Gresham pollen forecast - today, tomorrow and the week ahead
Pollen Today
Today (20 September) pollen in Gresham is high; ragweed is high; grass is moderate; mugwort is low.
Pollen Tomorrow
Tomorrow (21 September 2026) mugwort remains low, grass declining, ragweed declining.
7-Day Trend
Over the next week mugwort will increase; grass and ragweed will decrease.
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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 | 35/100 |
| Willow (Salicaceae) | None | 0/100 |
| Mugwort (Artemisia) | Low | 22/100 |
| Ragweed (Ambrosia) | High | 64/100 |
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) | Low | 16/100 | |
| Willow (Salicaceae) | None | 0/100 | — |
| Mugwort (Artemisia) | Low | 3/100 | |
| Ragweed (Ambrosia) | Moderate | 44/100 |
Local Environmental Conditions
Gresham's parks, including Nadaka Nature Park and the Sandy River corridor, feature alder, oak, and Douglas fir as the main tree pollen sources, with exposure varying between these green areas and the more built-up city centre. Surrounding Pacific Northwest woodland and Willamette Valley farmland contribute substantial tree and grass pollen, while the Columbia River Gorge channels easterly winds that can concentrate pollen across the Portland metro area.
Gresham's pollen season opens with alder from January, followed by birch and Douglas fir through March and April. Grass pollen peaks through June and July. The Columbia River Gorge channels easterly winds that can concentrate pollen across the Portland metro area, while surrounding Pacific Northwest woodland and Willamette Valley farmland contribute substantial tree and grass pollen. Frequent Pacific rainfall provides regular interruptions throughout the season.
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Frequently Asked Questions About Pollen in Oregon
Is pollen high in Gresham right now?
On 20 September 2026, overall pollen levels in Gresham are high. Ambrosia is the main contributor to airborne pollen, while grass and artemisia remain at lower levels
Will pollen levels change in Gresham tomorrow?
On 21 September 2026, pollen conditions in Gresham will be different from today, with artemisia at low levels and grass, ambrosia continuing to decline
Is pollen expected to decrease in Gresham this week?
According to the five-day outlook, grass and ambrosia pollen levels are expected to gradually decrease, while artemisia pollen will increase, suggesting mixed conditions later in the week.
Which pollen types are currently active in Gresham?
On 20 September 2026, Ambrosia is the dominant pollen source in Gresham. grass and artemisia are also present but at lower levels.
When is pollen season in Gresham?
How often is the Gresham pollen forecast updated?
The Gresham pollen forecast is updated four times daily. See the timestamp below for the most recent update.
Does pollen in Gresham 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 Gresham 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 Gresham, 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).
