Veraison began in the wine grape vineyards of San Joaquin County a few days later this year than last. Still, that’s more than a week earlier than usual, according to Paul Verdegaal, the county’s University of California Cooperative Extension farm advisor for viticulture in San Joaquin County.
However, as the berries here and elsewhere in the San Joaquin Valley take on color and sugar in transitioning from the growth stage to the ripening phase of the season, there is still work to be done in managing the crop between now and harvest. He offers some pointers:
During hot weather, keep drip irrigation rates at near full Et.
“Usually, I recommend turning on the pumps for a drip irrigation system whenever temperatures reach 100 degrees, whether or not you’re using deficit irrigation,” Verdegaal says. “When it gets this hot, a drip system can’t provide water fast enough to meet the vine’s demand. It’s best to watch the weather reports and begin irrigating just ahead of the actual hot spell.”
As veraison approaches, it’s better to forego any applications of nitrogen and potassium. During veraison, the vines are directing water and nutrients to the berries to fill the clusters. Nitrogen and potassium given to the vines now could end up in the clusters adversely affecting pH levels and quality of the grapes.
However, once veraison is completed, typically at about 18º to 20º Brix, nitrogen and potassium fertilization can resume. “That way, you won’t be complicating the job of the winemaker and these two nutrients will still be available for the vines to take up, since ripening demands for them can sometimes outpace their availability in soil, and for vine storage as soon as harvest is finished,” Verdegaal says.
In addition to the customary testing of leaves at bloom to check the vine’s nutrient status, he suggest doing it again during veraison. This takes advantage of research data and improved technology that now allow for more precise applications of water and nutrients than can be done based only on an early-season sample. A veraison leaf test offers a way to double-check the vine’s nutrient status and correct it, as needed, going into harvest.
Unlike flood irrigation, in which the berms remain dry and the lack of moisture helps limit weed growth, drip irrigation can contribute to weed problems by keeping the soils moist. This extra water not only encourages “escapes” to germinate and grow but it also accelerates breakdown of residual herbicides applied early in the season, reducing the length of their effectiveness. Keeping an eye on the berms and spot spraying as needed can help prevent perennial or noxious weeds from going to seed later in the summer. Plus, it helps minimize development of weed resistance to the herbicides, Verdegaal says.
In many cases, good soil moisture levels earlier in the season contributed to vigorous vine growth that may have helped keep mites from attacking the vines. However, with summer’s dry conditions and heat spells, their numbers can build quickly. That’s why it’s important to continue monitoring vineyards and be ready to respond promptly to prevent any flare-ups, Verdegaal notes.
As for pressure from various leafhopper species, by the time veraison has begun growers may have already helped reduce that threat when they treated their vineyards earlier in the season to control the vine mealybug. “Many growers who treat for the vine mealybug seem to get pretty good incidental control of leafhoppers,” he says.
Verdegaal offers one more tip to keep in mind before grape picking begins: “Remember,” he says, “stay in contact with your grape buyer or winery reps to avoid misunderstandings and major problems during the “controlled” confusion of harvest.”