Basic Plant Structures
Blackberries are thicket-forming root perennials with biennial canes. They grow in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on the species. Leaves are palmate, or hand-like, with fine-toothed edges. White flowers bloom in spring and early summer and turn to elliptical aggregate berries, meaning that each facet of the berry has its own seed even though it emerges from one flower. The majority of blackberry species have sharp thorns on the canes. There are three types of growth habits: erect, semi-erect and trailing.
The common blackberry (Rubus allegheniensis), also called the Allegheny blackberry, is primarily found in the eastern U.S. in USDA zones 5 and 6. Its erect purplish canes are 2 to 4 feet long, with leaves that are 3 to 7 inches long and pale on the underside. Blossoms are less than 1 inch around and ripen to berries in August. Northern dewberry (Rubus flagellaria) is a trailing berry found along coastlines in the East. It ripens from July into August and is very cold tolerant, stretching from USDA zones 3 through 9. The West Coast version of this crawling berry is the California, or trailing blackberry (Rubus ursinus), found in USDA zones 5 through 9. It has flattened prickles, sometimes giving it the moniker "thornless," and long arching red-brown canes.
While native blackberries can be a welcome addition to a home garden, there are a couple of invasive non-native blackberries that can easily escape cultivation and become a problem for surrounding plants. It's important to identify the difference between these plants and those you can grow without concern. The cut leaf, or evergreen blackberry (Rubus laciniatus), is a European import with very large berries that ripen in August. It has escaped cultivation and can be found scattered over USDA zones 5 through 10, along both the east and west coasts of the U.S. It's easy to identify this species by its deeply incised leaflets, the reason for the name "cutleaf." It can grow up to 15 feet tall with 40-foot-long canes. The Himalayan blackberry (Rubus armeniacus), another European import, can be found invading zones 4 to 7 in the U.S.. It has many of the same features as the cut leaf, but has compound leaves with oblong teeth along the edges, with five leaflets on a main stem.
This or That
Blackberries are not the only "black" fruit found in U.S. forests, which makes it easy to confuse them with other species of berry. The black raspberry (Rubus occidentalis) is found along the East Coast in USDA zones 4 to 9. The easiest way to tell it apart from the true blackberry is that raspberries are hollow inside. When you pull one off of the cane, it leaves behind a short receptacle, called a torus, and the berry will be completely hollow inside. Mulberries (Morus) are another similar looking elliptical berry that has species found across USDA zones 4 through 10. Mulberries grow on thornless trees, and the berries have small short stalks on the top where they attach to the tree.