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Alnus

Alder
Betulaceae
Deciduous, Trees

Moisture-loving trees that thrive in moist or wet soils, even tolerate periodic flooding. Good near creeks and other waterways. Very fast growing. In all species, clusters of tassel-like, greenish yellow male flower catkins give interesting display before leafout. Female flowers develop into small woody cones that decorate bare branches in winter; these delight flower arrangers. Seeds attract birds. Alders need little pruning except to remove suckers, crossing branches, and dead wood.

Alnus glutinosa

Native to Europe, North Africa. Probably best as multistemmed tree; grows moderately quickly to 70 ft. tall, 30 ft. wide. Roundish, 2–4-in., coarsely toothed leaves in lustrous. Makes dense mass from ground up. Good for screening.

Alnus rhombifolia

Native along streams throughout most of California’s foothills except along the coast; mountains of Oregon, Washington, British Columbia, and Idaho. This fast grower reaches 50–90 ft. tall and 40–60 ft. wide. Branches spread out, then droop at the tips. Coarsely toothed leaves are dark green above, paler green beneath. Tolerates heat and wind. Susceptible to tent caterpillars, borers, and mistletoe in its native range.

Alnus rubra

Native to stream banks and marshy places from Alaska south to Northern California; usually found in areas with maritime influence. This is the most common alder of low lands in the Pacific Northwest. It can grow to 90 ft. high but is usually seen at 45–50 ft. tall and 20–30 ft. wide. Attractive bark is light gray and smooth. Dark green leaves are rust-colored and hairy beneath; coarsely toothed margins are rolled under. Red alder can take brackish water and is useful wherever underground water is somewhat saline. It’s generally disliked in the Pacific Northwest because it is a favorite of tent caterpillars.

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