A trellis system offers a significant advantage to home gardeners cultivating blackberry bushes, transforming unruly thickets into manageable rows. Providing physical support for the canes maximizes both plant health and fruit production. Canes held off the ground receive greater airflow, which reduces humidity around the foliage and minimizes the risk of fungal diseases. By directing growth upward and outward, a trellis ensures that leaves and developing berries receive optimal sunlight exposure, necessary for fruit development and higher sugar content. This organized structure also simplifies harvesting and pruning, making the entire planting more accessible and productive.
Understanding Growth Habits
The need for a specific trellising system is determined by the natural growth habit of the chosen blackberry cultivar. Blackberries are categorized into three main types based on how their canes grow, each presenting different support requirements.
Erect varieties, such as ‘Arapaho’ or ‘Navaho,’ produce stiff, upright canes. While these types can be grown without a trellis, supporting them allows for taller growth and longer lateral branches, which increases overall yield.
Semi-erect varieties, like ‘Triple Crown,’ have canes that begin growing upward but eventually arch over. They require intermediate support to keep the fruiting wood off the ground.
Trailing varieties, including ‘Marion’ or ‘Columbia Star,’ have flexible canes that naturally trail along the ground. They must have a complete trellising system for all stages of growth. These canes can grow quite long, sometimes exceeding 15 feet, and would otherwise be highly susceptible to disease and damage if left unsupported.
Choosing the Appropriate Trellising System
The simplest option is the I-trellis, or single vertical wire system, which functions well for erect varieties that require only simple containment. This design uses posts with one or two horizontal wires strung between them, keeping the upright canes confined within a narrow hedgerow. The I-trellis is relatively inexpensive and easy to build, though it can sometimes lead to crowded canopies and reduced light penetration.
For semi-erect types, the T-trellis is a widely used and effective system. It features a cross-arm at the top of the post to support horizontally spaced wires. This configuration spreads the canes outward, improving airflow and sunlight exposure compared to a single-wire system. The T-trellis is typically constructed with posts set 20 to 30 feet apart in the row, with cross-arms creating a wide support area for the arching canes.
Trailing varieties often benefit most from systems that provide maximum separation between the canes, such as the V-trellis or the Rotational Cross-Arm (RCA) system. The V-trellis uses a pair of angled posts or wires to create a wide V-shape, which separates the new vegetative growth from the fruiting wood. The V-trellis allows the floricanes to be tied to the outside wires for easier harvest, while the primocanes grow freely in the center.
Step-by-Step Installation of Support Structures
The long-term stability of the trellising system relies on a sturdy foundation, starting with the installation of robust end posts. End posts should be set at least two feet deep into the ground to withstand the significant tension of the wires and the weight of a mature, fruit-laden canopy. A common post height is five feet above the ground. The end posts, which bear the most strain, should be braced, either with an angled support post or by anchoring a wire from the top of the post to a ground anchor.
Intermediate posts, or line posts, are generally spaced 10 to 25 feet apart along the row to support the wire and prevent sagging. For the T-trellis, cross-arms are attached to the posts to create the necessary horizontal spread for the wires. Galvanized, high-tensile wire, typically between 12.5 and 14 gauge, is the material of choice for its strength and longevity.
Wire tension is achieved using specialized hardware, such as ratchet tensioners or stainless steel turnbuckles, installed at one or both ends of the row. The wire is threaded through the posts or cross-arms and pulled hand-tight before the tensioning device is used to remove all remaining slack. A properly tensioned wire is taut and will not sag significantly under the weight of the canes, ensuring maximum support throughout the growing season.
Training and Pruning Canes onto the Support
Effective trellising requires a cyclical pattern of training and pruning that manages the biennial nature of blackberry canes. Primocanes are the first-year canes that grow from the crown and do not produce fruit until the following year, when they become floricanes.
Training begins in the summer when the new primocanes of erect varieties reach a height of 30 to 36 inches. At this point, the growing tip should be pinched off, a process known as tipping. Tipping encourages the development of fruit-producing lateral branches for the next season.
For semi-erect and trailing types, the flexible primocanes must be secured to the lower wires of the trellis as they grow, using soft twine or plastic tape to avoid girdling the cane. On a V-trellis, the new primocanes are generally allowed to grow in the center of the V, while the floricanes are trained to the outside wires. This separation keeps the new growth away from the fruiting wood, improving light exposure and facilitating the eventual removal of the spent canes.
After the harvest is complete, the floricanes, which have finished fruiting, must be removed entirely by cutting them back to the ground. This pruning step clears the trellis, minimizes the risk of disease carryover, and directs the plant’s energy toward developing the remaining primocanes. In the dormant winter season, the lateral branches on the remaining primocanes are typically pruned back to a length of 12 to 18 inches to encourage larger fruit size and maintain a manageable canopy shape.