The question of when allergy season occurs in Arizona does not have a simple answer like it does in many other parts of the United States. Unlike regions where a hard winter resets the biological clock, the mild climate of the lower desert areas allows for nearly continuous plant activity. This means the typical seasonal allergy calendar is stretched and blended, creating a complex, near year-round challenge for sufferers. The lack of a prolonged freeze enables various plants to bloom sequentially, ensuring that airborne allergens are present almost every month.
The Continuous Nature of Arizona Allergies
The primary reason for the extended allergy issues in the lower desert is the absence of a sustained, hard freeze during the winter months. Freezing temperatures in other climates typically kill off seasonal plants and force trees into a true dormancy, which clears the air of pollen for a period. In places like the Phoenix and Tucson metropolitan areas, the mild winter temperatures are not cold enough to stop the growth cycle of many allergenic plants.
This creates a scenario of continuous supply where plants take turns being the primary source of pollen, preventing a true dormant period for allergy sufferers. As one group of species finishes its bloom, another group begins to pollinate, ensuring there is always something in the air. The warm, arid conditions also contribute by allowing pollen and dust particles to remain suspended in the air for longer periods, which exacerbates exposure.
Spring and Early Summer Allergy Calendar
The most intense period for pollen in the low desert environment, which includes Phoenix and Tucson, begins early, often starting in January or February and peaking through June. This time is dominated by the release of tree pollen as numerous species exit their brief winter rest. Tree pollen is the first major wave of outdoor allergens that impact the region, with symptoms often starting well before the official spring equinox.
Prominent tree allergens during this phase include the Olive tree, which thrives in the arid climate and releases large quantities of irritating pollen from late April into May. Other notable tree pollens come from species such as Mulberry, Mesquite, and Cottonwood. Mesquite trees specifically pollinate from mid-March to late May. Following the tree bloom, grass pollen becomes a major contributor, with Bermuda Grass being a significant trigger that typically peaks in late spring and early summer, often throughout May and June.
Late Summer and Fall Allergy Calendar
As the extreme summer heat arrives, the primary allergens shift away from tree pollen and toward weed pollen and mold, roughly spanning July through December. A significant factor in this late-year calendar is the arrival of the North American Monsoon season, which typically runs from late June through September. While the initial heavy rains can offer temporary relief by washing pollen out of the air, the resulting increase in humidity and ground moisture creates an environment for mold and fungi growth.
Mold spores, which are present year-round, multiply rapidly in the damp conditions, becoming a major allergen during the monsoon and post-monsoon period. The late summer and fall are characterized by the dominance of weed pollen, which is often the most irritating allergen for many people. Common weed culprits include Ragweed, which causes significant issues from late summer through fall, and Russian Thistle, also known as tumbleweed, which is particularly active in the fall.
Regional Differences in Arizona Allergy Seasons
The vast differences in elevation across Arizona mean the allergy calendar varies significantly between the Low Desert Valleys and the High Country. In cooler, higher-elevation areas like Flagstaff and Prescott, the allergy season is significantly delayed and compressed. These regions experience colder, longer winters that enforce a true dormancy on plant life, pushing the onset of the season much later than in the south.
Allergy symptoms in the high country may not appear until late spring or early summer, and the season is shorter and less intense overall. The primary allergens in these mountainous zones also differ, with species like Pine and Aspen being more prevalent triggers than the desert-adapted plants found in the lower elevations.