Alnus rhombifolia aka White Alder

Alnus rhombifolia is white alder, also called Californian alder. This deciduous tree belongs to the birch family, Betulaceae. Its native range spans the western United States and northern Baja California. It grows beside streams, in canyon bottoms, and on lower mountain slopes. These sites often have rocky or gravelly soil. It grows from near sea level into the mountains.

Mature trees often stand 50–80 feet (15–24 metres) tall. One source gives a smaller height of 39 feet (12 metres). Record trees exceed 100 feet (30 metres). The crown spreads into a broad, open dome. Slender branch tips droop, and forest trees may have several trunks. It can also form a large shrub. Young bark is pale gray and fairly smooth. Older bark turns brown and breaks into plates near the base. Green twigs turn red-orange as they age.

The leaves grow alternately and fall each year. They are oval to diamond-shaped, with finely toothed edges. Their upper sides are glossy green; the undersides are paler. Each leaf measures about 2–4 inches long. Male and female flowers grow on the same tree. Male catkins hang in clusters and release wind-borne pollen. Female catkins are short and upright near branch tips. Flowering takes place in early spring, with March reported. Small brown, woody cones hold tiny, unwinged nutlets. Seeds ripen from autumn into winter, while cones can persist.

White alder grows fast but has a fairly short life. It reaches full size in about 50–60 years. Grow it in sun with moist or wet soil. It also accepts light woodland shade and heavy clay. Root nodules contain bacteria that fix nitrogen, helping it grow on poor sites. It tolerates heat and strong winds, but not coastal exposure. Hardiness reports differ: Oregon State gives USDA zone 6. Plants For A Future lists zones 8–11. Tent caterpillars and borers can attack it within its native range.

Its size suits woodland planting, groups, and wet garden sites. The roots have supplied basket material; bark supplies dyes and tannin. The soft, brittle wood serves mainly as fuel. Reported edible parts include bitter catkins and dried inner bark. Fresh inner bark can cause vomiting. Traditional remedies use bark for several ailments, but these reports do not establish safety or effectiveness.