Blackberry (Rubus spp.) is the name given to numerous species of semi-woody, fruit-bearing shrubs commonly called "brambles." Blackberries are hardy in U.S. Department of Agriculture plant hardiness zones 6 through 8. The plants produce multiple biennial stems, called canes, which grow vigorously in their first year and produce fruits in their second year. At the end of their second season, the canes die and are replaced by new canes.
The Rubus species commonly called dewberries include the northern dewberry (Rubus flagellaris), the Pacific dewberry (Rubus ursinus) and the southern dewberry (Rubus trivialis). The northern dewberry is well-adapted to cold climates and can be found as far north as Quebec and Ontario in Canada; it is hardy in USDA zones 3 through 7. The southern dewberry and the Pacific dewberry are much less tolerant of cold temperatures and are hardy in USDA zones 6 through 9.
Depending on the species or cultivar, the growth habit of blackberries may be either erect, in which case the plants' canes support themselves and grow in a tall, arching habit, or trailing, in which case the canes cannot support themselves and grow along the ground.
Dewberries, in comparison, always have a trailing or very low arching habit, and they remain close to the ground. While blackberry species often reach heights of 3 to 6 feet, dewberries seldom grow more than 2 feet tall and may remain under 1 foot high.
Both blackberries and dewberries spread when the tips of their canes touch the ground and take root and when new plants sprout from underground rhizomes. Dewberries often form low, dense mats, and blackberries commonly spread into large thickets. Both plants can be weedy and invasive when left uncontrolled.
Canes, Thorns and Fruit
Blackberry and dewberry fruits are very similar in appearance. The fruits are not simple berries, but rather each is an aggregate fruit made up of a cluster of very small fruits called drupelets. Both blackberries and dewberries' fruits are pale before they ripen, but they darken to a near-black color when ripe. In both fruits, each drupelet contains a single seed, but the seeds of dewberry are larger than those of the blackberry.
Dewberry and most blackberry canes are thorny, but some blackberry cultivars are thornless. Dewberry thorns are thinner and more flexible than those of the blackberry, and dewberry canes are often covered with fine hairs, while blackberry canes are hairless.