Tips For Canopy Management On Open-Gable Trellises
Dry-on-vine (DOV) is an important raisin production method, especially for new plantings. In DOV vineyards, canes bearing mature fruits are severed to initiate drying, and the dried raisins are then harvested directly from the vines by machine.
Because of the extended drying time associated with DOV, early-ripening raisin grape varieties such as ‘Fiesta’ or ‘Selma Pete’ are used instead of ‘Thompson Seedless.’ Most DOV raisin vineyards in California have an overhead arbor or open gable (Figure 1) trellis system.
The overhead arbor has a greater yield potential than the open gable, but an overhead trellis is more costly than a gable and requires specialized low-profile farming and harvest equipment. The original open gable DOV trellis uses steel posts topped at 4.5 feet with 6 feet-wide V-shaped steel cross arm assemblies supporting six fruiting wires, three on each side.
The bottom two wires on each cross arm support fruiting canes. Cordon support wires are affixed immediately below the base of the cross arm assemblies, and a vertical 1 foot-tall post extension is mounted in the center of the cross arm assembly to support a foliage catch wire.
‘Sun Canes’ More Fruitful
Separating the canopy into fruiting and renewal sections has always been a part of the open-gable concept. Former University of California viticulturist L. Peter Christensen knew ‘Thompson Seedless’ canes, which developed under sun-exposed conditions, “sun canes,” had better bud break and were more fruitful than shade canes.
As a result, he designed the gable to have moveable rake wires to pull the renewal shoots away from the fruiting shoots on the trellis arms and toward the catch wire in the center of the trellis. This center-divided canopy should help improve sun exposure on renewal shoots, and thus optimize bud fruitfulness.
Alternately, the canopy could be divided into renewal and fruiting zones by selective pruning such that canes and spurs are left on separate cordons, a method known as within-row-alternate-bearing (WRAB). However, the specific benefit that center-divided or WRAB canopies may have on the exposure of renewal shoots to sunlight, or on bud fruitfulness, has not been determined for any DOV varieties on open-gable trellises.
Therefore, I recently subjected vines to one of three different canopy separation methods: 1) center-divided, 2) non-divided (Figure 1, A), or 3) WRAB. Vines assigned to the center-divided or non-divided treatments were cane pruned, leaving six to eight 15-node canes, and approximately 10 two-bud spurs per vine.
Other Factors More Important
Canes and spurs were left on all cordons. The only difference between those treatments was the use of rake wires and a center-mounted foliage catch wire to separate renewal shoots from fruiting shoots in the center-divided vines. Vines assigned to the WRAB treatment were pruned such that the cordons between any two adjacent vines were either entirely spurs or canes, creating fruiting or renewal zones that alternated between pairs of vine trunks (Figure 1, B).
In general, the renewal shoots on vines with WRAB canopies had a higher proportion of external leaves, and fewer leaf layers, than vines subjected to other canopy management treatments. Light intensity in the renewal zone was also highest in vines with WRAB canopies in spring, but as the canopy developed, treatment effects on light intensity diminished.
Despite somewhat improved canopy structure and better light exposure in spring, none of the canopy separation treatments have improved raisin yield or quality in any of the three study years. Thus, other factors, such as irrigation and nutrient management, could be more important than canopy separation in regard to optimizing raisin grape yield on an open gable trellis.