Seasonal allergies, often called hay fever, are caused by the body’s reaction to microscopic airborne particles released by plants. Determining when this season concludes is a complex matter, as the “end date” is highly variable and depends on a mix of biological, geographical, and meteorological factors.
The Three Phases of Outdoor Allergy Season
The outdoor allergy season is generally divided into three distinct phases, each dominated by a different class of wind-pollinated plants. The first phase belongs to tree pollen, which is typically the earliest to appear and marks the start of the season for many people. This initial period commonly begins in late winter or early spring, depending on the region’s climate.
The second phase is characterized by grass pollen, which becomes the predominant airborne allergen as spring transitions into summer. Grasses release their pollen in late spring and remain highly active throughout the summer months.
Finally, the third phase is driven by weed pollen, most notably ragweed, which emerges in late summer and continues through the fall. Weed pollen is often the last major seasonal allergen and frequently causes the most intense symptoms for individuals whose allergies peak in autumn. These three distinct periods mean that the overall allergy season can span from February to November in many parts of the country.
Typical End Dates for Seasonal Pollen
The typical termination month for each pollen phase is highly dependent on local climate conditions, but general timelines can be established. Tree pollen, which is the first to emerge in the spring, usually begins to subside by late May or early June in most temperate regions.
The grass pollen season, which is active through the summer, typically starts to fade toward the end of August and is largely finished by early September. This decline offers relief to those sensitive to species like Kentucky bluegrass, rye, and fescue. The timing of this end date can shift, however, particularly in southern regions where warm weather extends the growing period.
Weed pollen, primarily ragweed, is the main cause of autumn allergies and is the last to conclude its release. For most of the country, the ragweed season lasts until the first major environmental event that stops plant growth, placing its end date typically in October or, in milder climates, extending into November.
Weather Events That Signal the End
The definitive end to the outdoor pollen season is signaled not by the calendar, but by specific meteorological events that physically halt plant reproduction. The most important of these is the arrival of the first hard frost, which effectively kills the remaining pollen-producing annual plants, like ragweed. A light frost, where temperatures dip briefly to 32 degrees Fahrenheit, may slow pollen release but does not stop it completely.
The necessary condition for a true end is a hard freeze, which is defined by temperatures dropping below 26 degrees Fahrenheit and remaining there for at least a few hours. The timing of this first hard freeze is what ultimately determines the final day of the seasonal outdoor allergy period.
Climate variability plays a significant role in shifting these end dates, as a delayed first frost directly translates to a longer allergy season. Regions experiencing a lengthening of the freeze-free period are finding that the weed pollen season, in particular, is extending later into the fall. Another weather factor that temporarily reduces pollen counts is heavy, sustained rainfall, which washes airborne grains out of the atmosphere.
Allergies That Persist Year Round
Indoor Allergens
When the pollen season finally concludes, not all allergy symptoms disappear, as certain allergens persist regardless of the outdoor temperature. These year-round, or perennial, allergies are often triggered by indoor substances that thrive in a controlled climate. The end of the pollen season simply shifts the focus of allergy management from outdoor exposure to controlling the environment within the home.
Common Perennial Triggers
Dust mites are a primary culprit, microscopic organisms that live in bedding, carpets, and upholstered furniture, feeding on shed human skin cells. Pet dander, which consists of tiny flakes of skin, hair, or feathers, is another common indoor allergen that continues to cause symptoms throughout the winter months. Mold spores can also be a factor, growing in damp indoor areas like basements, bathrooms, and kitchens, and they can also be found outdoors until sustained snow cover is present.