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Alder (Alnus)

Alders, birches, hazelnuts and hornbeams are all members of the Betulaceae family. Two species are native to Belgium: the common alder (european alder = Alnus glutinosa), and the grey alder (Alnus incana).

The common alder is the most frequent in our country, usually growing on moist soils, on river banks, along streams and ponds, in moist woods. The grey alder needs less water and is adapted to poorer soils; consequently it is often planted on slopes, heaps of fallen earth or used for afforestation.

Grey alder flowers from early to late January, the common alder in March. The male and female flowers are formed in separate catkins. Male catkins are long, pendulous and often grouped in 3. Female catkins are small and often grouped at the end of lateral shoots. The catkins are getting woody when growing old.

The first grains of alder pollen can be seen in our samples already at the beginning of January.

The clinical role of alder pollen in respiratory allergy has been demonstrated, but the frequency of accidents is rather low. Indeed densities of pollen grains released into the air depend essentialy on the winter conditions.

It should be noted that crossreactivity exists between the different pollens of the Betulaceae family; sufferers sensitized to birch pollen may thus experience symptoms at the beginning of the year when concentrations of alder pollen are very high. This is however rarely the case.