Question: What can cause seasonal allergies at different times of the year? (Early spring, late spring/early summer, fall?)
Answer: Clearly people can have allergies at various times of the year or throughout the year. What causes the symptoms during different times of the year depends on the months of the year. In early spring, the trees are classic pollinators. They begin to pollinate when the weather warms, and they go in to usually April or May depending on where you're at in the country. After the trees pollinate then the grasses begin to pollinate and they generally peak in June.
And then most allergy patients will have a respite in July and then in mid-August the weeds begin to pollinate heavily and they go on until the first frost. The caveat to this is depending on where you're at in the country and the local climate, the length of the pollen season and the pollen that's prevalent varies somewhat depending on the region.