Along with pollens from trees, grasses, and weeds, molds are an important cause of seasonal allergic rhinitis.
People allergic to molds may have symptoms from spring to late fall. The mold season often peaks from July to late summer.
Unlike pollens, molds may persist after the first killing frost. Some can grow at subfreezing temperatures, but most become dormant. Snow cover lowers the outdoor mold count dramatically but does not kill molds.
After the spring thaw, molds thrive on the vegetation that has been killed by the winter cold. In the warmest areas of the United States, however, molds thrive all year and can cause year-round (perennial) allergic problems. In addition, molds growing indoors can cause perennial allergic rhinitis even in the coldest climates.