The Short Answer
Allergy season in the US doesn't have a single start date — it has three overlapping waves that cover most of the year in most regions. If you're asking when YOUR allergy season starts, the answer depends on where you live and what you're allergic to. Here's the complete picture.
When Each Pollen Wave Starts — By Region
| Region | Tree Pollen Starts | Grass Starts | Ragweed Starts | Season Length |
|---|---|---|---|---|
| Deep South (TX, FL, GA) | January–February | April–May | August | 10–11 months |
| Mid-Atlantic / Southeast | February–March | May | August | 9–10 months |
| Northeast (NY, NE) | March–April | May–June | Mid-August | 7–8 months |
| Midwest | April | May–June | Early August | 7–8 months |
| Pacific Northwest | January–February (alder) | May–September | August–October | 9–10 months |
| Southwest (AZ, NM) | January–February (desert trees) | March–October | July–October | 10+ months |
What Triggers the Start of Tree Pollen Season
Trees don't bloom on calendar dates — they bloom in response to accumulated warmth, measured in "growing degree days" (GDDs). GDDs are calculated by tracking daily temperatures above a threshold. When a tree species accumulates enough GDDs, it blooms and releases pollen. This is why allergy season timing shifts year to year: a warm February accelerates GDD accumulation and triggers early blooming; a cold, wet spring delays it.
Climate Central's research shows average pollen season start dates have moved 20 days earlier across North America since 1990 as winters have shortened and springs have warmed. In practical terms, allergy season starts earlier today than it did a generation ago — in some regions, dramatically so.
The Three Waves in Detail
Wave 1: Tree Pollen (Late Winter – Late Spring)
The first pollen wave. Trees bloom before they leaf out, producing abundant pollen that travels long distances. In the South, this begins in January or February with cedar, elm, and early deciduous trees. Oak is the dominant species across much of the East and South, peaking in April or May depending on region. Birch dominates the Northeast and Pacific Northwest. Duration: roughly 8–10 weeks of significant exposure per region.
Wave 2: Grass Pollen (Late Spring – Summer)
The second wave overlaps with tree season's end. Grass pollen season runs from May through August across most of the US — with peak typically in June. Bermuda grass (South and Southwest), Timothy (Northeast and Midwest), and Kentucky bluegrass are the dominant species. Grass pollen has a unique nighttime peak between 10 PM and 2 AM. Duration: 12–16 weeks in most regions.
Wave 3: Ragweed and Weeds (Late Summer – First Frost)
The final wave. Ragweed is the primary allergen from August through October or November depending on when the first hard freeze arrives. A single ragweed plant produces up to 1 billion pollen grains per season, and the pollen travels hundreds of miles on wind. September is peak ragweed month across most of the continental US. Season ends definitively with the first hard freeze.
The "Shoulder Season" Gap Most People Miss
Late July is often the lowest-pollen period of the year across most of the US — after grass season has mostly concluded and before ragweed has begun in earnest. This 2–3 week window in late July to early August represents genuinely lower allergen burden in most regions. If you've been planning an outdoor event, travel, or outdoor project that's been delayed by allergy season, late July is your window.
How to Know When YOUR Season Starts
The most precise answer to "when does allergy season start for me" requires two pieces of information: your specific allergen sensitivities (from allergy testing) and your local pollen calendar for those specific species. Without testing, you're managing the generic season. With testing, you know that your season starts in April when oak appears — and that the pine pollen in March that turns cars yellow isn't actually your problem.
Know when your season starts and ends.
Anthos tracks your specific allergens daily and shows active season countdowns — so you know how many days until your worst allergen peaks, and how many until it's over.
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Anthos provides general wellness information only. Nothing in this article constitutes medical advice, diagnosis, or treatment. Always consult a licensed healthcare professional before making health decisions.