Alder (A. viridis) is active January through May, high pollen season is in the end of February in West Poland, East Ukraine, South of France and North of Spain. High pollen season in Finland and Estonia is in the middle of April. It flowers from end of May to June in Alpine regions at timber line. Ash tree is especially active in France in the end of March. It's active through the first 10 days of May. Cypress family is active entire year, but high season is February and March, especially in Spain, North Italy and Germany.
One of the most widespread in Europe, Birch pollen is observed from March till June, very high in April and May, especially in central and north Europe. cross-react with Alder and Hazel Oak's activity coincides with birch, but the territory is mostly on the West. Olive tree and Plane tree should be watched on the West and South, Sweet chestnut is mostly active in the end of July in France. Other trees such as Hazel display mild activities in February and March. Yew and Willow are also producing pollen in March in UK.
Grass season is May through August, partially in September. Be careful in June, in Spain, France and a part of East Polland. Weeds: mugwort is active in the end of August: Poland and Estonia; nettle/wall pellitory is all over Europe in July and August and can be still observed in winter in South Spain and Italy. Ragweed is especially active in Balkans in the end of August.