Allergy season follows pollen. Trees pollinate in spring, grasses in late spring and summer, and weeds like ragweed in late summer and fall, with a peak around mid-September. Because these seasons overlap, hay fever can last from spring into fall.
The simplest way to answer "when is allergy season" is to use one clear framework: the Pollen Calendar. Seasonal allergies, also called seasonal allergy symptoms, are an immune reaction to harmless pollen proteins, not an infection. Because three different pollen groups take turns through the year, your season depends on which pollen you react to. If you are new to the topic, it helps to first understand what pollen is and how it moves through the air.
"Hay fever" is the everyday name for allergic rhinitis, and it has nothing to do with hay or fever. It is your immune system reacting to pollen. Once you know when trees, grasses, and weeds release their pollen, you can predict your own worst weeks and plan around them instead of being caught off guard.
Tree, Grass & Weed Pollen Seasons
Pollen allergy is really three overlapping seasons, one for each major plant group. Trees come first in spring, grasses follow in late spring and summer, and weeds close out the year in late summer and fall. The Pollen Calendar below shows how they line up.
Pollen Calendar — Trees, Grasses & Weeds
Pollen group
🗓️ When it peaks
🌿 Notes
Trees
Spring
The first pollen of the year; opens the allergy season.
Grasses
Late spring & summer
Overlaps the tail of tree season into the warm months.
Weeds (ragweed)
Late summer to fall (Aug–Nov)
Peaks around mid-September; the main fall trigger.
Trees, grasses, and weeds pollinate in sequence, so symptoms can stretch from spring through fall.
Because these windows overlap, many people with hay fever feel symptoms across much of the year rather than in one short burst. Knowing which group is active helps you read a daily pollen count and understand why some days are far worse than others.
When Ragweed Season Peaks
Ragweed is the headline act of fall allergies. Ragweed and other weeds pollinate from late summer into fall, roughly August through November, and typically peak around the middle of September. If your symptoms flare after the summer heat fades, ragweed is the most likely cause.
What makes ragweed such a powerful trigger is sheer volume and range. A single ragweed plant can produce up to one billion pollen grains, and one grain can travel more than 100 miles on the wind. That is why ragweed can affect you even when none is growing near your home, and why fall symptoms can appear far from open fields.
How to Prepare for Your Season
The most useful move is to prepare before your season starts, not after symptoms begin. Once you know whether trees, grasses, or ragweed are your trigger, you can time your defenses. Simple, honest steps help most:
Check your local pollen count and stay indoors with windows closed on high-pollen days.
Pollen often peaks in the morning, so plan outdoor time accordingly.
Shower and change clothes after being outdoors, and use a HEPA filter indoors.
If you are considering allergy relief through immunotherapy, timing matters a great deal. Allergen immunotherapy, such as sublingual ragweed tablets, works to desensitize you over the long term, and ragweed treatment generally needs to begin about 12 weeks before the season starts. Because ragweed peaks around mid-September, that means planning in early summer, well ahead of the flare.
When to See a Doctor
Seasonal allergies are common, but you do not have to manage them alone. Consider seeing an allergist if any of the following apply:
Your symptoms return every year and disrupt sleep, work, or daily life
You want to know exactly what you are allergic to, confirmed by an allergist through skin or blood testing
You are considering immunotherapy and need to start it about 12 weeks before your season
Symptoms come with a true fever or last well beyond your usual pollen window, which may point to an infection rather than allergy
An allergist can confirm your triggers and help you build a plan. This article is educational and is not medical advice.
Frequently Asked Questions
When does allergy season start?
Allergy season begins in spring, when trees release their pollen. Grasses follow in late spring and summer, and weeds, especially ragweed, take over in late summer and fall. Because the three groups overlap, many people with hay fever have symptoms across much of the year, not just one short window.
When is ragweed season?
Ragweed season runs from late summer into fall, roughly August through November, and typically peaks around the middle of September. Ragweed is the main weed pollen behind fall hay fever, and a single plant can release up to one billion pollen grains that can travel more than 100 miles on the wind.
What months are worst for pollen allergies?
The worst months depend on what you are allergic to. Spring is worst for tree-pollen allergies, late spring and summer for grass-pollen allergies, and late summer through fall for ragweed and other weeds, with a common peak around mid-September. Tracking your local pollen count helps you see which pollen is high on a given day.
Is ragweed the same as hay fever?
Hay fever is the everyday name for allergic rhinitis, an allergic reaction to pollen; it has nothing to do with hay or fever. Ragweed is one common cause of hay fever, and it is the main trigger behind fall symptoms. Trees and grasses cause the same kind of hay fever earlier in the year.
How early should I prepare for allergy season?
It helps to prepare before your season starts rather than after symptoms begin. For allergen immunotherapy such as sublingual ragweed tablets, treatment generally needs to begin about 12 weeks before the season starts. An allergist can confirm what you are allergic to and when to begin, so plan ahead.
Can I be allergic to pollen all year?
Seasonal allergies are tied to pollen, so they come and go with tree, grass, and weed seasons. But because those seasons overlap from spring through fall, some people react across most of the year. Year-round symptoms can also point to perennial triggers like dust mites, so an allergist can help sort out the cause.
Why do my allergies start in the fall?
Fall allergies are usually driven by weed pollen, especially ragweed, which pollinates from late summer into fall and often peaks around mid-September. Because one ragweed plant can produce up to a billion pollen grains that travel long distances, ragweed can affect you even if none is growing nearby.
Sources
ACAAI — Ragweed Allergy (ragweed pollinates late summer into fall, peaks around mid-September; one plant produces up to a billion pollen grains that travel more than 100 miles; immunotherapy tablets begin about 12 weeks before season).
ACAAI — Allergy Facts (tree pollen in spring, grass in late spring/summer, weeds in late summer/fall).
Mayo Clinic — Hay fever (allergic rhinitis) (hay fever is an immune reaction to pollen, not an infection; exposure-reduction and medical options).