Growth Habit Differences
Blackberries, which are generally hardy in U.S. Department of Agriculture plant hardiness zones 6 through 8, belong to a group of shrubs commonly called "brambles." These plants produce multiple stems, called "canes," that may have, depending on the variety, either a self-supporting arching growth habit or a trailing growth habit in which the canes spread along the ground. Erect blackberry varieties may reach a height of 10 feet.
Species varieties of blackberries have thorny canes, but some modern cultivars, such as 'Triple Crown' (Rubus 'Triple Crown'), which is hardy in USDA zones 5 through 8, are thornless.
Mulberries are thornless, upright, mid-sized trees. Red mulberry (Morus rubra) may reach a height of nearly 50 feet, and white mulberry (Morus alba) grows up to 18 feet tall; both of those species are hardy in USDA zones 4 through 9. Black mulberry (Morus nigra), with a mature height of up to 32 feet, falls between the two other species in terms of size, and it is somewhat less cold-tolerant; it is hardy in USDA zones 5 through 9.
Blackberries have compound leaves made up of three to five leaflets. Each leaflet is oval-shaped with a pointed end. The leaves have a prickly texture, and their edges are sharply toothed.
Mulberry leaves are not compound, and their edges are bluntly toothed. They are often deeply lobed, although some individual leaves may not be lobed.
The fruits of both blackberries and mulberries are not actually berries but rather clusters of tiny, individual fruits called drupelets. The fruits of both plants are generally very pale when unripe but darken to a deep, purple-black when ripe; the fruits of white mulberries and red mulberries, however, may be lighter red or purple when ripe.
Unlike some other Rubus species in which a ripe fruit pulls easily away from the core stem around which it grows, a blackberry fruit pulls away less easily and takes the stem with it. Mulberry fruits also come away from the plants with their stems intact, but they drop very easily when ripe; some mulberry varieties may be harvested by placing sheets under the trees and shaking the branches so that the fruits fall onto the sheets.