the high range tree pollen counts, but not horrible. It has also been raining, in particular during the week of the historical peak pollen counts. Rain removes pollen from the air. Statistically, the number of new patients suffering from respiratory allergies is down. Naturally there is always more than one possible reason. In this case, the economy also means that fewer people go to the doctor for what is perceived as not life-threatening symptoms, which is true. To seeGreenville's 2009 pollen counts.