Common Alder or European Alder

(Betulaceae - Birch Family)

Form

• large shade tree, doubling as an ornamental tree in youth
• maturing at 60' tall by 30' wide
• upright pyramidal growth habit in youth, becoming upright oval or open and irregular with maturity, sometimes losing its strong central leader with age, and also sometimes found in multitrunked form
• rapid to medium growth rate

Culture

• full sun to partial sun
• prefers moist to wet soils of average fertility in full sun, but is adaptable to poor soils, dry soils, and soils of various pH
• species form is propagated primarily by seeds but also by rooted stem cuttings, while the cultivars are propagated by rooted stem cuttings or cuttings grafted onto species rootstock
• Birch Family, with branch cankers and tent caterpillars as occasional minor disease and pest problems, respectively
• low availability, in ball and burlap form

Foliage

• alternate, densely foliaged, and dark glossy green
• leaf blades are 3" long and 3" wide, oval to orbicular, doubly serrated, and distinctly notched at the apex when fully expanded, with impressed veins
• like most members of the Birch Family, its leaves flutter in the slightest breeze, due to its broad leaf blade base and the relatively long petiole
• fall color is green, chartreuse, or yellow-brown, and ornamentally ineffective

Flowers

• monoecious (separate male and female flowers on the same plant)
• clustered reddish-brown male catkins are present in Winter, to about 1" long, but elongate up to 4" in March, swaying in the early Spring winds
• miniature oval purple female flowers are barely noticeable in March, without any corolla but emerging with exserted pistils to accept pollen from the nearby catkins

Fruits

• small brown winged nutlet seeds are borne in green ovoid rough fruits that are about 0.5" long, changing to dark brown in Autumn
• the Winter-persistent clusters of opened fruits (strobiles) that resemble miniature pine cones are very characteristic of Alders, and often persist into the following season

Twigs

• green-brown to brown and smooth

Trunk

• bark is green-brown in youth, changing to brown and slightly ridged with maturity
• trees can be single leader or multitrunked, and are strongly pyramidal in youth, but more upright oval and sometimes losing the central leader with age

ID Summary

• alternate leaves emerge orbicular, on long petioles with doubly serrated margins, and develop a notch at the apex and a glossy upper surface when fully expanded, and flutter with the slightest breezes
• male catkins expand to 4" long in March, swaying in the wind, and pollinate the almost microscopic female flowers close by, which are devoid of a corolla; the resultant fruits are 0.5" long green knobs that release their seeds in Autumn, and persist into the Winter and following season as dark brown conelets that are an excellent identification feature
• growth habit of the species form is pyramidal to upright oval, with one or more trunks, and preferring wet sites

USAGE

Function
• shade, specimen, wet site, or deciduous windbreak tree, effective in solitary, group, or linear plantings

Texture

• medium texture in foliage and when bare
• thick density in foliage and when bare

Assets

• wet site or dry site tolerant
• rapid growth and establishment
• dark glossy green foliage that flutters in the slightest breeze
• can also be considered an ornamental tree in youth (due to its strongly pyramidal habit, Winter-persistent cone-like fruits, and late Winter elongated catkins)
• wood is not brittle as compared to other rapidly growing landscape trees, such as Silver Maple (Acer saccharinum), Callery Pear (Pyrus calleryana), Willows (Salix species), and Sweetgum (Liquidambar styraciflua), in terms of being prone to storm damage

Liabilities

• none serious, although branch cankers and tent caterpillars occasionally occur

Habitat

• zones 3 to 7
• native to Europe, Western Asia, and Northern Africa
• naturalizes along wet sites where it has escaped to and self-propagated by seeds

Alternates

• rapidly growing shade trees (Acer x fremanii, Betula nigra, Liquidambar styraciflua, Quercus acutissima, Zelkova serrata, etc.)
• shade trees for wet sites (Betula nigra, Celtis occidentalis, Populus deltoides, Quercus bicolor, Salix alba, Taxodium distichum, etc.)
• pyramidal trees (at least in youth) (Abies fraseri, Picea abies, Liquidambar styraciflua, Quercus palustris, Taxodium distichum, etc.)
• trees with dry fruit appeal in Winter (Gleditsia triacanthos [species form], Gymnocladus dioicus [female], Liriodendron tulipifera, Ostrya virginiana, etc.

Variants

• Alnus glutinosa 'Imperialis' - a cutleaf form, with light green leaves that have sinuses cut over halfway to the midrib, somewhat awkward looking in youth but maturing into a fine specimen tree
• Alnus glutinosa 'Pyramidalis' - an extremely upright columnar form, maturing at 40' tall by 10' wide, which can be used in rows as a deciduous privacy screen or individually as a distinct focal point or specimen tree; this columnar form (actually not a pyramidal form, as the cultivar name implies) is a possible substitute for Lombardy Poplar (Populus nigra 'Italica', which is extremely susceptible with age to a fatal trunk canker and has surface roots that sprout many suckers), but unfortunately is rarely available in the nursery trade

NOTES

Translation

• Alnus is the Latin name for Alder.
• glutinosa translates as "sticky", referring to the feel of the young shoots and leaves.

Purpose

• Common Alder is an excellent wet site or dry site shade tree, whose roots also fix atmospheric nitrogen as a self-made source of nitrogen fertilizer.

Summary

• Alnus glutinosa is an underutilized shade tree that is excellent for wet or dry sites, having glossy dark green summer foliage, a pyramidal shape and rapid growth in youth, and ornamental fruits and catkins in Winter.