Betula alleghaniensis is a medium-sized, typically single-stemmed, deciduous tree reaching 60–80 feet (18–24 m) tall (exceptionally to 100 ft (30 m)) with a trunk typically 2–3 ft (0.61–0.91 m) in diameter, making it the largest North American species of birch. Yellow birch is long-lived, typically 150 years and some old growth forest specimens may last for 300 years.
It mostly reproduces by seed. Mature trees typically start producing seeds at about 40 years but may start as young as 20. The optimum age for seed production is about 70 years. Good seed crops are not produced every year, and tend to be produced in intervals of 1–4 years with the years between good years having little seed production. The seeds germinate best on mossy logs, decaying wood or cracks in boulders since they cannot penetrate the leaf litter layer. This can lead to odd situations such as yellow birches with their roots growing around a tree stump, which when it eventually rots away leaves the birch standing atop stilt-like roots. Yellow birch saplings will not establish in full shade (under a closed canopy) so they typically need disturbances in a forest in order to establish and grow. The tree is fairly deep-rooted and sends out several long lateral roots.
The bark on mature trees is a shiny yellow-bronze which flakes and peels in fine horizontal strips. The bark often has small black marks and dark horizontal lenticels. After the tree reaches a diameter greater than 1 ft the bark typically stops shredding and reveal a platy outer bark although the thinner branches will still have the shreddy bark. There is an uncommon, alternate form of the tree (f. fallax) which grows in the southern part of the range. F. fallax has darker gray-brown bark which shreds less than the typical form.
The twigs, when scraped, have a slight scent of wintergreen oil, though not as strongly so as the related sweet birch (B. lenta), which is the only other birch in North America to also smell of wintergreen. However, the potency of the odor is not considered a reliable identification method unless it is combined with other characteristics.
The leaves are alternately placed on the stem, oval in shape with a pointed tip and often a slightly heart shaped (cordate) base. They are 2–5 in (5.1–12.7 cm) long and typically half as wide with a finely serrated (doubly serrate) margin. They are dark green in color on the upper side and lighter on the bottom, the veins on the bottom are also pubescent. The leaves arise in pairs or singularly from small spur shoots. In the fall the leaves turn a bright yellow color.
The leaf has a very short petiole1⁄4–1⁄2 in (1–1 cm) long.
The flowers are wind-pollinated catkins which open in later spring. Both male and female flowers will occur on the same tree making the plant monoecious. The male catkins are 2–4 in (5–10 cm) long, yellow purple, pendulous (hang downwards), and occur in groups of 3-6 on the previous year's growth. The female catkins are erect (point upward) and 1.5–3 cm (5⁄8–1+1⁄8 in) long and oval in shape, they arise from short spur branches with the leaves. The fruit, mature in fall, is composed of numerous tiny winged seeds packed between the catkin bracts.
The seed is a winged samara with two wings which are shorter than the width of the seed which matures and gets released in autumn.
Similarity to Betula lenta
Both yellow birch and sweet birch (B. lenta) have nearly identical leaf shape and both give an odor of wintergreen when crushed. Seedlings of the two species can be very hard to tell apart. To differentiate the two, the range, buds, or bark must be examined. The ranges do overlap in Appalachia where they commonly grow together, but sweet birch does not grow west of Ohio or north into Canada whereas yellow birch does. Sweet birch also has black non-peeling bark compared to the lighter, bronze colored, peeling bark of yellow birch. For young trees where bark has not yet developed, yellow birch can also be identified by its hairy buds and stems; sweet birch has hairless buds.