Seasonal allergies are a common immune response triggered by airborne particles, traditionally associated with the spring bloom. Many people assume the allergy season concludes in summer once the major producers like oak and birch have finished their pollination cycles. However, fall symptoms like sneezing, congestion, and itchy eyes can persist well past the summer months. While the peak tree pollen season occurs earlier in the year, certain tree-related factors and a few specific species do contribute to the autumn allergy experience.
Trees That Release Late-Season Pollen
Most tree species complete their reproductive cycle by late spring, but a few exceptions extend their pollination into the late summer and early fall. This delayed cycle means that tree pollen, although in much lower concentrations than in spring, can still be a source of discomfort. Specific varieties of the Elm family, such as the Chinese Elm (Ulmus parvifolia), are notable for their late-season activity. Studies have tracked significant levels of Chinese Elm pollen throughout August and September in certain regions, particularly in the Southeast United States.
The Chinese Elm is often utilized in urban landscaping, which places its pollen directly into populated areas. Certain Elm species in southern climates may continue to release pollen into the air as late as November, complicating the traditional understanding of the allergy calendar.
Another regional exception is the Mountain Cedar, or Ashe Juniper. This tree releases massive amounts of highly allergenic pollen starting in late fall, often beginning as early as November and peaking from December through February. While not a primary allergen during the core fall months, it illustrates how certain wind-pollinated trees challenge the typical seasonal allergy timeline. Its presence, especially in the Southwestern United States, extends the period of tree-related pollen allergy well past the first frost.
The Contribution of Leaf Mold and Decaying Matter
A major source of fall allergy symptoms that is directly related to trees is not their pollen, but the massive release of mold and fungal spores. As temperatures drop and leaves begin to fall from the trees, they create an enormous quantity of decaying organic material on the ground. This decomposing matter, especially when damp from autumn rains or morning dew, becomes a perfect incubator for various types of fungi.
Fungi reproduce by releasing microscopic spores into the air, and these spores are recognized by the immune system of allergic individuals as a threat. The conditions present in a pile of wet, fallen leaves are ideal for mold growth. Consequently, activities like raking leaves or walking through wooded areas can stir up large clouds of these allergenic spores.
The two most common outdoor molds responsible for fall allergy flares are Alternaria and Cladosporium. These fungi are ubiquitous, but their spore counts often peak in the late summer and fall, with Alternaria being particularly abundant during this time. Unlike pollen, which typically decreases after a heavy rain, mold spores can increase following wet weather due to the proliferation of the fungi in the damp environment. Mold spores are also far smaller than most pollen grains, allowing them to travel vast distances on air currents and easily infiltrate indoor spaces. Therefore, much of the misery people attribute to “tree allergies” in the fall is actually a reaction to the fungi thriving on the trees’ discarded leaves.
Identifying the True Primary Fall Culprit
Despite the presence of late-season tree pollen and the widespread issue of leaf mold, the single most significant cause of fall allergies across much of the United States remains a weed: ragweed. The ragweed plant begins to release its fine, lightweight pollen grains into the air starting in August and continues until the first hard frost.
A single ragweed plant is capable of producing up to a billion pollen grains in a season. These grains are so small and buoyant that they can travel hundreds of miles on the wind. The peak of the ragweed season typically occurs around mid-September, creating a potent and pervasive allergic trigger that overshadows the localized impact of tree pollen. This is why allergy symptoms can be severe even in urban areas where ragweed plants are not immediately visible.
Because the symptoms caused by tree pollen, mold, and ragweed are nearly identical—sneezing, itchy eyes, and nasal congestion—it can be impossible for a person to differentiate the source on their own. Medical professionals rely on diagnostic tools to pinpoint the specific allergen. The most common method is the skin prick test, which introduces small, standardized amounts of various allergens, including tree pollens, mold spores, and ragweed, onto the skin. A visible reaction, such as a raised, red welt, confirms sensitivity to that specific substance, providing clarity on the true source of a person’s fall allergy symptoms.