The Italian Alder Alnus cordata, native to Italy and Corsica, was introduced in 1820 and is now common in parks, gardens and urban areas. It grows rapidly and is highly tolerant of urban pollution. It has catkins like the other Alders but the female cone-like catkins are bigger than those on the Common and Grey Alder. It has glossy heart-shaped leaves that stay on the tree until November.
The Grey Alder Alnus incana is a medium sized tree that grows in poor soils and tolerates air pollution. It is native to the mountains of Europe and the Caucasus, except Britain, and was introduced in 1780. It is used on reclamation sites and in urban areas. Like all Alders it fixes nitrogen and improves the soil. It has catkins and can be confused with the Common Alder and Italian Alder but it has smaller cone-like catkins, a different leaf shape and smooth bark.
The Green Alder Alnus viridis is a bush or small tree native to the mountains of central and south-eastern Europe. It was introduced to Britain in 1820 but it is rare and only found in a few collections. It grows well on poor soils. In the Alps its natural habitat is on steep, north-facing slopes at elevations of 1000 to 2000m but it is increasingly colonising abandoned sub-alpine pastures.
The Italian Alder leaf is heart shape with a pointed end, quite different from that of the Common Alder which has a blunt indented end.
Close-up of the Italian Alder male catkins in March before shedding pollen. The red anthers are not yet open to release pollen.
Grey Alder male catkins, hanging down, with a small female catkin, sticking up at the top. Photo in October. They will shed pollen in the following February.
Woody, cone-like, female catkins of the Italian Alder are bigger than any other alder. This is a catkin that has released its seeds in autumn, photographed in the following July
Grey Alder female, cone-like, catkins in October. The one on the left is ready to shed seeds, those on the right have already shed seeds. The catkins are smaller than those of other Alders.