There are 120 known species within the dogwood family (Cornaceae) according to The Sibley Guide to Trees, 2009. These species include trees, shrubs and a few herbs. Most dogwood species in North America are shrubs but there are four native species of dogwood trees. Dogwoods can be divided into three major groups based on their flower structure: flowering dogwoods, shrub dogwoods and cornels.
In general, dogwoods can be recognized as a group by their curved leaf veins that follow the leaf edge back towards the middle-vein, their opposite leaf structure (two ovate leaves at each node), and their berry-like clusters of fruit. The subgenus of flowering dogwoods (Benthamidia), including Flowering Pacific and Kousa Dogwood have stalk-less flowers bordered by large showy white bracts, which are easily mistaken for petals. Members of the subgenus of shrub dogwoods (Swida), including Alternate-leaf and Rough-leaf, have small, white flowers in branched clusters without bracts. The subgenus of cornels (Cornus) has small yellow to greenish white flowers on short stalks, with small bracts. The Cornelian-cherry, and Black-fruit Dogwood are included in this group.
Because dogwoods do not grow thick trunks and are mostly shrub-like in stature, they are not generally used for commercial purposes. However, several of the species are grown commercially because of their ornamental popularity. The Flowering Dogwood for instance is often used to make boxwood wreaths. In their natural setting, these trees are important because they help to make up the understory in wetlands and thickets. They create habitat for birds, which feed off of their fruits.
Dogwoods commonly grow throughout the continental United States, and parts of Alaska and Canada. The Flowering Dogwood (Cornus florida), also known as the Boxwood, is native to eastern North America, as is the Alternate-leaf Dogwood (Cornus alternifolia). Some species such as the Rough-leaf Dogwood (Cornus drummondii) grows in USDA Hardiness Zones 4 to 8 in the Midwest and central United States. The Red Osier (Cornus stolonifera) grows across the entire span of the northern United States and in Canada below the Arctic Circle. The Pacific Dogwood (Cornus nuttallii), also known as the Mountain Dogwood, grows in USADA Zones 7 to 9 along the Pacific Northwest coast from northern California to southern British Columbia.
Most species of dogwoods thrive in woodland areas such as the commonly cultivated Flowering Dogwood, which is common in the understory of rich, open woods. The Alternate-leaf Dogwood grows less commonly in the understory of broadleaf or coniferous woods in moist to dry soil. The Pacific Dogwood is found growing in the understory at low elevations, especially on well-drained slopes. The Red Osier is commonly seen along watercourses.
Dogwoods are not large trees, and many of species are shrub-sized. When identifying trees it is always best to refer to a tree identification field guide, such as the well-illustrated, Sibley Guide to Trees, or The National Audubon Society Field Guide to North American Trees. Dogwoods can be identified by their leaves, bark, size and shape, berries and flowers, depending on the season. For example, the Flowering Dogwood is usually about 15 to 30 feet tall with a short trunk and an oval shaped crown of branches and sparsely layered leaves. Its flower clusters are green and the four surrounding bracts (which look like flower petals) can be white to pink, turning more pink as they age. The fruits are stalkless clusters of red berries, which may stay on the tree through winter.