It is the largest species of alder in North America and one of the largest in the world, reaching heights of 20-35 m. The official tallest red alder (1979) stands 32 meters tall in Clatsop County, Oregon (USA). The name derives from the bright rusty red color that develops in bruised or scraped bark. The bark is mottled, ashy-gray and smooth, often draped with moss. The leaves are ovate, 7-15 cm long, with bluntly serrated edges and a distinct point at the end; the leaf margin is revolute, the very edge being curled under, a diagnostic character which distinguishes it from all other alders. The leaves turn yellow in the autumn before falling. The male flowers are dangling reddish catkins 10-15 cm long in early spring, and female flowers are erect catkins which develop into small, woody, superficially cone-like oval dry fruit 2-3 cm long. The seeds develop between the woody bracts of the 'cones' and are shed in the autumn and winter.
In southern Alaska, western British Columbia and the northwestern Coast Ranges of the USA, red alder grows on cool and moist slopes; inland and at the southern end of its range (California) it grows mostly along streams and in swamps. It is associated with coast Douglas-fir Pseudotsuga menziesii subsp. menziesii, western hemlock Tsuga heterophylla, grand fir Abies grandis, western redcedar Thuja plicata, and Sitka spruce Picea sitchensis forests. Along streambanks it is commonly associated with willows Salix spp., red osier dogwood Cornus stolonifera, Oregon ash Fraxinus latifolia and bigleaf maple Acer macrophyllum.
To the southeast of its range it is replaced by white alder (Alnus rhombifolia), which is closely related but differs in the leaf margins not being rolled under. In the high mountains it is replaced by the smaller Sitka alder (Alnus viridis subsp. sinuata), and east of the Cascade Mountains by thinleaf alder (Alnus incana subsp. tenuifolia).
In moist forest areas Alnus rubra will rapidly cover a former burn or clearcut, temporarily preventing the growth of conifers but also improving soil fertility for future growth of conifers. It is a prolific seed producer, but the seeds require an open area of mineral soil to germinate, and so skid trails and other areas disturbed by logging or fire are ideal seedbeds. Such areas may host several hundred thousand to several million seedlings per hectare in the first year after landscape disturbance (Zavitkovski & Stevens 1972).
Twigs and buds of alder are only fair browse for wildlife, though deer and elk do browse the twigs in fall and twigs and buds in the winter and spring. Beavers eat the bark. Several finches eat alder seeds, notably common redpoll and pine siskin, and as do deer mice.
A russet dye can be made from a decoction of the bark and was used by Native Americans to dye fishing nets so as to make them less visible underwater. Native Americans used red alder bark (Alnus rubra) to treat poison oak, insect bites, and skin irritations. Blackfeet Indians used an infusion made from the bark of red alder to treat lymphatic disorders and tuberculosis. Recent clinical studies have verified that red alder contains betulin and lupeol, compounds shown to be effective against a variety of tumors.
Alnus rubra is an important forestry tree. Its rapid growth makes it useful in covering disturbed land, such as mine spoils. Alder leaves, shed in the fall, decay readily to form a nitrogen-enriched humus. It is being considered as a rotation crop to discourage the conifer root pathogen Phellinus weirii (Laminated root rot). The vigorous growth has in the past earned it the designation of a "trash tree" by the timber industry. Herbicide spraying of red alder over large areas of coastal Oregon and Washington has resulted in a number of lawsuits claiming it caused health problems, including birth defects and other side effects. The increased value of the wood, combined with a better understanding of the species' benefits to other trees, has largely led to a cessation of this practice.
The wood is not durable in outdoor use but due to its workability and ease of finishing it is increasingly in use for furniture and cabinetry. Historically it has not been considered of high value for timber but it is now becoming one of the western USA's more important hardwoods. Alder wood ranges from white through pinkish to light brown, is relatively soft, and has medium luster. It is easily worked, glues well, and takes a good finish.
Because of its oily smoke, A. rubra is the wood of choice for smoking salmon.
Alnus rubra are not commonly planted as ornamental trees but will do well in wet swales or on stream banks. If used domestically they should be planted well away from drainpipes, sewage pipes, and water lines, as the roots may well invade and clog the lines. Alnus rubra are also famed for growing easily in burned or destroyed land, and are used as "pioneering" or "reclamation" trees.
Additionally, red alder is often used by scientists as a biomonitoring organism to locate areas prone to ozone pollution, as the leaves will react to the presence of high ozone levels by developing red to brown or purple discolourations.