Blackberries are perennial plants which typically bear biennial stems ("canes") from the perennial root system.[7]
In its first year, a new stem, the primocane, grows vigorously to its full length of 3–6 m (9.8–19.7 ft) (in some cases, up to 9 m [30 ft]), arching or trailing along the ground and bearing large palmately compound leaves with five or seven leaflets; it does not produce any flowers. In its second year, the cane becomes a floricane and the stem does not grow longer, but the lateral buds break to produce flowering laterals (which have smaller leaves with three or five leaflets).[7] First- and second-year shoots usually have numerous short-curved, very sharp prickles that are often erroneously called thorns. These prickles can tear through denim with ease and make the plant very difficult to navigate around. Prickle-free cultivars have been developed. The University of Arkansas has developed primocane-fruiting blackberries that grow and flower on first-year growth as much as the primocane-fruiting (also called fall bearing or everbearing) red raspberries do.
Unmanaged mature plants form a tangle of dense arching stems, the branches rooting from the node tip on many species when they reach the ground. Vigorous and growing rapidly in woods, scrub, hillsides, and hedgerows, blackberry shrubs tolerate poor soils, readily colonizing wasteland, ditches, and vacant lots.[6][8]
The flowers are produced in late spring and early summer on short racemes on the tips of the flowering laterals.[7] Each flower is about 2–3 cm (0.8–1.2 in) in diameter, with five white or pale pink petals.[7]
The drupelets only develop around ovules that are fertilized by the male gamete from a pollen grain. The most likely cause of undeveloped ovules is inadequate pollinator visits.[9] Even a small change in conditions, such as a rainy day or a day too hot for bees to work after early morning, can reduce the number of bee visits to the flower, thus reducing the quality of the fruit. Incomplete drupelet development can also be a symptom of exhausted reserves in the plant's roots or infection with a virus such as raspberry bushy dwarf virus.
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