When does allergy season start?

Allergy season isn't one event. It's a four-leg relay that runs February through October, with each pollen type handing off to the next. Here's who runs when, and why your particular two bad months land where they do.

The short answer

For most of the United States, pollen allergy symptoms begin somewhere between late February and early April and continue — with one or two pollen-type handoffs — until the first hard frost, usually in October. Mold spores overlap the whole thing.

If you've ever felt like you have allergies all year and then briefly feel normal in December, that's because you do. The trigger just changes identity every couple of months.

The four-leg relay

1. Tree pollen — February through May (peaks in April)

Tree pollen is the opening leg. In the southern half of the country — Texas, Florida, much of the Southeast and Southwest — cedar fever can begin in late December and peak by February. Cedar, juniper, and ash are the Southern winter-to-spring sequence.

Everywhere else, tree pollen kicks off in March and peaks in April. The usual suspects: oak, birch, maple, alder, elm, willow, and pine. Oak is the one you can see — a yellow-green film coating cars and patio furniture in April is almost always oak pollen.

Tree pollen is the heaviest-hitter of the year for most allergy sufferers. It's small, wind-borne, and produced in absurd quantities. A single mature oak can release more than 100 million grains in a season.

2. Grass pollen — May through July

As trees ease off in May, grasses take over. Timothy, Bermuda, ryegrass, Kentucky bluegrass, Johnson grass, and fescue are the big US offenders. Grass season peaks from late May through early July, with daily pollen counts highest between 10 AM and 3 PM on warm, breezy days.

A freshly mowed lawn doesn't release less pollen — it often releases more, at least for a day or two. Unmowed, seeded grasses are worse yet. If you live somewhere humid and green, grass season tends to be harder than tree season.

3. Weed pollen — August through October

The third leg is dominated by one plant: ragweed. A single ragweed plant produces roughly one billion pollen grains in a single season, and its pollen can travel more than 100 miles on the wind. Ragweed triggers symptoms in roughly 15–20% of Americans, and its season runs from early August until the first hard frost — typically mid-to-late October depending on latitude.

Other contributors in this leg: mugwort, lambsquarters, pigweed, sagebrush, and tumbleweed (mostly Western US).

4. Mold spores — spring through fall, spikes after rain

Mold doesn't follow the same calendar. Outdoor mold counts — mostly Alternaria and Cladosporium — are highest when it's warm and wet: after rainstorms, in piles of wet leaves, in compost. Indoor mold can be year-round. Mold is the one trigger that tends to get worse after rain clears the pollen out.

Why it starts earlier than it used to

Pollen season is getting longer. A 2021 study in the Proceedings of the National Academy of Sciences (Anderegg et al.) found that across North America, pollen seasons are measurably earlier, longer, and more intense than they were a generation ago.

The drivers: warmer winters (plants reach pollen-producing temperature thresholds sooner), earlier springs, and elevated atmospheric CO₂ — which, counterintuitively, makes many plants produce more pollen per flower. Every winter with fewer hard freezes is a winter where next year's pollen season starts earlier.

If you're certain your seasonal allergies have been getting worse year over year, you're not imagining it.

How to tell which pollen is getting you

The single best diagnostic tool is a calendar. Symptoms that start in March and fade by June are almost certainly tree pollen. Symptoms peaking in June and July are grass. Symptoms that start in August and end with the first frost are ragweed. Symptoms that spike after every rain are mold.

If you want to know precisely which trees, grasses, or molds are the culprits, the gold standard is a skin-prick test at an allergist's office — roughly 40–60 surface scratches with purified allergens, read 15 minutes later. Blood testing for specific IgE antibodies is less precise but more convenient and can be ordered by your primary care doctor. See "When to see a doctor" below.

What to do when your season starts

• Start antihistamines one to two weeks before your typical symptom start. Second-generation oral antihistamines (Claritin, Zyrtec, Allegra, Xyzal) work best as a daily baseline through the season, not reactively after symptoms hit.
• Start Flonase or another steroid nasal spray early. These take 3–5 days of daily use to reach full effect; waiting for the first symptom means you're already behind.
• Check the forecast. The home page shows a 5-day forecast plus day-by-day severity for your location. Plan outdoor time on lower-count days when you can.
• Close windows in the morning. Pollen counts peak between 5 AM and 10 AM.
• Shower before bed. Pollen collects in hair and on clothing; a five-minute rinse before sleep measurably cuts overnight symptoms.
• Put one HEPA filter in the bedroom. One unit where you sleep makes a bigger difference than three spread across the house.

When to see a doctor

See a doctor — ideally a board-certified allergist — if any of the following apply:

• Over-the-counter medication hasn't given adequate relief after two to three weeks of consistent daily use
• Symptoms are interfering with sleep, work, or normal activity
• You want to know specifically what you're allergic to (testing)
• You're interested in immunotherapy — allergy shots or sublingual tablets — which can reduce or eliminate seasonal allergies over three to five years for the right candidates
• You develop wheezing, chest tightness, or any difficulty breathing (possible allergy-related asthma, which is a separate diagnosis)

Call your local emergency number immediately if you experience severe symptoms like difficulty breathing, tongue or throat swelling, or a significant drop in blood pressure. These can be signs of anaphylaxis, which is a medical emergency regardless of cause.

Sources

• Anderegg, W. R. L., et al. (2021). Anthropogenic climate change is worsening North American pollen seasons. PNAS 118(7).
• Asthma and Allergy Foundation of America. Allergy Facts and Figures.
• American Academy of Allergy, Asthma & Immunology. AAAAI Pollen Library.
• National Allergy Bureau. NAB pollen-count reporting stations.
• Centers for Disease Control. Allergens and Pollen.

Check today's pollen reading for your area on the home page, or browse the rest of our reference guides.