Cottonwood trees (genus Populus) are common across North America, especially near rivers and waterways. During late spring and early summer, these trees release a substantial amount of white, fluffy material into the air. The visible “cotton” often leads to the mistaken belief that this material is the pollen responsible for seasonal allergy symptoms. This article clarifies the distinction between the visible white fluff and the actual microscopic cottonwood pollen grain.
The Visible White Fluff
The easily observable white material floating through the air is not pollen, but the mechanism the tree uses to disperse its seeds. This cottony fiber is a filament of cellulose attached to a tiny seed, acting like a parachute for wind dispersal. This fluff originates exclusively from female cottonwood trees after the seeds have matured, typically from late May through June. Because the fluff is relatively large, it cannot penetrate human nasal passages or airways to cause an allergic reaction on its own. However, the sticky filaments can accumulate and harbor other microscopic allergens, such as dust, mold spores, or grass pollen.
Appearance of the True Pollen Grain
The actual cottonwood pollen grain is entirely invisible to the naked eye, as it is measured in micrometers. The grains are quite small, typically ranging from 25 to 40 micrometers in diameter, compared to a human hair’s width of roughly 100 micrometers. The morphology is generally spheroidal or slightly triangular, with a simple, smooth, or faintly granular surface structure. It lacks the deep grooves or spiky ornamentation found on many other highly allergenic pollens. This structure is characteristic of wind-pollinated species, allowing the lightweight grain to be easily lofted into the atmosphere.
Seasonal Release and Dispersal
Cottonwood trees are a dioecious species, meaning individual trees are either male or female, and only male trees produce the true pollen. Male trees bear catkins that release the pollen into the air in a concentrated burst during the early spring. This release generally begins in late February and can extend through March and April, varying based on latitude and local climate. As an anemophilous species, cottonwoods rely entirely on wind currents to carry the microscopic pollen from the male catkins to the female flowers. This wind reliance causes high concentrations of the grain to saturate the air during the peak season.
Allergic Impact
The true cottonwood pollen is considered moderately allergenic to sensitized individuals. Symptoms of an allergic reaction include sneezing, nasal congestion, runny nose, and watery eyes, similar to other common pollen allergies. The simple, smooth surface of the cottonwood pollen grain may make it less potent than the complex pollens of other plants, such as ragweed or grass. A difficulty in isolating the impact of cottonwood pollen is that its release coincides with the pollination of several other common early-spring trees, such as elm and maple. This overlap often leads to misdiagnosis, where the visible summer fluff is incorrectly blamed for symptoms caused by the earlier, invisible cottonwood pollen or other contemporary spring allergens.