Blackberries are often mistakenly referred to as trees due to their height and woody appearance, but they are technically perennial shrubs belonging to the genus Rubus. These plants feature a perennial root system, which lives for many years, supporting canes that are biennial. This growth habit, known as a bramble, produces new canes, called primocanes, each season to replace the two-year-old fruiting canes, known as floricanes. The total size a blackberry plant achieves is a dynamic balance between the plant’s genetic potential and active management practices.
Understanding Blackberry Growth Habits
The ultimate dimensions of a blackberry plant are determined largely by its specific growth classification, which dictates the cane’s structure and need for support. Blackberries are categorized into three main growth habits: erect, semi-erect, and trailing.
Erect varieties grow upright and are largely self-supporting, forming a bush-like structure that typically reaches a height of 4 to 6 feet. These types are generally the most manageable for smaller spaces and often send up new suckers from the roots to form a dense hedgerow if not controlled.
Semi-erect varieties, like ‘Chester’ or ‘Triple Crown,’ are more vigorous. They produce canes that start upright but eventually arch and require trellising for optimal production. These plants can reach heights between 5 and 7 feet and may spread to a width of 5 to 8 feet.
Trailing varieties, such as ‘Marion,’ are the most vine-like and vigorous. They produce long, flexible canes that are not self-supporting and must be trained onto a trellis. If left untrained, these sprawling canes can easily reach lengths of 10 feet or more, creating a wide, dense thicket.
Factors That Determine Maximum Size
The inherent size potential of a blackberry variety is significantly modified by its growing environment and the specific cultivar chosen. Within each growth habit, breeders have developed varieties with different levels of vigor, from standard types to compact, dwarfing cultivars like ‘Little Black Prince,’ which only grows to about 3 feet tall. A plant’s maximum size is also heavily influenced by the quality of the soil, as rich, well-drained soil promotes faster and more extensive cane growth.
Nutrient availability, particularly nitrogen, can lead to excessive vegetative growth, resulting in a larger plant mass but potentially lower fruit production. Optimal sunlight and consistent moisture are also necessary for the plant to reach its full genetic potential. Blackberries need at least six to eight hours of direct sun daily; if they receive less, their growth may be slower and the overall plant size more compact.
Practical Techniques for Size Management
Controlling the vigorous nature of blackberries is accomplished through annual pruning and training systems. The most important pruning action is the removal of floricanes, the two-year-old canes that have finished fruiting, which should be cut down to the ground immediately after harvest. Removing these dying canes redirects the plant’s energy into the new primocanes and prevents them from competing for resources and sunlight.
Summer tipping is another technique where the tips of new primocanes are pinched or cut off once they reach a height of about 3 to 4 feet. This action encourages the cane to stop growing vertically and forces the development of lateral branches, which increases the total fruiting area for the following year.
For semi-erect and trailing types, a trellis is used to physically restrict their spread and keep the long canes organized. Training the canes onto wires or a support structure prevents them from sprawling across the ground, which would otherwise allow them to root and create a wide, tangled thicket. A common two-wire trellis system can keep the fruiting canes confined to a vertical plane, often between 3 and 5 feet high.
For erect types that spread via suckers, maintaining the planting in a narrow hedgerow of about 12 inches wide is necessary to prevent the patch from aggressively expanding. In certain aggressive varieties, physical root barriers may be used to contain the spread of these underground shoots outside the desired planting area.