IsItAllergySeasonYet
—
Guide · Pollen by type

Grass pollen: the May–July grind

For the roughly 20% of Americans with grass pollen allergy, late spring is the rough stretch. Trees fade, grasses take over, and the season runs right through the best outdoor weather of the year. Here's what's actually causing your symptoms, when it peaks, and why some days are dramatically worse than others.

The short answer

Grass pollen season runs from roughly mid-May through early July across most of the US, peaking in late May and June. In the South, warm-season grasses like Bermuda extend the season into August. The main offenders are Timothy, Kentucky bluegrass, ryegrass, orchard grass, Bermuda, and Johnson grass — all wind-pollinated, and all producing small pollen grains that easily reach the lower respiratory tract.

Grass allergy is also medically significant beyond runny nose: roughly 80% of people with allergic asthma are sensitized to grass pollen, and grass pollen season correlates with measurable spikes in asthma emergency visits.

Why grass pollen hits harder than it looks

Most people picture grass pollen as something that comes from fields. In reality, the grass in your own backyard — and on every lawn, roadside, and park strip in your neighborhood — is the primary source. Grass pollen dispersal is highly local: most grains fall within 100 meters of the plant that produced them, meaning your exposure is largely determined by what's growing immediately around you.

Grass pollen is also extremely small — 20–35 microns in diameter — making it one of the most easily inhaled pollens. Unlike some tree pollen grains that deposit mostly in the upper airways, grass pollen can penetrate into the lower airways and trigger bronchospasm in sensitized individuals.

Finally, grasses exhibit a phenomenon called high-allergenicity concentration: even at moderate pollen counts (50–100 grains/m³), many people experience significant symptoms, unlike tree pollen where symptoms often require higher concentrations to trigger. This is why a "moderate" grass reading can feel worse than a "high" tree reading for some people.

The main offenders by region

Timothy grass — the national standard

Phleum pratense — Timothy grass — is the species used as the reference for grass allergenicity testing worldwide. It's a cool-season grass planted as hay and pasture grass across the Northern US, and it's now naturalized along roadsides and in disturbed areas throughout the country. Season: May through July. If you're allergic to "grass," Timothy is almost certainly one of your sensitizers.

Timothy is also the source of the only FDA-approved grass sublingual allergy tablet in the US (Grastek), which means it's well-studied — sensitization to Timothy is associated with cross-reactivity to most other cool-season grasses.

Kentucky bluegrass — the suburban lawn

Poa pratensis is the most common turfgrass in the Northern US — the standard lawn grass from New England to the Midwest. It's short, it gets mowed regularly, but mowing doesn't stop it from pollinating (more on that below). Kentucky bluegrass pollen is cross-reactive with Timothy and most other cool-season grasses. If you have a standard suburban lawn in the North, bluegrass is producing pollen in your yard from late April through June.

Bermuda grass — the Southern season extender

Cynodon dactylon is a warm-season grass that dominates lawns, golf courses, and roadsides across the South and Southwest. Unlike cool-season grasses that peak in May–June, Bermuda grass pollinates from May through September in warm climates. In Florida, it can pollinate nearly year-round. It's among the most allergenic warm-season grasses, and it's essentially inescapable in the Southern US.

Bermuda allergy is the main reason people in the South continue to have grass symptoms through July and August when counterparts in the Midwest have found relief.

Johnson grass — the roadside problem

Sorghum halepense is an invasive warm-season grass that has colonized roadsides, field edges, and disturbed soils across much of the South and Central US. It grows tall (up to 7 feet), produces abundant pollen, and its season runs from June through October — well past when most other grasses have finished. Johnson grass is considered an agricultural pest as well as an allergy problem.

The mowing myth

Regular mowing does not meaningfully reduce grass pollen production — and on mow days, it temporarily increases your exposure.

Here's why: grasses pollinate from the seed head (the tall spiky part above the lawn surface). Mowing removes those seed heads before they mature and release pollen, which does suppress pollination — but only until the grass grows the seed heads back, which happens within one to two weeks of mowing. Continuously mowed lawns still pollinate; the grass simply does it at a shorter height and at a lower total rate.

More importantly, the act of mowing releases already-deposited pollen from leaves and the ground, along with grass fragments and mold spores. For a grass-allergic person, mowing is one of the highest-exposure activities of the season. If you must mow during peak season, wear an N95 mask, shower immediately after, and change clothes.

Hiring someone else to mow during June is genuinely good allergy medicine.

Advertisement
336 × 280

Why afternoons are worse

Tree pollen peaks in the morning. Grass pollen peaks in the afternoon — typically between 11 AM and 4 PM — because cool-season grass releases pollen when the temperature rises and humidity drops. Hot, dry, breezy afternoons are the worst conditions for grass pollen exposure.

This means:

After rain, grass pollen counts drop sharply — the water weighs down airborne grains and washes deposited pollen into the ground. Counts rebound within an hour or two of the rain stopping.

Thunderstorm asthma: the real risk

During certain thunderstorms, grass pollen grains absorb moisture from the pre-storm air and rupture, releasing sub-pollen particles — fragments roughly 0.5–2.5 microns — that are small enough to penetrate deep into the bronchial tree. Combined with the air movement from storm downdrafts concentrating and spreading these particles, the result can be a sudden, mass asthma event.

The most documented event was Melbourne, Australia in November 2016: over 9,000 people presented to emergency departments with acute asthma over a single evening, linked to a severe thunderstorm during peak ryegrass season. Smaller events have been documented in other regions.

If you have asthma and known grass pollen sensitivity, have your rescue inhaler accessible during peak grass season, particularly before approaching thunderstorms on high-pollen days. This is the kind of situation worth discussing with your doctor if you haven't already.

Management during grass season

Sources

Check today's grass pollen reading for your area on the home page, or browse the rest of our reference guides.