Blackberries are cane berries that are biennial plants with perennial roots, meaning their canes live for two years. First-year canes, called primocanes, grow vegetatively before becoming floricanes in their second year, which produce the fruit. Building a supportive structure for these canes is known as trellising and is a common practice in fruit production. The primary purpose of a trellis is to manage the plant’s growth habit, preventing the long, flexible canes from sprawling onto the ground. This guide provides the steps necessary to build and utilize a wire-based trellis system for your blackberry patch.
Why Blackberries Need Structural Support
Trellising is necessary to promote plant health and maximize fruit yield. Most blackberry varieties are not naturally self-supporting, and without a structure, their canes will bend and trail along the soil surface. This sprawling habit is detrimental because it significantly increases the risk of fungal diseases, such as cane blight and fruit rot.
Keeping the canes elevated improves sunlight penetration to the plant canopy, which is essential for uniform ripening and sugar development. Enhanced air circulation around the foliage and fruit helps dry moisture quickly, reducing the environment where pathogens thrive. Furthermore, a trellis simplifies cultivation tasks like pruning, weeding, and harvesting the ripe berries, which are kept clean and within easy reach.
Selecting the Optimal Trellis Design
The best trellis design depends on the specific growth habit of the blackberry variety. Canes are classified into three types: erect, semi-erect, and trailing. Trailing varieties, which have long, flexible canes, and semi-erect varieties, which produce arching canes, require trellising to prevent ground contact.
The two most common designs are the T-Trellis and the V-Trellis, both using horizontal cross-arms to support multiple wires. The T-Trellis uses a single cross-arm to create a vertical plane of support, suitable for erect or semi-erect varieties. The V-Trellis, sometimes called a Shifted Plane Trellis, is excellent for semi-erect types because canes are trained up angled wires to create a V-shape, separating new primocanes from fruiting floricanes. For trailing varieties, a simple two-wire system spaced vertically on a single post line (I-Trellis) is often sufficient, as their canes are trained horizontally.
Step-by-Step Construction Guide
Construction of a durable wire trellis begins with setting sturdy end posts, which must anchor the entire system against the tension of the wires and the weight of the canes. End posts should be pressure-treated wood, typically 4×4 or 5-inch diameter posts, and at least eight feet long. These posts need to be embedded 2.5 to 3 feet deep, often with concrete or a packed gravel base, to ensure they remain stable.
Intermediate line posts, which can be less robust metal T-posts or wooden posts, should be installed every 15 to 20 feet along the row to prevent wire sag. If constructing a T-Trellis, a cross-arm (usually a two-by-four board about two feet long) is secured horizontally near the top of the posts, typically five feet above the ground. For V-Trellises, the posts themselves are angled outward to form the V-shape, or angled cross-arms are attached to the posts.
High-tensile, 12.5-gauge or 14-gauge galvanized wire is then strung between the end posts, using the cross-arms or posts to maintain the horizontal spacing. Wires are installed at multiple heights, such as 3 feet and 5 feet above the ground, to support the growth at different stages. It is beneficial to install in-line wire tensioners, like turnbuckles, at one end of each wire run to allow for annual tightening, which is necessary as the wood posts settle and the wires stretch over time.
Training Canes Onto the New Structure
Once the trellis is complete, the process of cane training begins to integrate the plant’s growth with the structure. The goal is to orient the biennial growth cycle by physically separating the floricanes (fruiting canes) from the primocanes (new vegetative canes). This separation maximizes light exposure and airflow to both sets of canes, which is important for disease control and fruit quality.
Canes must be loosely secured to the wires using materials that will not girdle or cut into the tender stems. Soft, stretchy garden tape, specialized trellis clips, or natural twine are preferred over rigid materials. For V-Trellises, floricanes are tied to the outside wires to keep the fruit accessible, while new primocanes are allowed to grow up through the open center of the V. In a T-Trellis system, the canes are often divided and tied to the wires on the left and right sides, creating an open fan shape to ensure uniform light distribution.