Seasonal allergies, like other types of allergies, develop when the body’s immune system overreacts to something in the environment, usually during spring, summer or fall when certain plants pollinate.

In many areas of the United States, spring allergies begin in February and last until the early summer. Tree pollination begins earliest in the year followed by grass pollination later in the spring and summer and ragweed in the late summer and fall. In tropical climates, however, grass may pollinate throughout a good portion of the year. Mild winter temperatures can cause plants to pollinate early. A rainy spring can also promote rapid plant growth and lead to an increase in mold, causing symptoms to last well into the fall.

The most common culprit for fall allergies is ragweed, a plant that grows wild almost everywhere, but especially on the East Coast and in the Midwest. Ragweed blooms and releases pollen from August to November. In many areas of the country, ragweed pollen levels are highest in early to mid-September.