Alnus rubra aka Red Alder

Alnus rubra, or red alder, belongs to the birch family. This deciduous tree comes from western North America. Its coastal range runs from Alaska south to California. It also grows inland, including in Idaho and Montana. It grows fast and spreads by seed. Frankia bacteria in its roots help it fix nitrogen. This lets the tree grow in poor soil.

Garden height estimates range from 40–62 feet [12–19 m]. NC State lists taller trees at 65–98 feet [20–30 m]. Virginia Tech gives an extreme of 120 feet [36.6 m]. Spread estimates range from 13–50 feet [4–15.2 m]. Trees in open ground have broad crowns and spreading branches. Forest trees have straighter trunks and narrower crowns. The gray bark stays smooth until plates form near the base. White lichens can cover much of the trunk. The inner bark turns red when exposed to air.

The simple leaves grow in an alternate pattern along twigs. They have toothed edges that curl under. Their undersides look paler and bear hairs along the veins. The leaves turn yellow in fall. Young twigs have angular sides and clear lenticels. The buds sit on short stalks and have hairy scales. Spring flowers form separate male and female catkins on one tree. Long male catkins hang in small groups. Female catkins are short and thick. Brown, woody, cone-like fruits hold tiny winged seeds. These fruits stay on the tree through winter.

Grow red alder in full sun with moist, well-drained soil. It also tolerates part shade and wetter ground. Suitable soils include sand, loam, clay, and chalk. It accepts acid, neutral, and alkaline soil. Sources also note drought tolerance. NC State lists USDA zones 5–6; RHS rates it H7. It needs little pruning. It suits screens, wildlife gardens, and sites near water. Its roots help hold riverbanks, and it supports woodland recovery. Finches eat its seeds; deer and elk browse its twigs. People use the wood for cabinets and furniture. Native Americans used the bark for dye and medicine.

Tent caterpillars, sawflies, and leaf beetles can damage foliage. These pests seldom kill the tree. Older trees can develop white heart rot. Other risks include Phytophthora and honey fungus. PFAF lists catkins, sap, and prepared inner bark as edible. Catkins taste bitter, while sap tastes sweet. Fresh inner bark can cause vomiting. PFAF states that inner bark needs drying before cooking.