What Allergies Are Bad This Month? Month-by-Month U.S. Pollen Calendar

U.S. Allergy Season Calendar: Which Allergens Peak Each Month?

Tree pollen usually dominates from late winter through spring, grass pollen from late spring through summer, and ragweed and other weed pollen from late summer through fall. Outdoor mold can overlap these seasons; dust mites, pets, and indoor mold can cause symptoms throughout the year. Mild climates and winter juniper in Central Texas are important exceptions. AAAAI: outdoor allergens

There is no single worst allergy month for the whole United States. Latitude, elevation, local plants, temperature, rain, and frost can move a city's peak by weeks. This calendar helps you identify likely triggers before comparing them with nearby measurements, forecasts, and your symptoms.

Editorial update: September 7, 2026. Monthly entries describe typical seasonal patterns. The September section below is seasonal guidance, not a live national pollen report.

What Allergies Are Bad This Month? September 2026

Ragweed is a leading September trigger, especially in the East and Midwest. In many areas, concentrations are highest in early to mid-September, although the exact peak varies. ACAAI: seasonal allergies

Depending on where you live, also check for:

• Other weed pollens: Sagebrush, mugwort, pigweed, lamb's-quarters, and Russian thistle can contribute, including in western and drier regions.
• Continuing grass pollen: Warm, irrigated, coastal, or cooler high-elevation locations may have grass exposure after the usual early-summer peak.
• Outdoor mold: Decaying vegetation, compost, crop material, and yard work can release spores. Some molds spread more readily in dry wind; others favor humid conditions.

Ragweed often continues into October and can last longer in mild areas. A killing frost usually reduces local ragweed production, but it does not end indoor allergies or eliminate outdoor mold. AAAAI: ragweed, AAFA: mold allergy

For today's decision: Check the local report's date, location, and allergen category. Check air quality separately when smoke or ozone is a concern. A seasonal calendar cannot establish that your city has high pollen today.

At-a-Glance 12-Month U.S. Pollen Matrix

These are broad planning windows, not measured monthly risk ratings. Mold produces spores, not pollen. Indoor allergens are included where they help explain symptoms outside the main pollen season.

How to use the matrix

Use the calendar to ask a better question:

Which category should I compare with my local data this week?

Then check:

• Tree, grass, and weed levels, plus mold counts where available.
• The dominant plant or category.
• Recent rainfall, heat, wind, drought, or frost.
• Whether symptoms followed outdoor or indoor exposure.
• Whether the same pattern repeats each year.

Use the categories to guide a symptom diary and focused testing when needed. A pollen-only forecast does not tell you the outdoor mold level.

Why the U.S. Allergy Calendar Is Shifting

Longer freeze-free growing seasons

Climate Central's March 2026 analysis used temperature records from 1970 through 2025. The freeze-free growing season lengthened in 173 of 198 analyzed U.S. cities, by 21 days on average among those cities. Regional averages also increased across the eight climate regions included in its regional comparison. Climate Central: 2026 analysis and methodology

This measures the interval between the last spring freeze and first fall freeze. It is a climate indicator, not a direct measurement of pollen-season length or a prediction of how severe September 2026 will be in your city.

What the 2026 CDC–NAB study adds

A study published on June 27, 2026 describes standardized pollen-season metrics developed from 2003–2024 National Allergy Bureau observations, with CDC and academic collaborators. It covers historical and annual timing measures for selected pollen types, plus grouped daily and weekly products. Manangan and colleagues, Atmosphere

Three limitations matter when using it:

• It describes historical exposure. A daily historical record is not a current-day forecast.
• Coverage varies. Locations, years, and pollen types have different reporting completeness. The paper's count of active stations refers to reporting through 2024, not a verified September 2026 network total.
• Some grouped grass metrics are excluded under NAB data-sharing policies. The products should not be presented as a complete, uniform tree–grass–weed feed for every location.

The study supports regional comparisons, including earlier tree-season starts in the Southeast than at higher latitudes. It does not establish one nationwide start date or a personal symptom threshold. The calendar here is an editorial synthesis, not a calculation from the underlying dataset.

Observed pollen changes are different from growing-season changes

A 2021 study of 60 U.S. and Canadian stations from 1990–2018 found that pollen seasons started about 20 days earlier. Its results section reports roughly 8 additional days of season length and about a 21% increase in the annual pollen integral, a measure based on accumulated pollen concentrations. Those are separate outcomes. Anderegg and colleagues, PNAS

The University of Utah's public summary rounds the lengthening to about 10 days. Neither source supports describing both the earlier start and the longer duration as 20-day changes. These historical findings also do not mean that every city's pollen rose by 21%. University of Utah research summary

Temperature, carbon dioxide, and weather interact

Warmer temperatures and altered frost timing can change when plants flower. Higher carbon dioxide can increase pollen production in some plants. Rainfall, plant species, land use, and local weather also affect exposure, so a difficult week cannot be attributed to one factor alone. CDC: allergens and pollen, updated August 11, 2026

A late killing frost can prolong ragweed exposure. Steady rain may temporarily remove pollen from the air, while subsequent plant growth or mold can contribute to later symptoms. See the weather section below for exceptions.

What AAFA's 2026 Allergy Capitals Ranking Means

Boise, Idaho, ranked first in AAFA's 2026 comparison of the 100 most populous metropolitan areas in the contiguous United States. The report combines tree, grass, and weed pollen scores with allergy medication use and access to board-certified allergists. AAFA: 2026 Allergy Capitals

The 2026 report uses 2025 data. Its pollen component weights days rated high or very high using automated Pollen Sense measurements. This is a different data source and method from the CDC–NAB historical study. AAFA report, methodology

Use the ranking to understand metropolitan allergy burden. It is not a live September ranking, a forecast, or a diagnosis of your triggers. A low-ranked city can still have difficult days for someone allergic to its local plants.

January: Winter Cedar and Indoor Exposure

January is not an allergy-free month.

Outdoor triggers

In Central Texas, Ashe juniper—commonly called mountain cedar—often creates a major winter pollen event. Texas A&M Forest Service describes a season that typically begins in mid-December, peaks around mid-January, and tapers toward early March.

Other possible January pollens include:

• Juniper and cedar in parts of the South and Southwest.
• Cypress in mild climates.
• Early alder or hazel in selected West Coast locations.
• Early trees after an unusually warm southern winter.

Indoor triggers

Spending more time indoors can increase contact with:

• Dust mites in bedding and upholstered furniture.
• Pet allergens.
• Indoor mold around condensation or leaks.
• Cockroach and rodent particles.

Smoke, fragrances, combustion fumes, and dry air can also irritate the nose and airways without being allergens.

Winter congestion should not automatically be blamed on pollen. Infection and dry heated air can produce overlapping symptoms.

February: Early Trees Move North

Tree season often expands during February.

Likely triggers

• Alder and hazel in parts of the Pacific Northwest and West Coast.
• Juniper and cedar in the South and Southwest.
• Elm, maple, cypress, and other early trees in warmer eastern regions.
• Continuing mountain cedar in Central Texas.

Weather signal

Several warm days after winter chill can accelerate bud development. A cold snap may pause release without resetting the entire season.

Someone who historically started medication in March may need an earlier plan after repeated warm winters. Use local counts rather than the old calendar date alone.

March: Tree Pollen Accelerates

March often marks a sharp expansion of spring tree pollen.

Common regional contributors

• Oak.
• Elm.
• Maple and box elder.
• Ash.
• Juniper and cypress.
• Alder.
• Poplar and cottonwood.
• Mulberry in some southern and western areas.

Regional pattern

The South and lower Mid-Atlantic may already be heavily active. The lower Midwest begins rising, while colder northern regions may still be early in the cycle.

Dry, warm, windy conditions can create abrupt count spikes. Rain may offer brief relief without ending the season.

April: The Broad Tree-Pollen Wave

April is one of the most difficult months for tree-sensitive people across large parts of the United States.

Frequent culprits

• Oak.
• Birch.
• Maple.
• Hickory.
• Ash.
• Sycamore.
• Poplar.
• Beech.
• Willow.

The yellow-dust problem

Visible yellow dust is often produced by pine or other conspicuous pollens. A visible coating does not prove that the most abundant dust is the allergen causing your symptoms. Less visible oak, birch, grass, or other pollen may be more clinically important.

The Tree vs. Grass vs. Weed Pollen Guide helps separate timing, plant groups, symptoms, and cross-reactions.

May: Tree and Grass Overlap

May often creates a double exposure: late tree pollen plus accelerating grass.

Common grass groups

• Timothy.
• Orchard grass.
• Ryegrass.
• Fescue.
• Kentucky bluegrass.
• Sweet vernal.
• Bermuda grass in warmer regions.

Why symptoms can suddenly change

A person allergic only to trees may begin improving while a grass-sensitive person becomes worse. Someone sensitized to both can experience one long continuous season.

Lawn mowing, sports fields, parks, and dry windy days can create concentrated personal exposure even when a regional index is moderate.

June: Grass Becomes the Main Story

June is a common grass-pollen peak in northern, central, and many temperate regions.

Higher-exposure activities

• Mowing.
• Field sports.
• Golf.
• Hiking through meadows.
• Hay handling.
• Outdoor events on grass.
• Driving or sleeping with windows open.

Warm-season grasses may dominate farther south, while cool-season grasses are more important in northern climates.

Use the Lawn Mowing and Yard Work Allergy Checklist before high-exposure chores.

July: Grass, Mold, and Early Weeds

July is not the same everywhere.

In northern or high-elevation regions

Grass may remain the leading trigger.

In warm, humid regions

Outdoor mold often becomes increasingly important. Spores can rise around:

• Crops.
• Compost.
• Cut grass.
• Soil.
• Decaying leaves or wood.
• Humid nights and warm days.

Early weed transition

Some weed pollen can appear locally in July, but August is a more typical start for the main ragweed season. Heat or drought alone does not reliably predict ragweed flowering.

Spore release is not determined by humidity alone: some molds disperse during dry, windy conditions, while others favor fog or dew. AAFA: mold allergy

The Mold Allergy Guide explains how outdoor spore exposure differs from indoor moisture-driven growth.

August: Ragweed Season Builds

August begins the main fall-pollen transition across much of the country.

Eastern and central regions

Ragweed becomes increasingly important, especially across:

• The Midwest.
• Northeast.
• Mid-Atlantic.
• Great Plains.
• Parts of the South.

Western and arid regions

Important weeds may include:

• Sagebrush.
• Mugwort.
• Pigweed or amaranth.
• Russian thistle and other plants commonly called tumbleweeds.
• Lamb's-quarters.

Symptoms to track

Ragweed can cause:

• Sneezing.
• Watery nasal drainage.
• Itchy eyes.
• Nasal congestion.
• Asthma symptoms.
• Pollen-food reactions to selected raw foods in sensitized people.

September: The Classic Ragweed Peak

In many regions, ragweed pollen is highest in early to mid-September.

That does not mean September is universally worst. Western weeds, warm-season grass, drought, smoke, and mold may create a different local pattern.

Factors that extend or intensify September symptoms

• Prolonged warm autumn weather.
• Delayed killing frost.
• Dry windy weather.
• Disturbed vacant lots and roadsides.
• Agricultural activity.
• Smoke or ozone irritating pollen-sensitive airways.

Prepare with the Ragweed Allergy Guide, which covers season timing, symptom patterns, treatment, and pollen-food cross-reactions.

October: Frost Determines the Finish Line

October can bring relief in a northern city and ongoing weed exposure in a southern one.

Before the first killing frost

Ragweed and other weeds may continue releasing pollen.

After frost

Local ragweed and many annual weed pollens often fall after a killing frost. A light frost may not end pollination, and symptoms can continue because of:

• Outdoor mold around fallen leaves.
• Pollen carried indoors on clothing and pets.
• Indoor dust mites or pet allergens.
• Ongoing inflammation after the exposure falls.
• Warmer pockets within a region.

Do not stop a prescribed seasonal plan solely because the calendar changed. Follow symptom, count, and clinician guidance.

November: Outdoor-to-Indoor Transition

In colder regions, November shifts attention indoors.

Common indoor problems

• Dust mites in bedding.
• Pet allergens accumulating with closed windows.
• Mold around condensation.
• Dirty humidifiers.
• Heating-system dust and irritants.
• Cockroach or rodent particles.

Indoor allergens remain relevant throughout the year. In winter, check bedding, pets, indoor dampness, and ventilation rather than assuming that every symptom comes from outdoor pollen.

For indoor mold prevention, fix moisture sources and aim for relative humidity around 30–50%. Dry water-damaged materials within 24–48 hours when possible. EPA: mold and moisture control

The Dust Mite Allergy Guide covers encasements, laundry, humidity, and bedroom source control.

December: Mountain Cedar Returns

Mountain cedar or Ashe juniper becomes a defining outdoor trigger in Central Texas and can affect surrounding areas through wind transport.

Typical Texas pattern

• Starts around mid-December.
• Often intensifies after dry cold fronts.
• Peaks around mid-January.
• Tapers toward early March.

These are typical dates from Texas A&M Forest Service, not a forecast of the coming winter's peak.

Nationwide pattern

Elsewhere, winter symptoms more often reflect indoor exposure, respiratory infections, dry air, or irritants. Mild southern and coastal regions can still have outdoor pollen.

Regional Allergy Calendars

Use these broad regional patterns alongside the monthly table. Within-state differences, especially along coasts and across elevations, can be substantial.

Northeast

A classic pattern is:

1. Early trees in March.
2. Broad tree peak in April or May.
3. Grass from May into July.
4. Ragweed from August until frost.
5. Outdoor mold around summer humidity and autumn leaves.

Warm coastal areas can start earlier than inland or northern locations.

Midwest and Ohio Valley

Common waves include:

• Maple, elm, oak, and other trees in spring.
• Grass in late spring and early summer.
• Ragweed and mixed weeds in late summer and fall.
• Strong outdoor mold around crops, soil, and decaying vegetation.

Agricultural activity and regional winds can create exposures not obvious from a city landscape.

Southeast and Gulf Coast

The season is longer and overlaps more heavily.

Possible patterns include:

• Tree pollen beginning in winter or very early spring.
• Oak and pine-related visible dust in spring.
• Bermuda and other grasses active for extended periods.
• Ragweed and weeds in late summer and fall.
• Outdoor mold through long warm, humid periods.

A single “spring allergy” label may miss year-round or multiple sensitizations.

South-Central and Texas

Distinctive features include:

• Mountain cedar from December into February or early March.
• Oak and other trees in spring.
• Bermuda and other grasses through warm months.
• Ragweed and weeds in late summer and fall.
• Mold and agricultural exposures.

A person can move from cedar directly into oak and grass with little symptom-free interval.

Southwest and Desert Regions

Common triggers can include:

• Juniper and cedar.
• Mulberry and olive where planted.
• Mesquite.
• Ash.
• Bermuda grass in irrigated landscapes.
• Sagebrush.
• Pigweed and amaranth.
• Russian thistle.

Desert does not mean pollen-free. Irrigation and urban planting create concentrated local sources.

Mountain West and High Elevations

Elevation compresses and delays seasons.

A valley can begin tree pollen weeks before a mountain town. Grass may remain active later in cool locations, while sagebrush and other weeds dominate late summer and fall.

Rapid weather shifts and wildfire smoke can complicate symptom interpretation.

Pacific Northwest

Possible patterns include:

• Alder and hazel in late winter.
• Birch, maple, cedar, and other trees in spring.
• Grass in late spring and summer.
• Outdoor mold during damp or decaying-vegetation periods.

The region's long moisture season makes it important to distinguish outdoor spores from indoor building dampness.

California and the Pacific Coast

California contains multiple pollen climates.

Coastal, inland-valley, desert, and mountain calendars differ. Depending on location, important sources may include:

• Alder.
• Cypress.
• Oak.
• Olive.
• Mulberry.
• Grasses.
• Sagebrush and weeds.
• Year-round outdoor mold in selected coastal environments.

Local city data is substantially more useful than a statewide month label.

Alaska and Hawaii

These states are not represented in AAFA's contiguous-U.S. Allergy Capitals ranking.

Alaska has a short but potentially intense tree and grass season that varies with latitude and spring thaw.

Hawaii's tropical climate can support long or year-round exposure to grasses, molds, and locally important plants. The continental winter-spring-summer-fall framework fits poorly there.

How to Track Your Personal Allergy Calendar

1. Use counts and forecasts for different jobs

A pollen count estimates airborne concentration from a monitoring instrument. Manual NAB sampling commonly covers about 24 hours before laboratory identification, so check both the collection date and publication date. AAAAI: how pollen counts are measured

A pollen forecast predicts conditions using inputs such as weather, vegetation, historical patterns, and observations. A forecast or index is not interchangeable with a measured concentration in grains per cubic meter.

Use the nearest recent, representative observation when available, while recognizing that a distant or old station report can miss neighborhood exposure. No report does not mean no pollen. Provider scales differ, and the same numerical concentration can fall into different categories for trees, grass, and weeds. Read each provider's legend.

The AAAAI National Allergy Bureau provides station-based pollen and mold reports. Not every location or date has a report, and a pollen-only service may omit mold.

The best strategy is to compare both with symptoms. The How to Read a Pollen Forecast Guide explains the difference in detail.

2. Track categories before individual species

Record:

• Tree score.
• Grass score.
• Weed score.
• Mold or spore information when available.
• Dominant allergen.
• Weather.
• Outdoor duration.
• Symptoms.
• Medication use.

After several weeks, patterns may suggest questions for an allergist, such as whether symptoms follow trees, grass, or mold. Overlapping seasons and delayed symptoms mean that a diary alone cannot confirm the cause.

3. Use a simple daily score

Use this informal 0–3 diary for nose and eye symptoms. It is not a validated diagnostic or asthma-control score:

Record medication names, doses used according to your plan, location, and outdoor exposure separately. Use the same symptom categories each day.

Track breathing symptoms separately and act on them. Wheeze, chest tightness, breathlessness, night waking, or increased reliever use should trigger your prescribed asthma action plan and appropriate medical advice. Do not wait for a high diary score before seeking help.

4. Prepare before the expected rise

Many allergy medicines work better when used consistently before inflammation becomes severe.

The correct lead time depends on:

• The medication class.
• Product label.
• Prior seasonal pattern.
• Age.
• Pregnancy or health conditions.
• Clinician instructions.

Intranasal corticosteroids generally need consistent daily use for several days to reach full effect. For predictable seasonal symptoms, AAAAI suggests beginning about one to two weeks before the expected season, with timing and suitability guided by the product label and your clinician or pharmacist. If symptoms have already started, ask about treatment now. AAAAI: nasal steroids

Allergen immunotherapy has a separate schedule. For example, the FDA label for the prescription ragweed tablet RAGWITEK specifies starting at least 12 weeks before the expected ragweed season and continuing through it. It is not immediate symptom relief; the first dose requires medical supervision. Grass tablets have their own product-specific schedules. FDA: RAGWITEK prescribing information

Use the Allergy Medicine Guide to match treatment class to symptom and onset.

5. Confirm the trigger when the calendar is not enough

Consider an allergist when:

• Symptoms overlap several seasons.
• Indoor and outdoor triggers are unclear.
• Medication is needed for many months.
• Cough or wheeze occurs.
• Symptoms disrupt sleep or work.
• Immunotherapy is being considered.
• The expected pollen category does not match the symptom pattern.

Skin prick and specific IgE blood tests show sensitization and must be interpreted with exposure and symptom history. A positive result alone does not establish that a particular pollen is causing your symptoms. AAAAI: rhinitis diagnosis

The Allergy Testing Guide explains how focused testing avoids unnecessary broad panels.

Weather Events That Break the Calendar

An unusually warm winter

Can bring early tree flowering and a longer overlap with later species.

A late freeze

Can delay, reduce, or damage flowering for some plants while leaving others less affected.

A rainy spring

Can promote vegetation growth and later pollen, while temporarily lowering airborne counts on rainy days.

Drought

Can suppress some plants but favor drought-tolerant weeds, dust, smoke, and highly variable pollen release.

Wind

Can move pollen from distant sources and create a high-exposure day before local plants appear active.

Wildfire smoke

Does not increase pollen by itself, but fine particles and gases can irritate airways already inflamed by allergy.

Thunderstorms

Some storms during high-pollen periods can concentrate respirable allergen particles in the gusty outflow. People with pollen allergy, especially those with asthma, may be vulnerable. Stay indoors with windows closed during these conditions and follow your asthma plan. AAAAI: thunderstorm asthma

The Thunderstorm Asthma and Pollen Guide explains the difference between rainfall-related relief and storm-related breathing risk.

First frost

A killing frost usually ends local ragweed and many annual weed seasons. A light frost may not, and transported pollen or outdoor mold can still cause exposure.

Common Calendar Mistakes

Treating national months as local facts

Compare the calendar with a recent, nearby report. Check whether it is an observation or forecast and whether its timing and location are relevant to you.

Assuming visible yellow dust identifies the culprit

Conspicuous pollen is not always the most allergenic or clinically relevant pollen in the air.

Calling every winter symptom “indoor allergy”

Mountain cedar and early trees are major exceptions, and infection remains common.

Assuming green means grass

Map colors, index colors, and pollen categories differ among providers. Read the legend.

Starting medicine only after the peak

Plan ahead when possible. Starting after symptoms appear can still help; the appropriate medicine and schedule depend on your symptoms and health history.

Stopping at the first rainy day

Rain changes short-term exposure; it does not end the plant's season.

Using the AAFA ranking as a personal diagnosis

The report compares metro-level burden using several factors. It does not reveal your sensitization.

When the Symptoms Are More Than Hay Fever

Pollen can trigger lower-airway inflammation and asthma.

Seek prompt assessment for:

• Cough during pollen exposure.
• Wheezing.
• Chest tightness.
• Shortness of breath.
• Night waking.
• Reduced exercise tolerance.
• Frequent prescribed reliever use.
• Falling peak-flow readings.

Call 911 for severe or rapidly worsening breathing trouble, inability to speak or walk normally because of breathlessness, blue or gray lips, confusion, fainting, or marked pulling in around the ribs or neck. Follow your prescribed rescue plan while help is coming. Symptoms that do not improve with prescribed rescue treatment also need urgent assessment. AAFA: asthma emergency signs

Build Your Seasonal Allergy Plan

A U.S. pollen calendar works best as a layered system:

1. Use the month to identify the most likely allergen group.
2. Adjust for region, altitude, and climate.
3. Check current local counts and forecasts.
4. Compare conditions with your symptom diary.
5. Start treatment according to the product and clinical plan—not a generic date.
6. Reassess when the season moves earlier, lasts longer, or no longer matches history.

Recheck your personal calendar when symptoms start earlier, last longer, or occur during a different pollen category than expected.

For persistent, overlapping, or asthma-associated symptoms, find a local allergist in the AllergyAva directory for a diagnosis-driven seasonal plan.

How This Calendar Was Updated

The September 7, 2026 editorial update checked current CDC, AAAAI, ACAAI, AAFA, EPA, and FDA resources, the 2026 Climate Central analysis, and the 2026 CDC–NAB study. Historical study periods and report years are identified separately from the current month. Monthly and regional entries remain general planning guidance; no live national pollen measurements were used to create the September overview.