The right combination of leaf removal, shoot thinning and mechanical box pruning can dramatically reverse declining production caused by a dense canopy, without sacrificing wine quality, according to preliminary results of a large-scale wine grape field trial.
At the same time, the management method can significantly lower labor costs compared to hand pruning.
Kaan Kurtural, a member at the Viticulture and Enology Research Center faculty at California State University, Fresno, is directing the three-year study, which began last year.
The research includes two 80-acre plots in southern Kern County. One is Pinot Gris, planted in 2003; the other, Syrah, is two years older.
Prior to the study, diseases or insects had not been a problem in the irrigated, mechanically-hedged vineyards. However, minimal canopy management had contributed to extensive vegetative growth at the expense of grape production.
“We wanted to see how opening the canopy to increase exposure of the buds to light and heat would affect grape yields and quality,” says Kurtural. “Our goal was to balance the amount of leaf area of the vines with the amount of fruit they could carry.”
In the first year of the trial, yields tripled and pruning costs were reduced by more than half, while maintaining wine grape quality.
In the Syrah plots, researchers used six cordons to hang individual walls of leaves, and separated the canopies horizontally. Also, leaves were removed on the east side of the walls on half of each plot. Light meter readings indicate that this exposed the grapes to 25 percent more sunlight.
“Before we did this, we found leaf areas of the vines were as much of 35 to 40 square meters per vine, in some locations,” Kurtural says. “They looked like peach trees.”
Half the vines in the trial are being hand-pruned to 22 two-bud commonly found in the San Joaquin Valley;the rest are machine-pruned to a 4-inch length.
With both manually and machine-pruned vines, researchers are thinning shoots to three different densities: low density of 7 shoots per foot; medium density, 10 shoots per foot; and high density, 15 shoots per foot.
The researchers are trying to provide the right amount of shading without shortening the productive life of the vineyard.
“Thinning too much can hurt the fruit by exposing it to too much heat in this warm climate,” Kurtural says. “But, if we protect it too much by not removing leaves or shoots, we end up with too much shade, fewer grapes and a shorter life cycle for the vineyard.”
In 2010, the first year of the trial, hand pruning resulted in the lowest grape yields — 3 to 4 tons per acre — in both the Pinot Gris and Syrah plots.
The highest yield, 11 tons per acre, was achieved with mechanical hedging in Pinot Gris, and 12 tons per acre with cane pruning in Syrah.
Leaf removal, mechanical box pruning and medium shoot density, Kurtural says, produced the best overall results.
“Removing the leaves to provide three- to four-leaf layers in the canopy, gave us a decent architecture, and resulted in smaller berries, but more berries set per cluster for more fruitful production.”
Yields with this treatment ranged from 10 to 11 tons per acre. “At this level, we’re producing almost 10 pounds of fruit per pound of pruning weight,” he says. “That’s an optimum balance between vegetative and reproductive growth?”
Grapes were processed at Cottonwood Creek winery in Madera, Calif. “In the case of the Pinot Gris grapes, rose and banana aromas were prominent in the wine from the grapes of the hand-pruned vines that had leaves removed,” Kurtural says. “Also, we were able in increase the color of the Syrah, while keeping tannins at an acceptable level.”
Mechanical pruning not only produced higher yields, it was also more economical than hand pruning. Last year, mechanical hedging in the trials cost 24 cents per vine — 30 cents per vine, or 44 percent less than the cost of hand pruning.
This works out to a total cost of $380 per acre to hand-prune the plots, compared to $150 per acre (equipment lease, maintenance and fuel expenses) for mechanical pruning, Kurtural says.