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 in season.

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

In September, ragweed pollen is in season in San Francisco, peaking Aug–Oct. On San Francisco's mild Mediterranean California calendar the typical September level is Moderate, led by ragweed pollen — a seasonal estimate, not a live count.

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.

When does the next pollen season start in San Francisco?

Tree pollen is next in San Francisco: it opens around week 1 (early January), peaks around week 11, and runs about 122 days.

What is the allergy forecast in San Francisco today?

With no live reading for San Francisco today, its allergy forecast is the seasonal estimate above: a typical September level of Moderate, led by ragweed pollen.

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 September, ragweed pollen is in season in San Francisco, peaking Aug–Oct.

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 34 and a 274-day combined pollen season — its heaviest load is tree pollen, running 122 days.

#42of 100

Load index: 34
Combined season: 274 days
Heaviest load: Tree · 122 days

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

Typical now — Septemberseasonal estimate, not a live reading

Ragweed pollen typically leads in San Francisco in September, at an estimated Moderate level.

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–Apr

GrassMar–Jun

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 17 (late April), grass from week 10 (early March) to week 26 (late June), and ragweed from week 31 (early August) to week 43 (late October) here — a 274-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.

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 count scale and the other pollen & allergy guides.

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 in season.
What's in the air in San Francisco right now?: In September, ragweed pollen is in season in San Francisco, peaking Aug–Oct. On San Francisco's mild Mediterranean California calendar the typical September level is Moderate, led by ragweed pollen — a seasonal estimate, not a live count.
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.
When does the next pollen season start in San Francisco?: Tree pollen is next in San Francisco: it opens around week 1 (early January), peaks around week 11, and runs about 122 days.
What is the allergy forecast in San Francisco today?: With no live reading for San Francisco today, its allergy forecast is the seasonal estimate above: a typical September level of Moderate, led by ragweed pollen.