Colorado’s unique blend of arid climate, high altitude, and varied topography creates a distinct and often prolonged allergy season. The state’s environmental factors continually circulate airborne particulate matter, meaning that seasonal allergy symptoms can begin unexpectedly early and persist well into the fall. Understanding the timing of pollen release requires recognizing that the allergy season is a chronological sequence of three major phases, each dominated by a different type of allergen. This sequence typically does not conclude until the first significant cold snap arrives.
The Three Phases of Colorado Allergy Season
The annual allergy cycle begins with the tree pollen phase, which commonly starts in late February or early March, triggered by warming temperatures. This initial phase can be intense, with major tree culprits including Juniper, Cottonwood, and Siberian Elm releasing their fine, powdery pollen into the dry air. Spring tree season generally reaches its peak activity throughout April and May.
As the tree pollen subsides in late spring, the grass pollen season seamlessly takes over, typically starting in late May or early June. Grasses like Kentucky Bluegrass, Timothy, and Ryegrass are the primary sources of allergens across the plains and lower elevations. The grass phase continues to produce high pollen counts through July and often into August.
The final phase, the weed pollen season, begins its rise in late summer, usually around August, and remains active until a hard freeze occurs. Ragweed is the most widely recognized allergen, but other common weeds include Sagebrush and Tumbleweed. Weed pollen production can extend the allergy season deep into the fall, sometimes as late as mid-October.
Geographic Differences in Colorado Allergy Timing
The general statewide timeline for pollen release is heavily modified by Colorado’s significant variations in altitude and geography. The Front Range, which includes the Denver metropolitan area and other urban centers, often experiences the earliest onset of the allergy season. Lower elevations and the “urban heat island” effect mean trees begin pollinating here as early as late February, leading to a longer overall season.
In contrast, the high altitude and Mountain areas experience a distinct delay in seasonal timing due to persistent snowpack and cooler average temperatures. Spring tree pollen season in mountain valleys may start weeks later than on the plains, leading to a shorter, but sometimes more compressed and intense, period of high pollen activity. The unique flora at these elevations also shifts the dominant local allergens.
The Western Slope, encompassing regions like Grand Junction, presents its own unique timing and allergen profile in its lower valley areas. Tree pollen, such as Juniper and Cottonwood, can begin releasing in early March, comparable to the Front Range. While Ragweed is a primary concern statewide, the Grand Valley has a lower concentration, with arid-land weeds like Kochia and Sagebrush taking prominence in the fall.
Environmental Factors Affecting Severity and Duration
Beyond the chronological timing of plant life, Colorado’s specific environmental conditions significantly amplify the severity and duration of allergy symptoms. The state’s naturally arid climate and frequent high winds contribute to a persistent circulation of airborne particles. This lack of moisture means pollen grains remain suspended in the air for longer periods and are carried over vast distances, making it nearly impossible to avoid exposure.
Rapid and unpredictable temperature fluctuations, common in Colorado’s spring and fall, can also affect the pollen season. An unexpected early warm-up can cause a sudden, massive release of pollen, which is then often followed by a cold snap that temporarily halts the process, leading to repeated waves of high pollen counts. The infamous Chinook winds, which descend the mountains, can also carry concentrated pollen and dust into populated areas.
Moisture-related allergens, specifically mold spores, present another environmental trigger, often peaking outside the traditional pollen seasons. Mold activity increases significantly during the spring snowmelt and after summer rain events, thriving in damp areas and decaying organic matter. Furthermore, the first hard freeze, while ending the outdoor pollen season, drives people indoors, where allergens like dust and pet dander can intensify symptoms during the winter months.