The only member of its genus (Thuja) native to North America, the Western redcedar is not a true cedar.  Belonging to the Cypress family, the redcedar probably gets its name from its similarity to Old World cedars.  A graceful and impressive tree, the redcedar can grow as high as 70 meters and may have a trunk up to five meters wide at the base.  It is hard to gauge the age of redcedars accurately, because they have a tendency to grow buttresses and to develop multiple tops, but one sample of a tree in Washington suggests that they can reach at least 1460 years of age.  The bark of redcedars is greyish or reddish brown, fissured, fibrous and peeling.  The branches are arching.  Its foliage is a glossy green with a lighter, striped underside, arranged in flattened sprays like fans.  Its leaves are evergreen and scaled, and have a pleasant, spicy scent when crushed or chewed.  Wild redcedar populations do not show much genetic variation, but a number of cultivars have been bred by human beings, primarily to produce ornamental plants.

Western redcedars reproduce by seed, by branches that fall and take root, and by branches on fallen trees growing into trees themselves.  These trees have both male and female cones; the reddish male cones are close in to the trunk, while the greenish female cones are distributed farther out on the branches and higher up the tree.  Although individual cones may only contain three to six seeds, the great number of cones on a given tree means that seedfall can be quite large.  Typically redcedars begin to produce cones after a decade or two, reaching peak cone production at an age of seventy or eighty years.  Cone production can continue for several centuries, with strong seed crops produced in two to three year intervals.  The trees produce pollen in the spring, from April to June depending on location, and seedfall happens in autumn in October and November. 

The seeds that fall on disturbed soil in sheltered locations, or on rotted wood in contact with the soil, stand the highest chance of germinating.  Germination usually occurs in the fall, winter and spring, if conditions are right (if not, seeds generally will not germinate the next year, unless they are stored, in which case they will keep up to seven years).  Seedlings are intolerant of temperature extremes, and are vulnerable to sunburn; they prefer shaded, moist areas, making them competitive in brushy areas and in mature forests.  Redcedars tend to lack taproots, growing instead a dense network of smaller roots that generally outmasses the root formation of associated species in mixed forests.  The lack of a taproot makes them vulnerable to windfall; the fallen trees become nursebeds for many species besides their own (indeed, mature redcedars may be festooned with seedlings of other species even while standing).

The range of the Western redcedar is limited by precipitation and temperature; they are not resistant to frost or freezing temperatures and prefer moist locations and acidic, well-drained soil.  Ravines, depressions, and swamps are common habitats for redcedars, as are stream valleys, although they may also be found on rocky slopes.  They tend to grow in mixed coniferous forests rather than in pure stands, with douglas-fir and Western hemlock being the primary associated species.  (Pacific rhododendron and salal are frequent associates in coastal areas.)  There are two main clusters of Western redcedar.  One is a coastal arc stretching from northwestern California up to southeastern Alaska along the Cascades and Coast Range; the other is centered on the Rocky Mountains from Idaho and Montana up into British Columbia.  Canada has the greatest volume of Western redcedar, about 824 million m3 compared to 228 million m3 in the United States.