When does ragweed allergy season start and when does it peak?

Ragweed doesn't wait for cold weather to arrive. It's a short-day plant, so it won't flower until after the summer solstice.¹ As the nights grow longer in late summer, its flowers mature and begin releasing pollen.² That's why the start depends on the calendar more than the thermometer. In most of the country, pollen begins showing up in early August, climbs to its highest levels around mid-September, and fades through October.³ The whole season usually runs six to ten weeks.³

The season ends after the first hard frost kills the plants.¹ Until then they keep pumping out pollen, and because that date is local, so is your end date. Where frost comes late, ragweed can keep releasing pollen into November.¹ And in many parts of the country, the season now starts earlier and runs later than it used to.⁴

During ragweed season, pollen counts run highest in the morning.⁵ That's the opposite of the spring tree and grass pollen that peaks in the evening.⁵ Counts also surge on warm, windy days and right after a thunderstorm.³ Plan around those mornings and those days if ragweed bothers you.

Moving away rarely helps. Ragweed grows in nearly every state, and a single plant can release up to a billion pollen grains.⁴ Its grains are light enough to ride the wind for long distances, so they reach you even where little grows nearby.⁴

Your exact start and end dates shift with where you live and the weather each year, but the pattern itself is dependable.