A season is a broad pattern
A pollen calendar describes when plants commonly release pollen. It does not predict the concentration in a particular city on a particular day. Seasons can overlap, and the timing varies with location and weather. The CDC’s overview explains that both geography and season affect airborne pollen, with some plants pollinating year-round.
United Kingdom
The Met Office describes a broad sequence: tree pollen in spring, grass from later spring into summer, and weeds from summer into early autumn. Its calendar gives typical windows of late March to mid-May for trees, mid-May to July for grass, and late June to September for weeds. Early tree species can appear before these general windows.
These are approximate patterns, not fixed start and finish dates. See the Met Office’s pollen-season explanation and the UK government’s pollen calendar resources.
United States
There is no single calendar that fits the whole US. NIEHS describes the general spring-tree, summer-grass and autumn-weed pattern, while noting that trees can release pollen as early as January in southern states and as late as May or June in northern states. Ragweed commonly has a late-summer and autumn season.
The NIEHS pollen overview provides the regional context. These broad descriptions should not be treated as a daily forecast for every state.
Use a calendar alongside dated information
A calendar helps explain why a type might appear in a forecast. A dated local forecast provides the short-term outlook. Neither should be interpreted as a complete inventory of everything in the air.
PollenCount’s city directory lists its verified UK and Ireland coverage; it does not yet publish US city forecasts. Pages marked as demos contain synthetic values. Our city directory shows the available coverage. For the six types used in our pages, see the pollen-type guide.