When Is Allergy Season? A Month-by-Month U.S. Pollen Calendar
August 13, 2026
Allergy season usually runs from late winter through fall, but the timing depends on where you live and which pollen bothers you. The typical U.S. pattern moves from tree pollen to grass pollen to weed pollen.
- Tree pollen: late winter through spring (oak, birch, maple, cedar)
- Grass pollen: late spring through summer (Timothy, Bermuda, bluegrass)
- Weed pollen: late summer through fall, until frost (ragweed)
Warmer regions start earlier and run longer, while colder areas usually wind down after the first frost. Take the quick quiz below to match your home to the right filter for pollen season.
Pollen-season tip: The EPA recommends a MERV 13 filter, or the highest-efficiency filter your HVAC system can accommodate, to help capture more airborne particles, including pollen.
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Allergy season varies by location and allergen. Tree pollen often starts in late winter or spring, grass pollen becomes more important in late spring and summer, and weed pollen (especially ragweed) commonly peaks in late summer and fall. Warmer regions can see pollen earlier and for longer stretches, while northern areas typically have shorter seasons limited by frost.
When Does Allergy Season Start?
There isn't one universal start date for allergy season in the U.S. The CDC confirms that pollen amount and type depend on both the time of year and geographic region, and some plants can pollinate nearly year-round in milder climates.
Generally speaking, tree pollen is the first allergen to appear, often beginning in late winter or early spring. Grass pollen becomes the dominant allergen as temperatures rise through late spring and into summer. Ragweed and other weed pollens typically take over in late summer and persist into fall. In warmer parts of the country, this cycle can start weeks (or even months) earlier than it does in northern states, and it may extend later into the year before frost brings it to a close.
Seasonal Allergy Calendar by Month
The table below outlines a general U.S. pattern for pollen activity. It's not a prediction for any specific city: local weather, latitude, elevation, and the plant species growing nearby can all shift these windows earlier or later.
| Month | Typical Pollen Activity |
|---|---|
| January | Cedar and juniper pollen can appear in warmer regions |
| February | Tree pollen begins increasing in southern and warmer areas |
| March | Tree pollen becomes widespread across much of the country |
| April | Often a major tree-pollen month in many regions |
| May | Tree pollen continues as grass pollen starts increasing |
| June | Grass pollen becomes a prominent allergen |
| July | Grass pollen remains significant; some weed pollen begins |
| August | Ragweed and other weeds start increasing |
| September | Often a major month for ragweed and weed pollen |
| October | Weed pollen continues until frost in many areas |
| November | Pollen generally declines in colder regions; may persist in mild climates |
| December | Cedar and juniper pollen can begin in parts of the South and South Central U.S. |
This calendar reflects a general seasonal allergy pattern for the U.S. (actual timing in your area can vary from these ranges).
Tree vs. Grass vs. Weed Pollen Seasons
Three main types of pollen drive most seasonal allergy symptoms, and each tends to follow its own calendar.
Tree pollen comes from species like oak, birch, maple, and cedar, and typically peaks in spring. In warmer climates, certain trees (like cedar and juniper) can release pollen as early as winter.
Grass pollen from species such as Timothy, Bermuda, and bluegrass typically becomes most active in late spring and summer, when temperatures climb and grasses actively grow.
Weed pollen, with ragweed as the most notable example, typically dominates in late summer and fall. This season usually ends when frost arrives, since cold temperatures halt pollen production for most weed species.
The typical U.S. pattern moves from tree to grass to weed pollen as the year progresses, though exact timing varies widely by region and can overlap in transitional months.
Allergy Season by U.S. Region
Because climate, latitude, and native plant life differ so much across the country, allergy season looks different depending on where you live. The regional patterns below are generalized and shouldn't be treated as official or precise pollen calendars.
Northeast: Tree pollen typically arrives in spring, grass pollen follows in late spring and early summer, and ragweed pollen often continues from late summer until the first frost.
Southeast: Tree pollen can begin as early as winter in some areas, grass season tends to run long due to a warmer climate, and weed pollen can persist well into fall.
Midwest: This region typically sees a strong tree pollen season in spring, followed by significant grass pollen in late spring and summer, and often substantial ragweed activity in late summer and fall.
South Central: Cedar and juniper pollen frequently appear in winter, tree pollen continues through spring, grasses pollinate through the warm months, and ragweed typically peaks in late summer and fall.
Southwest: Desert tree species, Bermuda grass, and weeds like ragweed and sagebrush all contribute to allergy symptoms, with timing shaped by the region's arid climate.
Mountain West: Pollen timing here tends to follow elevation and local climate patterns, with tree, grass, and weed seasons generally mirroring the broader tree-grass-weed progression but shifted by altitude and temperature.
Pacific Northwest: Alder and birch are common early tree pollen sources, and grass pollen season tends to be notably strong. Mold is also a significant separate concern in this region due to higher humidity and rainfall.
California/Pacific: This region is distinct from the rest due to its dramatically varying microclimates: coastal, desert, and mountain areas within the same state can have very different pollen timelines.
What About Mold Allergies?
Mold spores can trigger allergy symptoms both indoors and outdoors, and mold activity doesn't follow the same plant-pollination calendar as tree, grass, and weed pollen. Instead, mold levels tend to fluctuate with moisture, humidity, rainfall, and decaying vegetation, which means mold exposure can spike at times when pollen counts are otherwise low.
When Does Allergy Season End?
There's no single nationwide end date for allergy season. In colder regions, weed pollen (including ragweed) usually declines sharply after the first hard frost, which halts most pollen production for the year. In warmer climates, however, certain trees, grasses, or weeds can continue producing pollen much later into the year, and some plants may pollinate nearly year-round, according to the CDC.
Why Allergy Season Changes From Year to Year
Pollen levels don't follow an identical pattern every year. Several factors influence how intense (and how long) a given allergy season turns out to be:
Temperature: Warmer-than-average conditions can trigger earlier pollen release.
Frost timing: Later first frosts in fall can extend weed pollen season; earlier frosts cut it short.
Rainfall: Steady rain can temporarily lower airborne pollen counts by washing particles out of the air, but this effect is temporary; pollen levels often rise again once conditions dry out.
Wind: Windy conditions can carry pollen over longer distances and keep it airborne longer.
Geography: Latitude, elevation, and proximity to specific plant populations all shape local pollen timing.
Climate trends: Longer-term shifts in temperature and precipitation patterns can influence pollen behavior across multiple seasons.
Is Allergy Season Getting Longer?
According to the CDC, climate change may shift when pollen season starts and ends, lengthen the overall season, and increase pollen concentrations. These effects are linked to changes in temperature, precipitation patterns, the number of frost days, and rising atmospheric CO₂ levels, all of which can influence how much pollen plants produce and how long they stay active.
When Is Ragweed Season?
Ragweed commonly becomes a significant allergen starting in late summer, with activity typically peaking in early-to-mid fall. The season generally continues until cold conditions or frost bring pollen production to a halt in a given region. Timing tends to start earlier and last longer in warmer climates compared to colder ones.
Ragweed is a notably prolific pollen producer: a single ragweed plant can generate up to about a billion pollen grains over the course of a season, according to the CDC, which helps explain why ragweed allergies are so widespread during fall allergy season.
What Allergy Symptoms Can Pollen Cause?
Pollen exposure can trigger two primary allergic conditions, according to the CDC:
Allergic rhinitis: Symptoms include sneezing, nasal congestion, and a runny nose.
Allergic conjunctivitis: Symptoms include red, watery, and itchy eyes.
Symptoms can range from mild and occasional to persistent and disruptive, depending on pollen exposure levels and individual sensitivity.
How Pollen Gets Inside Your Home
Pollen doesn't stay outdoors. It can enter a home through open windows and doors, cling to clothing, shoes, hair, and pets after time spent outside, and get pulled inside through HVAC systems that draw in outdoor air. Once pollen settles on surfaces, furniture, or carpeting, it can remain there and continue to affect indoor air quality until it's cleaned or filtered out.
How to Reduce Pollen Indoors During Allergy Season
Filtration can reduce the amount of airborne pollen circulating indoors, but it works best when paired with other exposure-reduction habits. Consider combining these approaches during high-pollen periods:
Keep windows and doors closed when outdoor pollen counts are high.
Change clothes and rinse off after spending extended time outdoors.
Clean floors, bedding, and upholstery regularly to remove settled pollen.
Use HVAC filtration to help capture airborne particles as air circulates through your system.
If symptoms are persistent, severe, or interfere with daily activities, it's worth discussing treatment options with a healthcare professional, since indoor exposure reduction is just one part of managing seasonal allergies.
What MERV Rating Is Best for Allergy Season?
The EPA recommends using a MERV 13 filter, or the highest-efficiency filter your HVAC system's fan and filter slot can accommodate, to help capture more airborne particles, including pollen.
MERV ratings measure a filter's particle-capture efficiency: they don't indicate medical suitability for specific conditions like asthma or severe allergies. Here's how common MERV ratings compare:
| MERV Rating | Filtration Performance |
|---|---|
| MERV 8 | Captures many larger particles, including larger pollen grains |
| MERV 11 | Offers greater capture efficiency for smaller particles than MERV 8 |
| MERV 13 | Provides stronger fine-particle filtration; the EPA’s preferred upgrade when your system can support it |
It's worth noting that a high MERV rating can't filter air that bypasses the filter entirely. The EPA recommends ensuring a snug, properly sealed fit so air is forced through the filter media rather than around it.
As for how often to replace your filter, follow the guidance provided by your filter and HVAC manufacturer, and inspect the filter periodically, especially during heavy pollen periods when it may need more frequent attention than usual.
HVAC Filter vs. HEPA Purifier
Central HVAC filtration and portable HEPA air cleaners serve different but complementary roles in reducing indoor pollen exposure. A portable HEPA air cleaner can supplement your central HVAC filter by targeting a specific room, such as a bedroom or home office, where you spend significant time. The EPA recommends selecting a portable air cleaner based on the size of the room and its Clean Air Delivery Rate (CADR).
| Feature | HVAC Filter | Portable HEPA Air Cleaner |
|---|---|---|
| Coverage | Central system, whole-home | Room-level |
| Operation | Works when HVAC fan runs | Runs independently |
| Rating system | MERV-rated | HEPA/CADR-rated |
| Best for | Multi-room circulation | Targeted, room-specific filtration |
Many households use both: a higher-MERV HVAC filter to address whole-home air quality, and a portable HEPA unit in rooms where allergy symptoms are most disruptive, such as a bedroom during peak pollen season.
Choosing a Filterbuy Filter for Pollen Season
Selecting the right air filter can make filtration part of your seasonal allergy routine. Filterbuy offers a range of MERV-rated filters designed to fit standard HVAC systems, including options at MERV 11 and MERV 13 for households looking to capture more fine airborne particles during high-pollen months.
When choosing a filter for allergy season, start by checking your HVAC system's compatible MERV range; not every system can support a MERV 13 filter, so it's worth confirming compatibility before upgrading. From there, match the filter size precisely to your system's filter slot to ensure a snug fit, since gaps allow unfiltered air to bypass the filter media entirely. Finally, plan to inspect your filter more frequently during heavy pollen periods, since elevated particle loads can cause filters to need attention sooner than they might during lower-pollen months.
Filterbuy's filter finder tool can help you identify the correct size and MERV rating for your specific HVAC system, making it easier to build filtration into your broader approach to managing seasonal allergies.
Building a Seasonal Allergy Routine That Works for You
Seasonal allergies are shaped by a mix of factors that are largely outside your control: climate, regional plant life, and year-to-year weather patterns all play a role in when and how intensely pollen affects you. But understanding the general rhythm of tree, grass, and weed pollen seasons can help you anticipate high-exposure periods and plan accordingly, whether that means adjusting outdoor activities, tightening up your indoor filtration, or simply being ready with symptom relief before peak season hits.
Pairing awareness of your regional pollen calendar with practical steps indoors (like choosing an appropriate HVAC filter and reducing pollen entry points) can make a meaningful difference in your day-to-day comfort throughout allergy season.
Frequently Asked Questions
What Month Does Allergy Season Start?
There's no single start month for allergy season nationwide. Tree pollen often begins in late winter or early spring, with warmer regions seeing activity as early as December or January.
What Months Are Worst For Allergies?
This varies by allergen and region. Spring months often bring the heaviest tree pollen, while late summer and early fall tend to be worst for ragweed and other weed pollen.
When Is Tree Pollen Season?
Tree pollen season typically runs from late winter through spring, though certain species like cedar and juniper can release pollen as early as winter in warmer climates.
When Is Grass Pollen Season?
Grass pollen season typically runs from late spring through summer, as grasses actively grow and pollinate in warmer temperatures.
When Does Ragweed Season Start?
Ragweed season commonly starts in late summer, with activity typically peaking in early-to-mid fall before declining after frost.
When Does Allergy Season End?
There's no single end date nationwide. In colder regions, weed pollen typically declines after the first frost. In warmer climates, pollen can continue later into the year or persist nearly year-round for some plants.
Why Does Allergy Season Vary By Region?
Climate, latitude, elevation, and the specific plant species native to an area all influence when pollen is released and how long each season lasts.
Can Pollen Season Start In Winter?
Yes, particularly in warmer regions, where trees like cedar and juniper can begin releasing pollen as early as December or January.
Is Allergy Season Getting Longer?
According to the CDC, climate change may shift pollen season timing, lengthen seasons, and increase pollen concentrations due to changes in temperature, precipitation, frost days, and atmospheric CO₂.
Does Rain Reduce Pollen?
Rain can temporarily lower airborne pollen levels by washing particles out of the air, but this effect is often short-lived, and pollen levels can rise again once conditions dry out.
What MERV Rating Is Best For Pollen?
The EPA recommends MERV 13, or the highest-efficiency filter your HVAC system's fan and filter slot can accommodate, for capturing more airborne particles including pollen.
Can HVAC Filters Remove Pollen?
HVAC filters can help capture airborne pollen as air circulates through the system, with higher MERV-rated filters generally offering greater capture efficiency for fine particles.
Is HEPA Better Than MERV 13 For Allergies?
HEPA and MERV 13 serve different purposes: HEPA air cleaners target a specific room independently, while a MERV 13 HVAC filter addresses whole-home air as it circulates. Many households use both to address different parts of their indoor space.