Blackberry plants require a support system to manage their vigorous growth and prevent an impenetrable tangle of canes. Trellising keeps the canopy open, significantly improving air circulation and sunlight penetration. This reduces humidity, which helps prevent fungal diseases and ensures a higher quality fruit set. A well-designed trellis also lifts the fruiting canes off the ground, making harvesting easier and preventing fruit damage. The specific trellis design depends directly on the growth habit of the chosen blackberry variety: erect, semi-erect, or trailing.
Understanding Blackberry Cane Growth and Support Needs
Blackberry plants exhibit a biennial cane cycle; individual canes live for two years, though the root system is perennial. First-year canes, called primocanes, are vegetative and focus on growth without producing fruit. In the second year, these canes become floricanes, which produce fruit before dying back after harvest.
The three primary growth habits dictate the required support structure. Erect varieties, such as ‘Ouachita’, have stiff, upright canes needing minimal support, mainly for the weight of the fruit-laden laterals. Semi-erect types, like ‘Chester Thornless’, have thick, arching canes that bend and trail without support. Trailing blackberries, such as ‘Marion’, have long, flexible canes that require continuous support along their entire length.
Simple and Fixed Trellis Systems
The simplest and most economical support structure is the Post and Wire System, also called the I-trellis. This system uses sturdy, vertical posts set in a straight line with one or two horizontal wires strung between them. For most erect and semi-erect varieties, a single wire placed 4 to 5 feet high is often sufficient to support the fruiting laterals.
A two-wire configuration, typically at 2 and 4 feet above the ground, allows canes to be secured between the wires, creating a narrow hedgerow. Posts should be spaced approximately 25 to 30 feet apart, with the canes tied loosely to the wires. While easy to build, the I-trellis can lead to a crowded canopy, increasing disease risk and making harvesting challenging.
A variation offering improved canopy management is the T-Trellis, which adds cross-arms to the top of the posts. A 3-foot-wide cross-arm is typically mounted at 5 feet high, with a wire attached to each end. This design trains the canes outward into a wider canopy, improving sunlight exposure and air flow through the center of the row.
The T-trellis is well-suited for vigorous, semi-erect varieties, as the lateral support helps to evenly distribute the weight of a heavy crop. The expanded canopy width helps mitigate crowding issues common with single-post systems, leading to better fruit quality and disease management.
Specialized and High-Density Trellis Designs
More complex systems are designed to manage the vigorous growth of trailing varieties or maximize light interception and harvest efficiency. The V-Trellis is a fixed system that uses two parallel rows of angled supports to form a ‘V’ shape over the row. This design is highly effective for separating the biennial canes, training primocanes up the center and fruiting floricanes to the outer wires.
The V-trellis, often using posts angled 20 to 30 degrees from vertical, allows for greater light penetration and air movement than hedgerow systems, which can result in higher yields. This separation simplifies pruning and harvesting, as the fruit is concentrated on the more accessible outer sides of the canopy.
For trailing varieties or regions with severe winters, the Rotating Cross-arm Trellis (RCA) offers a significant advantage. This design uses a hinged cross-arm that rotates from a vertical position to a horizontal position near the ground. The ability to lay the floricanes down horizontally allows the entire plant to be covered for cold protection during the winter months.
Once rotated vertically for the growing season, the RCA creates a narrow canopy that increases air flow and concentrates the fruit onto one side of the trellis, improving harvest speed and efficiency. These movable systems are often more expensive and require more labor for cane training, but they provide unparalleled cold protection and management flexibility.
Trellis Construction Materials and Placement
The stability of any trellis depends on the materials chosen for the posts and wires. For long-term durability, line posts should be made from treated wood or heavy-duty metal T-posts, which resist rot and the strain of a loaded row. End posts, which bear the tension of the wires, need to be significantly sturdier, typically 8-foot-long wooden posts driven 3 feet into the ground and braced.
Line posts should be set at least 2 feet deep into the soil, with about 5 feet remaining above ground for the fruiting zone. Post spacing should not exceed 25 to 30 feet to prevent excessive wire sag under the weight of the fruit. High-tensile, galvanized steel wire is the recommended choice for the load-bearing top wires, with 12.5-gauge wire being sufficiently strong for primary support.
A lighter 14-gauge wire can be used for the lower wires, which serve primarily for cane positioning rather than load bearing. For wide canopy systems, adequate row spacing is crucial, often requiring 8 to 10 feet between rows to accommodate the canopy and allow for equipment access.