Alnus incana Seeds | Grey Alder
LATIN NAME: Alnus incana
COMMON NAME: Grey Alder
Plant Profile
Alnus incana is the alder of cold, fast water, and it grows where the bank is washed every spring. The trunk is often short and the crown broad, and the bark is smooth and grey, which is the easiest way to tell it from the common alder, whose bark is dark and cracked. The leaves are oval, doubly toothed and grey-white underneath, and they stay that way through the summer.
The species grows along mountain streams from Scandinavia to the Caucasus, and it is one of the first trees to colonise gravel bars and landslides. It is short-lived for a tree, seldom past sixty years, and it leans and roots from the stem where silt builds up around it. The catkins open in early spring, before the leaves, and the cones that follow stay on the tree through winter.
Science & Biotechnology
Alder is actinorhizal rather than leguminous. It fixes nitrogen through a partnership with the actinobacterium Frankia, which lives in nodules on the roots and converts atmospheric nitrogen into a form the tree can use. That is why alder grows on gravel and on mine spoil where nothing else will, and why its fallen leaves are rich in nitrogen and improve the soil beneath it.
The tree also tolerates waterlogging through aerenchyma in the roots, air channels that carry oxygen down from the stem. Grey alder differs from common alder in its ability to resprout from the stump, which makes it a useful coppice species, and in its tolerance of cold, which takes it further north and higher up the mountains.
Uses & Benefits
The wood is soft, light and resistant to water when submerged, and it was used for piles, for water pipes and for clogs. It burns well and is used for charcoal, and the bark yields a red-brown dye and a tannin used in leatherwork. The tree is planted to stabilise riverbanks, to reclaim spoil heaps and to shelter livestock.
In folk medicine the bark was used as an astringent and as a gargle for sore throats, and the leaves were applied to inflamed skin. The tree is also used in agroforestry, where its nitrogen improves the soil for the crop beside it, and as a nurse for more valuable trees on poor ground.
Growing from Seed
Seed is small and short-lived. Sow it on the surface in spring, keep it moist, and it germinates in two to three weeks without pretreatment. Cuttings of hardwood taken in winter root readily, and the tree also suckers, which is how it holds a bank.
The tree wants full sun and wet ground, and it tolerates flooding, gravel and poor soil. It is hardy to about minus thirty-five. Plant it where the roots can reach water, and do not expect it to live long: it is a pioneer, and it will decline as the site matures. Coppice it every ten to fifteen years to keep it young.
Our Take
A riverbank pioneer that fixes its own nitrogen, holds a bank through a flood and then hands the site to something longer-lived.
Related Reading
Gardeners who grow Grey Alder often pair it with these related subjects: