San Marcos pollen forecast - today, tomorrow and the week ahead
Pollen Today
Today (20 September) pollen in San Marcos is high; ragweed is high; grass is moderate; mugwort is low.
Pollen Tomorrow
Tomorrow (21 September 2026) grass remains moderate, ragweed remains high, mugwort no longer expected.
7-Day Trend
Over the next week grass, mugwort, and ragweed will increase.
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Pollen Risk
Today - 20 September
| Pollen species | Level | Value |
|---|---|---|
| Alder (Alnus) | None | 0/100 |
| Birch (Betula) | None | 0/100 |
| Hazel (Corylus) | None | 0/100 |
| Cypress (Cupressaceae) | None | 0/100 |
| Grass (Poaceae) | Moderate | 39/100 |
| Willow (Salicaceae) | None | 0/100 |
| Mugwort (Artemisia) | Low | 10/100 |
| Ragweed (Ambrosia) | High | 70/100 |
Tomorrow - 21 September
| Pollen species | Level | Value | 7-Day Trend |
|---|---|---|---|
| Alder (Alnus) | None | 0/100 | — |
| Birch (Betula) | None | 0/100 | — |
| Hazel (Corylus) | None | 0/100 | — |
| Cypress (Cupressaceae) | None | 0/100 | — |
| Grass (Poaceae) | Moderate | 29/100 | |
| Willow (Salicaceae) | None | 0/100 | — |
| Mugwort (Artemisia) | None | 0/100 | |
| Ragweed (Ambrosia) | High | 61/100 |
Local Environmental Conditions
San Marcos' parks and the San Marcos Creek corridor feature oak, eucalyptus, and introduced trees as the main pollen sources, with exposure varying between these green areas and the more built-up city centre. Surrounding San Diego County chaparral and the Palomar Mountain foothills contribute substantial grass and scrub pollen, while the enclosed inland valley concentrates pollen under prevailing offshore winds.
San Marcos' pollen season in Texas opens with cedar and elm from December, with mountain cedar producing intense early-season pollen. Oak follows through March and April, with grass peaking through May and June. Ragweed extends the season into October. The Blanco and San Marcos river valleys channel airflows, while surrounding Texas Hill Country contributes substantial cedar and grass pollen under prevailing southerly winds.
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Resource Hub
Pollen forecast neighborhoods 1 neighborhood
Sports venues in San Marcos 2 sports venues
Frequently Asked Questions About Pollen in California
Is pollen high in San Marcos right now?
On 20 September 2026, overall pollen levels in San Marcos are high. Ambrosia is the main contributor to airborne pollen, while grass and artemisia remain at lower levels
Will pollen levels change in San Marcos tomorrow?
On 21 September 2026, pollen conditions in San Marcos will be different from today, with ambrosia at high levels, grass at moderate levels
Is pollen expected to decrease in San Marcos this week?
According to the five-day outlook, ambrosia, artemisia, and grass pollen levels will increase later in the week.
Which pollen types are currently active in San Marcos?
On 20 September 2026, Ambrosia is the dominant pollen source in San Marcos. grass and artemisia are also present but at lower levels.
When is pollen season in San Marcos?
How often is the San Marcos pollen forecast updated?
The San Marcos pollen forecast is updated four times daily. See the timestamp below for the most recent update.
Does pollen in San Marcos come from nearby or far away?
Studies show that most pollen exposure (often 70–90%) comes from nearby sources, such as local trees and vegetation. A good pollen forecast therefore depends on accurately representing local vegetation. Pollen can travel long distances, but this usually matters only early in the season or during specific wind events.
Are pollen levels in San Marcos based on measurements or forecasts?
Pollen data is often misunderstood. Most "pollen levels" are not real-time measurements, but forecasts.
Direct pollen measurements are typically delayed by 1-2 days because they require manual analysis. To show current conditions in San Marcos, modern forecasts combine recent measurements, weather data, and information about local vegetation.
This allows us to estimate pollen levels for today and the coming days, not just report the past.
Airmine’s atmospheric monitoring platform was developed with support from the European Space Agency (ESA)’s Business Applications and Space Solutions (BASS) team, using satellite and Earth observation technologies to improve emissions monitoring.
Understanding the Scale
The 0–100 scores represent risk levels, not direct measurements of pollen or air pollutant concentrations. Higher scores indicate higher risk. Pollen and air quality use different thresholds.
Pollen
| None | 0 | No or negligible risk of exposure. Pollen/mold exposure is expected to be minimal. |
| Low | 0-25 | Low risk of exposure. Small amounts of pollen or mold spores may be present. |
| Moderate | 25-50 | Moderate risk of exposure. Pollen/mold exposure is expected to be noticeable. |
| High | 50-75 | High risk of exposure. Elevated pollen/mold exposure is expected. |
| Very High | 75-100 | Very high risk of exposure. Pollen/mold exposure is expected to be at or near its highest levels. |
Get More in the Airmine Pollen app
Get 15-day pollen forecasts with morning, afternoon and evening detail, plus 5-day air quality forecasts. Explore interactive pollen risk maps, estimated pollen season start, route profiles and a symptoms diary.
Features and forecast availability vary by region. Currently covering Europe, the contiguous US, southern Canada, South Africa, Australia, Indonesia (Java & Bali), Brazil, Paraguay, Uruguay and the Buenos Aires area.
Developed with support from the European Space Agency (ESA) Business Applications and Space Solutions (BASS).
