When Is Pollen Season? Tree, Grass & Ragweed Allergy Calendar (2026)
Written by:Â Â Dr.Muhammad Ihsan Ullah, PhD
Medically reviewed by:Â Dr. Jamal ud Din Khan, FCPS
Last updated on May 20,2026
Pollen season is no longer a short, predictable spring event. In many regions, it now starts earlier, lasts longer, and shows higher airborne pollen concentrations than in previous decades. Climate change, urban air pollution, and shifts in plant biology have all contributed to this extended allergy burden (Zhang & Steiner, 2022).
For people sensitive to tree pollen, grass pollen, weed pollen, or ragweed pollen, understanding when each season begins and peaks is essential for planning outdoor activity and reducing exposure.
This 2026 pollen allergy season calendar explains when pollen season starts and ends, when tree pollen season starts, when grass pollen is highest, and when ragweed season becomes worst.
When does pollen season start and end?
Pollen season does not follow a single fixed timeline. It changes based on climate, region, altitude, and plant species.
In general:
- Tree pollen season begins in late winter or early spring
- Grass pollen season peaks in late spring and summer
- Weed and ragweed pollen season dominates late summer and fall
Warmer climates often experience earlier starts and longer durations, while colder regions have shorter but more intense peaks (DâAmato et al., 2020).
Why are pollen seasons changing?
Modern research confirms that climate change is reshaping pollen patterns worldwide.
Key changes include:
- Earlier spring pollen release
- Longer pollen seasons
- Increased pollen production per plant
- Higher atmospheric pollen concentration
- Expanded ragweed distribution
Rising carbon dioxide levels enhance plant growth and pollen output, particularly in highly allergenic species like ragweed (Zhang & Steiner, 2022).
What months are worst for pollen allergies?
The severity of allergy symptoms depends on pollen type and overlap between seasons.
Typical peak periods:
- Tree pollen â March to May
- Grass pollen â May to July
- Weed pollen â August to October
- Ragweed pollen â September (peak)
September is often the most difficult month for seasonal allergy sufferers due to intense ragweed exposure combined with lingering weed pollen.
Tree pollen season calendar
Tree pollen is the first major wave of the allergy year and often begins before leaves fully develop.
When does tree pollen season start?
- Warm climates: JanuaryâFebruary
- Temperate climates: FebruaryâMarch
- Cold climates: MarchâApril
Early-season trees include cedar, juniper, alder, and elm.
What months is tree pollen highest?
Peak tree pollen periods:
- March: oak, birch, ash
- April: maple, oak, pine
- May: late oak and birch overlap
Oak and birch are among the most clinically important tree allergens.
Which trees cause the strongest allergies?
Common high-impact tree allergens include:
- Oak
- Birch
- Cedar
- Juniper
- Ash
- Elm
- Maple
- Poplar
These trees produce lightweight wind-dispersed pollen that can travel long distances.
Why is tree pollen worse in spring?
Tree pollen is released before full leaf development, allowing it to disperse freely. Dry, windy spring conditions significantly increase airborne concentration.
Grass pollen season calendar
Grass pollen follows tree pollen and dominates late spring and summer.
It is one of the most widespread causes of seasonal allergies globally.
When is grass pollen highest?
Grass pollen peaks:
- Late spring (May)
- Early summer (JuneâJuly)
In warmer regions, grass pollen may extend into August.
What months does grass pollen peak?
- May: rising levels
- June: peak intensity
- July: sustained high levels
- August: declining or extended depending on climate
Which grasses cause allergies?
Common allergenic grasses include:
- Timothy grass
- Bermuda grass
- Ryegrass
- Kentucky bluegrass
- Johnson grass
Orchard grass
Why is grass pollen worse in summer?
Hot, dry, and windy conditions enhance pollen dispersal. Lawn mowing can also sharply increase local airborne pollen exposure.
What time of day is grass pollen highest?
Grass pollen is often highest:
- Early morning
- Late afternoon
Outdoor activities during mowing periods can significantly increase exposure.
Weed pollen season calendar
Weed pollen season begins in late summer and continues into fall.
Many individuals underestimate fall allergies, assuming spring is the only allergy season.
When does weed pollen season begin?
- Warm regions: July
- Temperate regions: August
What months is weed pollen worst?
- August: early rise
- September: peak exposure
- October: decline phase
Which weeds cause fall allergies?
Common weed allergens include:
- Ragweed
- Mugwort
- Pigweed
- Sagebrush
- Lambâs quarters
- Russian thistle
Why do weed allergies worsen in late summer?
Dry air, wind activity, and reduced rainfall increase airborne pollen levels. Overlapping mold spores may also worsen symptoms.
Ragweed pollen season calendar
Ragweed and weed pollen calendar 2026 infographic showing fall allergy peak months, weed pollen timeline, symptoms, and ragweed exposure risks
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Ragweed is one of the most potent seasonal allergens worldwide.
A single plant can release billions of pollen grains during one season.
When does ragweed season start and end?
- Starts: August
- Peaks: September
- Ends: October or first frost
In warmer climates, ragweed may persist into November.
What month is ragweed pollen highest?
September is the peak ragweed month in most regions.
Why is ragweed allergy so severe?
Ragweed pollen is:
- Extremely lightweight
- Highly wind-dispersed
- Produced in massive quantities
- Highly allergenic at low exposure levels
Its ability to travel long distances contributes to widespread seasonal outbreaks.
How far can ragweed pollen travel?
Ragweed pollen can travel hundreds of kilometers through atmospheric currents, affecting regions far from its origin.
Why is ragweed season getting longer?
Rising temperatures and delayed frost extend the growing season, allowing longer pollination periods (DâAmato et al., 2020).
Monthly pollen allergy calendar 2026
- January: cedar, juniper
- February: alder, elm
- March: oak, birch
- April: oak, maple
- May: tree peak + grass start
- June: grass peak
- July: grass + early weeds
- August: weeds + ragweed onset
- September: ragweed peak
- October: weed decline
- November: late weed pollen (warm regions)
- December: minimal pollen
Which pollen types dominate each season?
Spring
Tree pollen dominates.
Summer
Grass pollen dominates.
Fall
Weed and ragweed pollen dominate.
Which regions have the longest pollen seasons?
Warmer regions typically experience:
- Earlier pollen onset
- Longer seasonal duration
- Higher annual pollen load
Colder regions have shorter but sharper seasonal peaks.
Does weather affect pollen levels?
Yes, weather strongly influences pollen concentration.
Wind
Increases airborne pollen transport.
Rain
Temporarily reduces pollen but may increase mold exposure.
Heat
Increases plant growth and pollen production.
Humidity
May reduce dispersal but increase indoor allergen persistence.
Why are allergies worse in the morning?
Yes, weather strongly influences pollen concentration.
Wind
Increases airborne pollen transport.
Rain
Temporarily reduces pollen but may increase mold exposure.
Heat
Increases plant growth and pollen production.
Humidity
May reduce dispersal but increase indoor allergen persistence.
Why are allergies worse in the morning?
Many plants release pollen early in the day. Cooler air and lower dispersion rates allow pollen to accumulate near ground level.
Is climate change making pollen seasons worse?
Yes. Multiple studies confirm:
- Longer pollen seasons
- Higher pollen production
- Earlier seasonal onset
- Increased allergen potency
These changes are strongly associated with rising COâ levels and global warming (Zhang & Steiner, 2022).
Why do seasonal allergies feel worse today?
Several environmental factors contribute:
- Air pollution exposure
- Longer pollen seasons
- Urban heat islands
- Cross-reactive allergens
- Higher pollen concentrations
Together, these factors increase both exposure and immune sensitivity.
Pollen season comparison (quick guide)
Pollen Type | Start | Peak | End | Severity |
Tree | JanâApr | MarâMay | MayâJun | High (spring spike) |
Grass | AprâJun | MayâJul | JulâAug | ModerateâHigh |
Weed | JulâAug | AugâOct | OctâNov | Moderate |
Ragweed | Aug | Sep | OctâNov | Very High |
Peak allergy severity by month (2026 overview)
Month | Dominant Pollen | Risk Level |
JanâFeb | Cedar, early trees | LowâModerate |
MarâApr | Tree pollen | Very High |
MayâJun | Tree + grass overlap | High |
Jul | Grass + weeds | ModerateâHigh |
Aug | Weeds + ragweed start | High |
Sep | Ragweed peak | Very High |
Oct | Ragweed + weeds | High |
NovâDec | Minimal pollen | Low |
Key takeaways
- Tree pollen dominates early spring
- Grass pollen dominates late spring and summer
- Weed and ragweed pollen dominate fall
- September is the peak ragweed month
- Climate change is extending pollen seasons
- Weather strongly influences pollen exposure
- Regional climate determines allergy timing
Medical Disclaimer
This article is for educational and informational purposes only and is not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider or allergist for personalized guidance regarding allergy symptoms or medical conditions.
Frequently Asked Questions (FAQs)
1. When does pollen season start and end?
Pollen season usually starts in late winter or early spring with tree pollen, peaks in late spring and summer with grass pollen, and continues into fall with weed and ragweed pollen.
2. What month is pollen the worst?
September is often the worst month for pollen allergies due to peak ragweed exposure, especially in temperate regions.
3. When does tree pollen season begin?
Tree pollen season typically begins between January and April depending on the climate, with early pollens like cedar, alder, and elm appearing first.
4. When is grass pollen highest?
Grass pollen is usually highest from May to July, with peak levels occurring in late spring and early summer.
5. Why are allergies worse in the fall?
Fall allergies are mainly caused by ragweed and weed pollens, which release large amounts of airborne pollen during late summer and early autumn under dry, windy conditions.
References
- Zhang, Y., & Steiner, A. L. (2022). Projected climate-driven changes in pollen emission season length and magnitude over the continental United States. Nature Communications, 13, 1234. https://doi.org/10.1038/s41467-022-28764-0
- DâAmato, G., Vitale, C., De Martino, A., et al. (2020). Effects of climate change on allergic diseases: A review of the literature. International Journal of Environmental Research and Public Health, 17(9), 3077. https://doi.org/10.3390/ijerph17093077