Portland's Allergy Identity
Portland has two distinct allergy seasons that together cover most of the calendar year. Alder — the Pacific Northwest's defining tree allergen — begins as early as January in mild years, making Portland's spring allergy season one of the earliest in the country. Then, from May through September, the Willamette Valley's vast grass seed farms — producing some of the highest grass pollen concentrations anywhere in the world — pump pollen into the Portland metro on south winds. It's a remarkable combination: ultra-early spring followed by ultra-intense summer.
Portland Monthly Pollen Calendar
The Willamette Valley Grass Problem
The Willamette Valley produces a significant share of the world's grass seed — primarily ryegrass, fescue, and orchardgrass — on hundreds of thousands of acres between Portland and Eugene. During June and July, when these grasses are in full pollen production, south winds carry Willamette Valley grass pollen directly into Portland, generating some of the highest grass pollen counts recorded anywhere in North America. June is Portland's worst allergy month for many residents — not March or April as might be expected from a northern city.
Alder: Portland's Defining Spring Allergen
Red alder is the Pacific Northwest's most clinically significant tree allergen. It blooms earlier than virtually any other allergenic tree in the US — beginning in January in mild years, typically February in average years. Portland residents who experience winter allergy symptoms starting in the grey months when nothing appears to be blooming are almost certainly reacting to alder. Alder pollen is fine, travels easily on wind, and is highly allergenic to a large portion of the sensitized population.
Alder shares allergenic proteins with birch, hazel, and hornbeam — cross-reactivity between these species is common in Portland-area allergy patients. Testing positive for alder may predict reactivity to the others.
The Rainy Season Advantage and Post-Rain Trap
Portland's rain genuinely suppresses pollen during precipitation. But the city's characteristic weather pattern — long dry spells interrupted by significant rain events — creates post-rain pollen spikes that can exceed surrounding days dramatically. The first dry, warm day following a multi-day rainy period in May or June often produces the highest pollen count of the month as accumulated Willamette Valley grass pollen releases simultaneously. Watch the forecast: the day after rain in Portland often requires more precautions than the dull grey days preceding it.
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Anthos tracks alder, birch, and Willamette Valley grass for your exact Portland location — with the species-specific data that matters in the Pacific Northwest.
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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.