Managing crop load and canopy
For every "tweak" a grower makes, the grapevine will compensate in another area.
Grape growers have many tools to help them manage crop load and develop vine "balance," from matching site and variety to using trellis design, pruning, nutrition, irrigation, and canopy management techniques. But as growers manipulate the crop load and canopy, they must remember the grapevine will compensate for such changes.
Growers must keep in mind the "rules of play" that guide the vine's response to canopy manipulation, Dr. Markus Keller said during a canopy management session at the annual meeting of the Washington Association of Wine Grape Growers in February.
Overall vine capacity is limited, he said, adding that there's only so much light, temperature, and water to go around in a given season. Pruning tends to decrease the vine capacity, but increases vigor, while fruit production tends to decrease vigor and capacity in the long term. Increasing the number of shoots and clusters can decrease the vine's vigor, but vigor can also be affected by shoot orientation. Shoots forced downward or horizontal are less vigorous than those orienting upward. Vine vigor can improve bud fruitfulness in the next season, but if vigor continues increasing, bud fruitfulness will eventually decline.
"Vines are extremely good at self-regulating," Keller said. "This leads us to the principle called yield component compensation. If you change any one of the components—say you change the shoot number—the vine will compensate and grow more or less fruits or bigger or smaller berries… So, whatever you do to diminish your crop will not result in an equal decrease in yield at the end of the season."
For example, if you drop half the crop at bloom, you will not get half the yield at the end of the season, he explained.
"And whatever you do at any one time will affect both the fruit growth and shoot growth as well as next year's crop," he said.
Keller said that vines are also good at compensating for the number of flowers per cluster. The more flowers per cluster, the fewer flowers that will ultimately set fruit. Research has also found that the more berries per vine, the smaller the individual berries will be at the end of the season.
"That's one argument not to thin the crop if you are after small berries," he said, adding that regulated deficit irrigation can also be used to keep the size of berries small.
Pruning determines the bud number and bud position and can be used to maintain the canopy and vine shape. Pruning also sets the limit on yield potential.
In sharing his "Golden Rules of Pruning," Keller urged growers to aim for leaving 15 buds per pound of pruning weight; five shoots per foot of canopy; and avoid cross-row shading.
"You need to apply all three rules simultaneously," he said.
"The fourth rule is if you don't have sap flow at the end of March or early April, then you need to irrigate because the soil is too dry. This is very important in eastern Washington."
Cane or spur pruning? Usually spur pruning is appropriate in most locations, he said, noting that cane pruning tends to lead to apical dominance.
He advised growers not to panic if cold winter temperatures damage the vines. Minimal pruning after winter damage usually results in a crop and gives the vine a chance for recovery.
Shoot thinning, a tool used to improve light interception and regulate crop load, is not as critical in eastern Washington where light is abundant as it is in vineyards on the west side of the state. Keller warned eastern Washington growers not to overdo shoot thinning, aiming for about five shoots per foot of canopy length. Shoot thinning should be done as early as possible. Hedging can be used to trim shoots and can help reduce competition and redirect sugar to clusters, but it can promote lateral growth.
"In eastern Washington, you can turn off the water to stop lateral shoot growth, but don't overdo it," he said. "Leave some leaves to help you ripen the clusters."
Leaf removal is a tactic used to improve the microclimate of clusters, promote lateral growth, improve spray coverage, and reduce bunch rot. Leaf removal can increase phenolics in the berries, but can result in sunburn if berries are overexposed.
Keller suggested that leaves be removed two to four weeks after fruit set and under cloudy conditions to give the grapes a chance to harden off. "The earlier you remove leaves, the more time you give the grapes a chance to produce flavonols, which act as a sunscreen for the berries," he explained.
But removing leaves too early can impact fruit set.
Fruit thinning is done to fine-tune the crop load and may improve fruit composition and increase fruit size. But he warned that if it is done too early, it can increase shoot growth. When clusters are thinned late in the season, there is a greater effect on yield than when done earlier in the growing season.
Keller suggested that growers use fruit thinning at bloom on varieties with tight, compact clusters—pinching off one-third to one-half of the flowers—to elongate and loosen up the clusters.