How warmer fall temperatures are prolonging late season allergies 

Spring-time allergy season often gets the most attention for prolific pollen counts as flowers, trees, and grasses burst forth with new growth. For some, the secondary allergy season of late summer and fall can be just as impactful. Like much of the lower 48 states, warmer fall temperatures have been extending fall allergy season over the past several decades across Northwest Missouri and Northeast Kansas.

According to a study by Climate Central using weather observations from the past 55 years, St. Joseph has seen a 9-day average increase in consecutive freeze-free fall days from 1970 to 2025. This boils down to later fall freezes, and a longer growing season for fall allergens to thrive. While the timing of the first killing frost of fall always varies from one year to the next, the overall trend has been pushing this date later into the cool season. 

Ragweed is one of the biggest culprits behind fall allergy symptoms, blooming for months at a time. Peak bloom for ragweed regionally is August and September, although ragweed can also bloom well into October in this part of the country. Weeks like this one with well above average temperatures and sunshine following a prior period of wet weather can promote additional ragweed blooms. 

Dry, windy weather can also keep ragweed pollen and other allergens airborne for longer, making for easy transport over greater distances. The same idea applies to dust and organic particulates stirred up by harvesting of fall crops, that can fill the air and also travel long distances.

Another common offender when it comes to late season allergies is mold. As leaves and vegetation decay, moisture is often trapped underneath, the perfect breeding ground for mold spores to collect and spread. Unlike plant-related allergens, mold can be an issue after temperatures have dipped below freezing. While subfreezing air can make mold go dormant, daily warm-ups common in late fall and early winter can keep it alive and well.