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Gray Alder

Alnus incana

Alnus incana (Grey or Speckled Alder) is a species in the Betulaceae (Birch) family with a wide range across the cooler parts of the Northern Hemisphere. In California it is found primarily in the Sierras. It is a small to medium size tree 15-20 meter tall with smooth grey bark even in old age, its life span being a maximum of 60-100 years. The leaves are matte green, ovoid, 5-11 centimeter long and 4-8 centimeter broad. The flowers are catkins, appearing early in spring before the leaves emerge, the male catkins pendulous and 5-10 centimeter long, the female catkins 1.5 centimeter long and one centimeter broad when mature in late autumn. The seeds are small, 1-2 millimeter long, and light brown with a narrow encircling wing. The Grey Alder has a shallow root system, and is marked not only by vigorous production of stump suckers, but also by root suckers, especially in the northern parts of its range. The wood resembles that of the black alder, but is somewhat paler and of little commercial value.

Mountain Alder

Alnus incana ssp. tenuifolia

Mountain Alder (Alnus incana ssp. tenuifolia) is a native tree or shrub that grows in northern, southern and central California. It is fast growing and moderately long-lived. It grows in an upright form to a height of 20 feet, with active growth during the spring and summer. Flowers are purple and bloom in the early spring. Leaves are dark green and deciduous. It tends to grow in wet places, at elevations from 4500-8000 feet.

White Alder

Alnus rhombifolia

White alder (Alnus rhombifolia) is a fast-growing deciduous tree in the birch family (Betulaceae), native to western North America at elevations of 300–7,900 feet. It thrives in moist soils, often found along streambanks and other wet areas.

This tree provides excellent shade, with dark green leaves in summer that drop in winter. In early spring, before the leaves emerge, white alder produces noticeable male catkins, which hang in clusters and release large amounts of pollen. The female catkins mature in autumn, resembling small cones that may persist on the tree for up to a year.

White alder is heat and wind tolerant but requires deep watering to establish. Its roots can be invasive, so it should be planted with caution, especially near structures or small residential gardens. However, it is an excellent choice for woodland plantings and riparian areas.

This tree has high wildlife value. It serves as a larval food plant for the Western Tiger Swallowtail butterfly and provides habitat and food for various bird species.

Red Alder

Alnus rubra

Alnus rubra (red alder) is a deciduous broadleaf tree in the Betulaceae (Birch) family native to western North America. In California it is found primarily along the coast from San Luis Obispo County northward. In southern california Alnus rhombifolia is the more commonly found alder. Red Alder is the largest species of alder in North America and one of the largest in the world, reaching heights of 20-30 meters. The official tallest red alder (1979) stands 32 meters tall in Clatsop County, Oregon (USA). The name derives from the bright rusty red color that develops in bruised or scraped bark. The bark is mottled, ashy-gray and smooth, often draped with moss. Large trees are often unbranched for a considerable distance from the ground. The leaves are ovate, 7-15 centimeter long, with bluntly serrated edges and a distinct point at the end; the leaf margin is revolute, the very edge being curled under, a diagnostic character which distinguishes it from all other alders. The leaves turn yellow in the autumn before falling. The male flowers are dangling reddish catkins 10-15 centimeter long in early spring, and female flowers are erect catkins which develop into small, woody, superficially cone-like oval dry fruit 2-3 centimeter long. The flowers are wind pollinated, and a large amount of airborne pollen is produced in early spring. The seeds develop between the woody leaves of the 'cones' and are shed in the autumn and winter.

Surface roots can be problematic if the tree receives frequent, shallow watering. Better to water deep and infrequently. Established trees with deep roots can be surprisingly drought tolerant. This is a large tree that requires plenty of space. Best suited for restoration or large gardens with bioswales or streams.

Green Alder

Alnus viridis

Alnus viridis (Green Alder) is an alder with a wide range across the cooler parts of the Northern Hemisphere. It is a large shrub or small tree 3-12 meter tall with smooth grey bark even in old age. The leaves are shiny green, ovoid, 3-8 centimeter long and 2-6 centimeter broad. The flowers are catkins, appearing late in spring after the leaves emerge (unlike other alders which flower before leafing out); the male catkins are pendulous, 4-8 centimeter long, the female catkins 1 centimeter long and 0.7 centimeter broad when mature in late autumn, in clusters of 3-10 on a branched stem. The seeds are small, 1-2 millimeter long, light brown with a narrow encircling wing.

Siberian Alder

Alnus viridis ssp. fruticosa

Sitka Alder

Alnus viridis ssp. sinuata

Sitka Alder (Alnus viridis ssp. sinuata) is a native tree or shrub that grows in Northern California, primarily in the North Coast, the Klamath Mountain Range and North Coast Range regions. It is slow growing and long-lived. It grows in an upright form to a height of 16 feet, with active growth during the spring and summer. Leaves are light-green and deciduous. It tends to grow in creek and meadow edges, at elevations from 3300-8900 feet.

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