Timothy Grass Pollen: The Classic Hay Fever Grass Guide
When scientists picture hay fever, they usually picture timothy grass pollen. Timothy grass (Phleum pratense) is the most studied allergenic grass in the world — its pollen is the standard reference material in allergy research, and its major allergens are so well characterised that they have their own naming system. For millions of people across the northern United States, timothy grass pollen is the main reason late spring and early summer mean itchy eyes, sneezing fits, and a nose that will not quit.
Key takeaways
- Timothy grass pollen is the most studied allergenic grass in the world, and its pollen is the standard reference material in allergy research.
- A study in Multidisciplinary Respiratory Medicine found its major allergen Phl p 1 recognised by 99 percent of the grass-allergic children tested.
- Its season is a late-spring and early-summer event in the North: flowering begins in May and peaks in June and early July.
- Timothy and Bermuda divide the country between them — cool-season timothy in the North, warm-season Bermuda in the South — so grass season means June in Minnesota and August in Texas.
Why timothy grass pollen became the reference allergen
Timothy grass pollen earned its central place in allergy science honestly: it is both extremely common and strongly allergenic. Timothy was introduced from Europe as a hay and pasture grass — its name is traditionally credited to Timothy Hanson, an American farmer who promoted it in the early 1700s — and it became one of the continent's most widely planted forage grasses. Fields of it still cover large areas of the Northeast, Great Lakes, and northern Plains.
The Phl p allergens behind timothy's reputation
Its pollen carries a well-mapped set of allergen proteins, designated Phl p 1, Phl p 5, and others in the World Health Organization allergen nomenclature. A peer-reviewed review of grass immunotherapy in PubMed Central reports complete cross-inhibition of binding to nine other grass pollen species using timothy extract — laboratory shorthand for just how representative this one grass is. A study of timothy allergenic molecules in Multidisciplinary Respiratory Medicine found Phl p 1 recognised by 99 percent of the grass-allergic children tested. Because those proteins are shared, with variations, across the northern pasture grasses — Kentucky bluegrass, orchard grass, ryegrass, sweet vernal, and meadow grasses — timothy became the stand-in for the whole group. Researchers describe this as cross-reactivity: the immune system recognises similar proteins in related grasses, so a person sensitised to timothy often reacts to its cousins too. The AAFA's allergist Q&A on grass reactions lists timothy among the few grasses responsible for most grass allergy symptoms. That is one reason a single "grass pollen" number is meaningful even though dozens of grass species may be flowering at once.
Timothy in the daily Grass Index
On Pollen Atlas, timothy and its relatives are tracked in the daily Grass Index, scored 0 to 100 — Low from 0 to 33, Moderate from 34 to 66, and High from 67 to 100. Pollen Atlas data shows northern cities climbing into the Moderate and High bands as the pasture grasses peak in early summer.
When timothy grass pollen season runs
Timothy grass pollen season is a late-spring and early-summer event. In most northern states, flowering begins in May and peaks in June and early July, tapering as the summer heat builds and hay fields are cut. Higher elevations and more northern locations run later — a mountain or Canadian-border community can see its timothy peak weeks after a city a few hundred miles south.
| Area | Timothy grass pollen timing |
|---|---|
| Most northern states | Flowering begins in May and peaks in June and early July, tapering as summer heat builds |
| Higher elevations and more northern locations | The peak can come weeks later than in a city a few hundred miles south |
| Denver, at elevation | The grass season runs later and longer than in lower cities at similar latitudes |
| Southern states | The northern band of grass values fades southward, where Bermuda and other warm-season grasses take over |
The daily rhythm within timothy season
The season has a rhythm within the day as well. Grasses release pollen in the morning, and levels often build through the late morning and afternoon as air warms and mixes. Dry, sunny, and breezy weather produces the highest counts; cool, rainy stretches press them down temporarily. A second, smaller release can occur in some grasses in the evening, which is why some people notice symptoms again around dinner time on high days.
How hay cutting changes local counts
Cutting matters locally. Timothy is a hay crop, and mowing a field at peak flower dumps a large share of its season's pollen into the air at once. Rural and exurban readers may notice their worst days line up with hay-making in nearby fields more than with any citywide pattern.
Where timothy grass pollen hits hardest
Timothy's stronghold is the cool half of the country: the Northeast, the Great Lakes, the upper Midwest, the northern Plains, and higher ground in the West. Cities like Chicago, Illinois and Minneapolis, Minnesota sit in prime pasture-grass country, and their Grass Index peaks reflect it. Denver, Colorado, at elevation, runs its grass season later and longer than lower cities at similar latitudes, while Columbus, Ohio sits in classic hay country in the Ohio Valley.
The pollen heatmap makes the geography visible in June: a broad northern band of elevated grass values, fading southward where Bermuda and other warm-season grasses take over. That north–south handover is the single clearest pattern in American grass pollen, and it is why "grass season" means June in Minnesota and August in Texas.
Timothy grass pollen vs. Bermuda grass pollen
The comparison is worth spelling out, because the two grasses divide the country between them. Timothy is a cool-season grass: it greens up early, flowers in late spring, and sulks in extreme heat. Bermuda is a warm-season grass: it wakes late, flowers through the hottest months, and browns at the first frost. Their allergens overlap only partly, because the two belong to different grass subfamilies, so local testing in each region weights them differently.
Timing is the practical difference
For readers, the practical difference is timing. If your worst weeks are May through July, northern pasture grasses led by timothy are the likely drivers. If your worst weeks are July through September in a southern city, warm-season grasses are more likely. Our companion post on Bermuda grass pollen covers that side, and grass pollen season puts both into one national calendar.
Tracking timothy grass pollen in your city
Because the timothy season is concentrated, a few peak weeks do most of the damage — which makes daily tracking especially valuable. A morning check of your city's Grass Index and 5-day forecast on Pollen Atlas tells you whether today is a peak day to plan around or a Low day in between fronts. Watch for the pattern of several High days in a row in June: that is the heart of the season, and it passes faster than it feels like it will.
If grass season flattens you every year and you want to know exactly which grasses you react to, a specialist can test for that — you can find an allergist near you through the Pollen Atlas directory. This article is general information only and is not medical advice.
Frequently Asked Questions
When is timothy grass pollen season?
Timothy grass pollen season is a late-spring and early-summer event. In most northern states, flowering begins in May and peaks in June and early July, tapering as the summer heat builds and hay fields are cut.
Why is timothy grass pollen the reference allergen?
Timothy grass pollen is the standard reference material in allergy research because it is both extremely common and strongly allergenic. Its major allergens are so well characterised that they have their own naming system in the World Health Organization allergen nomenclature.
Does timothy grass pollen cross-react with other grasses?
Yes — timothy's allergen proteins are shared, with variations, across the northern pasture grasses such as Kentucky bluegrass, orchard grass, ryegrass, sweet vernal, and meadow grasses. A peer-reviewed review in PubMed Central reports complete cross-inhibition of binding to nine other grass pollen species using timothy extract.
How is timothy grass pollen different from Bermuda grass pollen?
Timothy is a cool-season grass that flowers in late spring and sulks in extreme heat, while Bermuda is a warm-season grass that flowers through the hottest months and browns at the first frost. Their allergens overlap only partly, because the two belong to different grass subfamilies.
Sources: PMC review, multiple grass mixes vs single grasses for immunotherapy; Multidisciplinary Respiratory Medicine (2013), sensitization to timothy allergenic molecules; AAFA Ask the Allergist, grass pollen reactions.
Conclusion: timothy grass pollen, the original hay fever
Timothy grass pollen is the original hay fever allergen — the grass that gave the condition its name, its research standards, and, for the northern half of the country, its hardest early-summer weeks. Its season is short, predictable, and intense. Know your weeks, watch your city's number, and those weeks get easier to navigate. Check your Grass Index free today — counts and forecasts for your city, worldwide, at PollenAtlas.com.