When is pollen worst in San Francisco?

Tree pollen jumps the gun in San Francisco, opening as early as Jan–Apr while colder regions are still dormant. Grass follows Mar–Jun and ragweed closes the year Aug–Oct. The early tree start is the trap — symptoms can begin before you expect them. Currently, ragweed pollen is what's driving counts this month.

What's in the air in San Francisco right now?

In August, ragweed pollen is in season in San Francisco — the dominant allergen you're likely reacting to right now. A live count, when available, confirms the day's actual reading; this reflects the typical peak windows for San Francisco's mild Mediterranean California climate.

Is tree or grass pollen higher in San Francisco in spring?

In spring, tree pollen leads in San Francisco — trees pollinate Jan–Apr, ahead of grass (Mar–Jun). The handoff is the tail of the tree window: tree counts taper as grass climbs, so an early-spring flare is more likely tree pollen and a late-spring one more likely grass.

What makes San Francisco's pollen season distinctive?

San Francisco sits in the mild Mediterranean California zone, which means an early start — mild winters pull tree pollen forward into late winter, ahead of a spring grass peak and a relatively contained ragweed fall. That shapes when symptoms hit and which allergen to watch.

How do I reduce pollen exposure in San Francisco?

Through San Francisco's peak windows (tree Jan–Apr, grass Mar–Jun, ragweed Aug–Oct), keep windows shut and run AC on recirculate; counts run highest on dry, warm, windy mornings, so push outdoor activity to late afternoon or just after rain, which clears pollen from the air. A HEPA purifier indoors, a saline rinse after being outside, showering before bed, and starting antihistamines a week or two before your worst local window all measurably cut symptoms.

What pollen index counts as high?

Pollen is reported on a categorical scale — None, Low, Moderate, High, and Very High. "High" and above means most allergy sufferers notice symptoms even with brief outdoor exposure, and sensitized people should limit time outside and pre-medicate. "Low" to "Moderate" usually only affects highly sensitive individuals.

What is the allergy forecast in San Francisco today?

The allergy forecast is the airborne-pollen outlook that drives hay fever — exactly what this page tracks. Today's pollen level for San Francisco is shown above, broken into the tree, grass, and ragweed pollen behind most seasonal allergies. When the reading is High or Very High, plan for symptoms and pre-medicate; the seasonal calendar below shows which allergen leads in each part of the year.

San Francisco, California Pollen Count & Allergy Forecast

San Francisco pollen count and allergy forecast — tree, grass, and ragweed seasons and what’s pollinating now

San Francisco, CA · Pollen season

In August, ragweed pollen is in season in San Francisco — the dominant allergen you're likely reacting to right now.

Based on the seasonal pollen calendar for this region.

San Francisco on the WeatherStory Allergy Index

San Francisco ranks #42 of 100 U.S. metros on the WeatherStory Allergy Index, with a modeled load index of 41 and a 304-day combined pollen season — its heaviest load is tree pollen, running 152 days.

#42of 100

Load index: 41
Combined season: 304 days
Heaviest load: Tree · 152 days

See the full 100 worst U.S. cities for pollen — a modeled seasonal load ranking (see methodology).

Typical now — Augustseasonal estimate, not a live reading

Ragweed pollen typically leads in San Francisco this time of year, at an estimated Low level. No live count is wired up; this reflects the seasonal calendar.

Pollen by type this season

TreeOut of seasonpeaks Jan–Apr
GrassOut of seasonpeaks Mar–Jun
Weed / RagweedIn seasonpeaks Aug–Oct

What pollinates near San Francisco

The tree, grass, and weed species that drive San Francisco’s pollen seasons, with each plant’s typical peak.

Trees

Oak Quercuspeaks March–April
Olive Olea europaeapeaks April–May
Coast live oak Quercus agrifoliapeaks March–April
Walnut Juglanspeaks April–May
Ash Fraxinuspeaks February–March
Mulberry Moruspeaks February–March

Grasses

Bermuda grass Cynodon dactylonpeaks April–June
Wild oat Avena fatuapeaks April–May
Perennial ryegrass Lolium perennepeaks April–June
Bromegrass Bromuspeaks April–May

Weeds

Common ragweed Ambrosia artemisiifoliapeaks August–September
Russian thistle Salsola traguspeaks July–September
Pigweed Amaranthuspeaks August–September
Sagebrush Artemisiapeaks August–September

Species are the region’s common wind-pollinated plants; peak months are a seasonal estimate, not a measured count — see methodology.

San Francisco’s allergy year

Typical month-by-month pollen intensity for San Francisco, tuned to its latitude and climate region — darker means higher typical counts. The highlighted column is the current month.

JFMAMJJASOND

TreeJan–May

GrassMar–Jul

RagweedAug–Oct

How San Francisco compares to the U.S. average

Each colored bar is San Francisco’s typical season against the national-average window (grey). The offset is the per-city timing shift.

Tree~7 wks earlier

Grass~7 wks earlier

Ragweedabout average

“U.S. average” is the textbook tree Mar–May, grass May–Jul, ragweed Aug–Oct calendar. The shift is a phenology estimate (Hopkins’ bioclimatic law), not a measured count — see methodology.

How San Francisco’s pollen count works

The calendar above is tuned to San Francisco’s mild Mediterranean California climate, not a national average: tree pollen runs from week 1 (early January) to week 18 (early May), grass from week 10 (early March) to week 27 (early July), and ragweed from week 31 (early August) to week 40 (early October) here — a 304-day combined pollen season. Those windows are why ragweed pollen is the one in season in San Francisco right now.

No live count is wired up for San Francisco today, so the seasonal calendar above is your guide to which allergen is in season. Counts run highest on warm, dry, windy mornings and drop after rain, which washes pollen out of the air — reported on the None / Low / Moderate / High / Very High scale.

Frequently asked

When is pollen worst in San Francisco?: Tree pollen jumps the gun in San Francisco, opening as early as Jan–Apr while colder regions are still dormant. Grass follows Mar–Jun and ragweed closes the year Aug–Oct. The early tree start is the trap — symptoms can begin before you expect them. Currently, ragweed pollen is what's driving counts this month.
What's in the air in San Francisco right now?: In August, ragweed pollen is in season in San Francisco — the dominant allergen you're likely reacting to right now. A live count, when available, confirms the day's actual reading; this reflects the typical peak windows for San Francisco's mild Mediterranean California climate.
Is tree or grass pollen higher in San Francisco in spring?: In spring, tree pollen leads in San Francisco — trees pollinate Jan–Apr, ahead of grass (Mar–Jun). The handoff is the tail of the tree window: tree counts taper as grass climbs, so an early-spring flare is more likely tree pollen and a late-spring one more likely grass.
What makes San Francisco's pollen season distinctive?: San Francisco sits in the mild Mediterranean California zone, which means an early start — mild winters pull tree pollen forward into late winter, ahead of a spring grass peak and a relatively contained ragweed fall. That shapes when symptoms hit and which allergen to watch.
How do I reduce pollen exposure in San Francisco?: Through San Francisco's peak windows (tree Jan–Apr, grass Mar–Jun, ragweed Aug–Oct), keep windows shut and run AC on recirculate; counts run highest on dry, warm, windy mornings, so push outdoor activity to late afternoon or just after rain, which clears pollen from the air. A HEPA purifier indoors, a saline rinse after being outside, showering before bed, and starting antihistamines a week or two before your worst local window all measurably cut symptoms.
What pollen index counts as high?: Pollen is reported on a categorical scale — None, Low, Moderate, High, and Very High. "High" and above means most allergy sufferers notice symptoms even with brief outdoor exposure, and sensitized people should limit time outside and pre-medicate. "Low" to "Moderate" usually only affects highly sensitive individuals.
What is the allergy forecast in San Francisco today?: The allergy forecast is the airborne-pollen outlook that drives hay fever — exactly what this page tracks. Today's pollen level for San Francisco is shown above, broken into the tree, grass, and ragweed pollen behind most seasonal allergies. When the reading is High or Very High, plan for symptoms and pre-medicate; the seasonal calendar below shows which allergen leads in each part of the year.

Pollen & allergy guides

For the science behind San Francisco’s pollen seasons — how counts are measured, when each type peaks, and what the numbers mean — see the pollen & allergy guides.

More for San Francisco

See the full San Francisco, CA weather forecast — hour-by-hour outlook, NOAA radar, satellite, and air quality.