Grape bloom usually occurs in mid May in the San Joaquin Valley of California and two weeks later on the Central Coast for the same varieties. The date when grape bloom will occur varies from year to year and is driven primarily by the temperaturesin the spring after bud break. In 1997 with a very warm spring, bloom started 10 days earlier than normal. This was followed in 1998 by a very cool spring in which bloom was two to three weeks later than normal (1998 was the latest bloom we have had in the last 10 years). Last year, in 2000, bloom was a few days earlier than normal. Since the time of bloom can vary by as much as three to four weeks in any given year, techniques that can improve the accuracy of predicting bloom can be quite valuable to grape growers. For a grape grower, knowing when bloom will occur is important for planning the application of pre-bloom foliar nutrient sprays and timing plant growth regulators. A pre-bloom foliar spray for zinc must be applied within the two weeks priorto bloom to have maximum effectiveness. Zinc foliar applications after bloom have no effect on the fruit symptoms of the deficiency. Zinc deficiencies can result in poor berry set and is one of the most common nutrient problems in California vineyards.Growers of table grapes and some raisin grape growers use a combination of pre-bloom, bloom, and post-bloom applications of gibberellic acid to manipulate the size and shape of the clusters and berries. The timing of these foliar nutrients and growth regulator sprays can also impact the schedule for fungicide and pesticide applications as well as the timing of irrigations.
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