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A Washington State University study found that traditional spacings for juice grapes—around six feet between vines and nine feet between rows—was best of four vine density treatments in the trial. Dead and dying leaves, like these pictured, were commonly found in the three-feet vine spacing. Nutrient analysis showed that potassium from the dead leaves was being recycled back into the fruit and resulted in high pH juice in the three-feet spacing.

A Washington State University study found that traditional spacings for juice grapes—around six feet between vines and nine feet between rows—was best of four vine density treatments in the trial. Dead and dying leaves, like these pictured, were commonly found in the three-feet vine spacing. Nutrient analysis showed that potassium from the dead leaves was being recycled back into the fruit and resulted in high pH juice in the three-feet spacing.

Although high-density plantings have boosted profits in other fruit crops, a narrow spacing between Concord grape plants can reduce yields and returns, according to Washington State University viticulturist Dr. Markus Keller.

The best overall spacing for juice grapes in a Washington State University Concord grape planting trial, which compared four vine densities and two row spacings, was six feet between vines and nine feet between rows, Keller reports. The worst, a high-density spacing of three feet between vines, reduced both yield and fruit ­quality.

Keller, based at the Irrigated Agriculture Research and Extension Center in Prosser, looks for ways to improve economic productivity for juice grape growers who often contend with low prices, high production costs, and high labor costs. His juice grape studies have focused on maximizing yield while maintaining quality, minimizing inputs, and increasing mechanization.

He set up a spacing trial in 2003 when a new Concord research vineyard was planted near WSU’s Prosser research center. The vineyard, sponsored by the Washington State Concord Grape Research Council, involves 8 acres, 2 of which were planted at densities of 3, 6, 9, and 12 feet between vines and 8 and 9 feet between rows. Three replicates of each combination were planted. The research vineyard is drip irrigated, and vines are ­mechanically pruned.

Keller wanted to learn if high densities used in European wine grape vineyards to impose competition between vines and decrease vigor could increase juice grape yields over the long-term. “The idea I had was that if you planted a lot of vines in a small space, you would end up with a lot of trunk volume,” he said, adding that the vine may be able to draw reserves from neighboring trunks in cool years to help ripen the crop.

Reduced yield and fruit quality

His expectations were that the vines on the densely planted three-foot spacing would fill the cordon quickly and outproduce vines in the wider spacings. In the first production year (year 3), the three-foot spacing did just that, producing about 2.5 tons more per acre than the other spacings of 6, 9, and 12 feet. But in each of the next five years, the 3-foot spacing produced nearly 5 tons less per acre than the 6-, 9-, and 12-foot spacings, for an ­average of 38 percent less production.

Average production for 2007 to 2010, when the vines were mature and yields leveled off, was 9.3 tons per acre for the 3-foot spacing compared with 14.3 tons for the 6-, 9-, and 12-foot spacings. The 12-foot spacings required a year longer to fill out the cordon wire than the other spacings.

Keller found little yield difference between the 8- and 9-foot wide rows, although the narrower 8-foot row spacing was more difficult to drive and maneuver equipment through later in the season when the canopy was full.

As Keller examined the yield components, such as cluster weight, he gained insight into the yield differences and why the high-density spacing was unproductive. He was surprised to find that the number of clusters per foot of canopy was only 22 in the 3-foot spacing but averaged 29 in the 6-, 9-, and 12-foot spacings. “That was opposite of what I expected,” he said. Additionally, in the high density 3-foot spacings, there were fewer berries per cluster (an average of 35 berries compared with 44 in the 6-, 9-, and 12-foot spacings), and cluster weights were lighter at 76 grams per cluster, compared with the average of 90 grams in the 6-, 9-, and 12-foot spacings. The number of shoots or canes per foot was also less in the high-density spacing.

“The bigger vines (6-, 9-, and 12-foot spacings) had bigger clusters and more clusters down the row,” he explained. “The high-density vines (3-foot spacings) had lower percentage of bud break from fewer shoots per foot, lower bud fruitfulness from fewer clusters per shoot, and reduced fruit set from smaller clusters.”

The smaller vines (3 feet apart) always had lower cropping, and the bigger vines always supported heavier crops, Keller said, adding that he saw no effects of ­biennial ­bearing in any of the spacings.

Competition for light

“What caused the low or light crop in the 3-foot spacing was canopy density,” he said. “I thought there would be competition for water and nutrients, but instead the vine, with such dense canopy, was competing for light. The dense canopy didn’t have enough light, so the vine was growing more and bigger leaves in the hope of better sun exposure.”

When he measured the amount of sunlight getting into the fruiting zone, Keller found there was little light penetration into the canopy and fruiting zone of the 3-foot spacing.

“There’s no competition for nutrients and water underground, like I thought I’d see. Only competition for light above the ground.”

Fruit quality differences

Keller assessed fruit composition of the different spacings for six years, and found no effect on Brix, titratable acidity, and color. However, the pH of juice was always higher in the 3-foot spacing—an average of 3.52 compared to 3.31 in the 6-, 9-, and 12-foot spacings. “It’s well known in wine grapes that higher potassium in the grape will drive up pH. This is true in Concords as well,” he said, adding that high pH in juice is not ­desirable.

Keller found dead and dying leaves falling off the outside of the canopy in the high-density vines. In measuring the nutrient concentration of the leaves, he found that 75 percent of the vine’s carbon was being recycled out of the leaves and back into the vine before the leaves fell off. More importantly, half of the potassium was recycled from the leaves and sent back to the fruit, essentially giving the fruit a double dose of potassium from both the soil and leaves.

He concludes that because there was little difference in yield and quality between the 8- and 9-foot row widths, growers could save money by using the wider 9-foot spacing. Wider rows would reduce the total number of plants needed and reduce the amount of trellis material. Traditional spacings of ­Concord vineyards in Washington State have been from 6 by 9 feet to 9 by 9 feet.

Keller believes that the same trends would apply to wine grapes planted in high-density, narrow spacings, except that wine grape canopies are not usually allowed to grow as big as juice grapes, so the effects would not be as acute. He recently visited vineyards in Bordeaux, France, with vines planted a meter apart with a meter between rows. The canopies were hedged continually and looked like a broom, he said, and many of the leaves were gone. “It’s never a good situation to have the vines competing for light above the ground.”

Keller presented his findings during the annual meeting of the Washington State Grape Society held last fall.