Alders are trees that compose the genus Alnus in the birch family Betulaceae. The genus includes about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few species extending into Central America, as well as the northern and southern Andes.
Alders can be trees or large shrubs. Depending on the species, they can grow from small, multi-stemmed shrubs to tall trees up to 35 meters in height. The leaves of alder trees are usually simple, alternate, and can range from broadly ovate to elliptical. They often have serrated or double-serrated edges. Some species have leaves with a distinctive shape, like the deeply lobed leaves of certain cultivars. The color is typically a vibrant green, though some species or varieties might show variations like greyish or bluish hues beneath.
Young alders have smooth bark, often with a grey or greenish color. As they age, the bark can become more textured, fissured, or even scaly. Some species have bark that’s used for natural dyes.
Alders are monoecious, they have both male and female flowers on the same tree. Male catkins are long, pendulous, and appear in clusters. Female catkins are smaller, upright when pollinating, and develop into woody cones that contain seeds. After pollination, female catkins develop into small, woody, cone-like structures that persist on the tree, often through winter. These cones release tiny winged seeds.
Many alder species are deciduous, losing their leaves in autumn. Before falling, leaves might turn a yellowish color, though this isn’t always particularly vibrant. Alder species thrive in wet soils and are often found along riverbanks, in wetlands, or in floodplains. They can tolerate flooding and help stabilize riverbanks.
Alder Trees
Common Alder (Alnus glutinosa)
- Description: A medium-sized tree that can grow up to 20-30 meters tall. It has dark, fissured bark, and its leaves are rounded, with a wavy or slightly toothed margin. It produces catkins; male catkins are long and pendulous, while female catkins are smaller and become woody cones.
- Distribution: Native to most of Europe, extending into North Africa and Western Asia. It’s also been introduced in other parts of the world.
Red Alder (Alnus rubra)
- Description: This is the largest species of alder, reaching heights of up to 35 meters. The leaves are ovate, with serrated edges and a pointed tip. The bark is mottled, light gray to whitish, often covered with mosses and lichens.
- Distribution: Native to the Pacific Northwest of North America, from British Columbia, Canada, to northern California, USA.
Italian Alder (Alnus cordata)
- Description: Known for its heart-shaped leaves that are glossy green. It grows quickly, up to 25 meters, with a conical shape when young, broadening with age.
- Distribution: Native to southern Italy, Corsica, and Albania. It’s been widely planted elsewhere for its rapid growth and soil improvement properties.
White Alder (Alnus rhombifolia)
- Description: Grows to about 15-30 meters. The leaves are diamond-shaped to slightly rounded, dark green, with a finely serrated edge. Unlike many alders, it doesn’t thrive in very wet conditions but prefers riparian zones.
- Distribution: Native to western North America, from Washington to California.
Speckled Alder (Alnus incana rugosa)
- Description: A shrub or small tree, typically growing to 10 meters. Its leaves are ovate with a doubly serrated margin and often have a wrinkled appearance.
- Distribution: Found in northeastern North America, particularly in wet areas.
Grey Alder (Alnus incana)
- Description: Can reach up to 20 meters. It has greyish bark and leaves that are more pointed than those of the common alder, with a greyish underside, giving the tree its name.
- Distribution: Widespread in Europe, from Scandinavia to the Mediterranean, and east into Russia.
Black Alder (Alnus glutinosa ‘Laciniata’)
- Description: A cultivar of the common alder, distinguished by its deeply lobed leaves, which give it a more ornamental look.
- Distribution: Cultivated variety, so found where planted, originally from Europe.
Japanese Alder (Alnus japonica)
- Description: This tree can grow up to 25 meters. Its leaves are slender, elongated, and have a finely toothed margin. The bark is smooth and gray.
- Distribution: Native to Japan, Korea, Taiwan, and parts of China.
Green Alder (Alnus viridis)
- Description: Often more of a shrub than a tree, rarely exceeding 10 meters. Its leaves are bright green, rounded to ovate, with sharp serrations.
- Distribution: Found in mountainous regions of Europe, North America, and Asia.
Sitka Alder (Alnus viridis sinuata)
- Description: Often grows as a shrub or small tree, with thin, serrated leaves that have a shiny surface. Known for its ability to fix nitrogen.
- Distribution: Western North America, particularly in the Pacific Northwest and Alaska.
Mountain Alder (Alnus tenuifolia or Alnus incana tenuifolia)
- Description: Typically grows in a shrub form or small tree, with thin, ovate leaves that have a slightly blue-green hue.
- Distribution: Found in western North America, particularly in mountainous and subalpine areas.
Alder Buckthorn (not a true alder, but often confused with them, Frangula alnus)
- Description: A shrub or small tree with glossy leaves, and inedible berries that turn from red to black.
- Distribution: Native across Europe, into North Africa and Western Asia.
Hazel Alder (Alnus serrulata)
- Description: Small tree or shrub with fine-toothed, elliptic leaves. It’s notable for its smooth bark and its winter catkins.
- Distribution: Eastern United States.
Seaside Alder (Alnus maritima)
- Description: Can grow as a small tree or large shrub, with leaves that are more oval and less toothed than other alders.
- Distribution: Limited to the coastal plain of the southeastern United States.
Nepalese Alder (Alnus nepalensis)
- Description: A fast-growing tree that can reach up to 30 meters. It has a silvery bark and leaves that are dark green above and lighter below.
- Distribution: Native to the Himalayas, from Pakistan through Nepal to Southwest China, thriving in subtropical highlands.
Peruvian Alder (Alnus acuminata var. peruviana)
- Description: Large tree with smooth, gray bark and toothed leaves.
- Distribution: Native to Peru and Ecuador.
Manchurian Alder (Alnus mandshurica)
- Description: Large tree with smooth, gray bark and deeply lobed leaves.
- Distribution: Found in northeastern China, Korea, and Russia.
Seaside Alder (Alnus maritima)
- Description: Smaller than other alders, with leathery leaves and a tolerance for salt.
- Distribution: Found along the Pacific coast of North America, from British Columbia to California.
Black Alder (Alnus glutinosa)
- Description: Dark bark, sticky leaves when young, and cone-like fruits.
- Distribution: Found throughout Europe, North Africa, and parts of Western Asia. In North America, it’s introduced and naturalized in some regions.