All species have windborne pollen which has been reported to be a significant allergenic cause. People sensitive to one alder species are usually sensitive to all species.
Alders are deciduous trees or large shrubs that often grow together in moist soils in wooded areas of North America. All species have separate drooping male and upright female flowers on catkins of the same plant. Flowering precedes the opening of leaf buds and usually occurs during January to April depending on latitude and elevation.
Pollen grains of Alnus are oblate to oblate-spheroidal and usually 4-5 porate. Their exinous thickened bands that extend in pairs from aperture to aperture distinguish the grains.
Pollen grains vary in size from 19-21 x 23-30 micrometers.
The shaded areas on the map indicates where the genus has been observed in the United States.
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