Tree of the Week: The fast-growing white alder is a thirsty native

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The white alder -- Alnus rhombifolia

The white alder is an inhabitant of ‘the other California,’ that small and shrinking part of the state where the streams are always flowing and water is never in short supply. The tree is native to stream beds in Western states, where it is an important contributor to the variety of riparian woodland species and supports abundant local wildlife. The tree grows very fast, sometimes while standing in water. Often it is the pioneer vegetation in denuded soils, and because it is able to manufacture its own nitrogen thanks to helpful bacteria living in its roots, it can grow in very poor soils.

Because it needs a constant supply of water, this alder finds only limited use as a street tree but is excellent for rehabilitating an abandoned stream site.

White alder leaves slightly smell of tannic acid, which is more noticeable on a foggy fall morning. Indians used the white alder to produce a red dye for their baskets, and to create a perspiration-inducing tea. The wood is used for firewood.

The white alder grows very fast to 40 to 80 feet tall and 40 feet wide. Initially it is pyramidal but later becomes more oval in shape. Slender green twigs turn to red-orange on maturation, and the spreading branches droop at the tip. The pale gray bark on the straight trunk is smooth but breaks into irregular plates near the base of the trunk. The tree is deciduous, but in our mild climate the leaves may cling to it for most of the year.

If it is allowed to grow unpruned, the resulting dense shade would kill most undergrowth. The somewhat leathery, elliptic, 2- to 4-inch-long leaves are simple in shape, placed alternately, and have toothed or double-toothed edges. They are paler green below than on top. The flowers appear before the leaves and show the tree’s relationship to the birch family. Slender male catkins hang down in clusters of two to five, while the short and thick female catkins on the same tree sit upright. These develop into half-inch-long, persistent, woody brown cones, which drop thousands of tiny seeds in fall. The tree tolerates heat and wind but not drought. It will take most soils and wants sun or part shade. Surface roots are usually not a problem. The white alder may be attacked by tent caterpillars and flat-headed borers.

-- Peter Severynen

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