Is it allergy season in California?

Most US states have a clear start and end to allergy season. California doesn't. The combination of a Mediterranean climate, a 900-mile latitudinal span, and a mix of native and invasive introduced plants (mulberry, olive, eucalyptus) means someone in the state is reacting to something in nearly every month of the year.

California is enormous — Eureka and San Diego are farther apart than Boston and Atlanta. Climate, elevation, and the mix of native versus introduced ornamental trees creates four meaningfully different allergen environments.

Northern California

Birch, alder, and oak — a tree-heavy spring

NorCal follows a more traditional allergy calendar. Alder and birch kick off in February, peak in March, then yield to oak in April. The wet winters that NorCal needs for its water supply are the same conditions that prime heavy spring pollen releases — warm, dry April days after a wet February are peak suffering.

Oak · Apr–May Birch · Mar–Apr Alder · Feb–Mar Grass · May–Jul

Bay Area

Extended season, moderated by marine layer

The Bay's coastal fog actually helps allergy sufferers — it suppresses pollen counts on foggy mornings and washes pollen out of the air. Inland valleys (Concord, Livermore, San Jose) see higher counts than coastal neighborhoods in the same metro. The overlap of oak, grass, and ornamental trees (olive, mulberry, London plane) in April–May makes that the hardest stretch.

Oak · Mar–May Olive · Apr–Jun Grass · May–Jul Marine layer helps

Central Valley

The state's most intense allergen environment

The San Joaquin and Sacramento Valleys have some of the highest grass pollen concentrations recorded in the United States. Orchard farming (almonds, pistachios, walnuts) creates massive bloom events in February–March, and commercial olive groves add a May–June punch. There's no marine layer to provide relief — hot, dry, windy days push counts to Very High for weeks at a stretch.

Grass · May–Aug Olive · Apr–Jun Walnut / Almond · Feb–Mar Ragweed · Aug–Oct

Southern California

Near-year-round activity with a winter eucalyptus wave

SoCal is the toughest zone for allergy sufferers because there's rarely a real break. Eucalyptus (introduced from Australia and planted widely as a windbreak) blooms in January–February. Mulberry — planted as a fast-growing ornamental — follows in March and is one of the most potent spring allergens in LA. Bermuda grass runs all summer, and sagebrush carries fall into November. Ragweed season is milder than the Midwest but still significant in inland areas.

Eucalyptus · Jan–Mar Mulberry · Feb–May Bermuda grass · May–Sep Sagebrush · Jul–Nov

Each row shows a full year of pollen for one region — trees in blue, grasses in green, weeds in amber. Look for where all three rows go quiet at the same time — that's your window.

Oral Allergy Syndrome (also called pollen-food allergy syndrome) causes tingling or mild itching in the mouth when you eat certain raw foods during the relevant pollen season. The proteins in the food are similar enough to the pollen protein that your immune system cross-reacts. Cooking usually deactivates the offending protein, so the same food cooked may cause no reaction.

Oak & Birch pollen

Stone fruits & apples

Apples, peaches, cherries, plums, pears, almonds, hazelnuts, carrots, celery

Peak risk March–May in CA. Peeling fruit helps — most of the cross-reactive protein is in the skin.

Grass pollen

Tomatoes, potatoes & melons

Tomatoes, potatoes, kiwi, watermelon, cantaloupe, oranges (in some cases)

Peak risk May–July. Central Valley sufferers often notice this at the same time grass counts spike in the heat.

Ragweed / Mugwort

Melons & bananas

Watermelon, cantaloupe, honeydew, banana, zucchini, cucumber, sunflower seeds

Peak risk August–October. Mugwort is the dominant fall weed allergen in inland CA and cross-reacts more broadly than ragweed alone.

Not medical advice. If you suspect OAS, speak with an allergist — it can sometimes progress, and symptoms that extend beyond the mouth should be evaluated.