Sweet Pepperbush

Growing to 1.5–3 m (4 ft 11 in – 9 ft 10 in) tall, it is a deciduous shrub. The leaves are obovate to oblong, 4–10 cm long and 2–4 cm broad, with a serrated margin; they are green turning yellow-golden during the autumn. The leaves are alternately arranged. The flowers are white or very pale pink, 5–10 mm in diameter, and have a sweet, somewhat cloying fragrance. The flowers, which are attractive to bumblebees, are produced in racemes up to 15 cm long and 2 cm broad in late summer, depending on the cultivar. The scent of C. alnifolia is described as sweet. The "pepper" part of the common name derives from the mature fruits, capsules which have a vague resemblance to peppercorns, however with no element of spiciness.

It grows in wet forests, pine flatwoods, wetlands, bogs and can be seen alongside woodland streams. It has moderate salt tolerance and can be found inland in coastal areas. It prefers a neutral to acidic soil. The Nova Scotian disjunct population is small and endangered, and C. alnifolia is listed as a threatened species under the Canadian Species at Risk Act.

C. alnifolia is typically used as a shrub for natural gardens, or is placed alongside a stream or pond to delay erosion. It does not leaf out until very late in the season, limiting its landscaping use. Several cultivars have been selected for garden use, including 'September Beauty'. Both 'Ruby Spice', with strongly pink flowers, and the dwarf 'Hummingbird' have gained the Royal Horticultural Society's Award of Garden Merit (confirmed 2017).

The Latin specific epithet alnifolia means "with leaves like alder (Alnus)". This gave rise to another common name, common white alder, though the two plants are not closely related.