Alder (Alnus spp.)

Family: Betulaceae

Flowering: January - March



The tree can become 20 to 30 meters height. The leaves are alternate and rounded. The trunk is gray-brown and mostly smooth but the bark can break up into rectangular plates. The male flowers are clustered in long terminal and drooping catkins (3-12 cm). Catkins are often grouped by three. The female flowers are grouped in small catkins at the end of lateral shoots (2 cm maximum).


Two alder species are native to Belgium: the common or black alder (Alnus glutinosa), and the grey alder (Alnus incana). Their flowering period is different, while the grey alder lowers already from the end of January, the flowering period of the common alder is in March. The common alder is the most widespread in our country. It usually grows on moist soils, typically on pond and river banks and in moist woods. The grey alder is adapted to poor and dry soils. It is therefore typically used for reforestation on slopes and heaps of fallen earth.


This is one of the first pollen wave of the year. In this period, there is a confusion between the symptoms of hay fever and those of a common cold.

Cross-reactive allergy: there is cross-reactive allergy between the pollens of the different Betulaceae species (alder, hazel and birch). Therefore, individuals allergic to birch pollen may also experience allergy symptoms at the beginning of the year, when the concentrations of alder and hazel pollens are very high.