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 coast; mountains of Oregon, Washington, British Columbia, and Idaho. This fast grower reaches 50–90 ft. tall and 40–60 ft. wide. Branchesspread out, then droop at thetips. Coarsely toothed leavesare dark green above, palergreen beneath. Tolerates heatand wind. Susceptible to tentcaterpillars, borers, and mistletoe in its native range.

Alnus rubra

Native to stream banks andmarshy places from Alaska south to Northern California;usually found in areas withmaritime influence. This is themost common alder of lowlandsin the Pacific Northwest. It cangrow to 90 ft. high but is usuallyseen at 45–50 ft. tall and20–30 ft. wide. Attractive barkis light gray and smooth. Darkgreen leaves are rust-coloredand hairy beneath; coarselytoothed margins are rolledunder. Red alder can take brackish water and is useful wherever underground water issomewhat saline. It’s generallydisliked in the Pacific Northwestbecause it is a favorite of tentcaterpillars.