Managing fruit set; an important early step in producing quality table grapes

Grapes in the San Joaquin Valley are blooming.  The flowers are not particularly impressive to look at—they bear a superficial resemblance to brocolli—but the have a fresh, clean, invigorating scent.  That’s good, because there’s a lot of work to be done in the vineyard around bloom.  Table grape growers are among the busiest of viticulturists at bloom, as they must take steps to limit the amount of fruit set, so that the remaining berries will have ample room to grow without becoming excessively crowded.  If too many berries are set, the cluster will become excessively compact, full of misshapen and cracked berries that are highly susceptible to rot.  Home growers can thin their grapes by brushing the flower clusters with a scalp brush until about half the flowers are removed, but this approach is impractical for most commercial growers.

Instead, most growers apply a natural plant growth regulator gibberellic acid (GA₃) during bloom to reduce fruit set.  Generally, between 0.5 to 20 grams/acre GA₃ is applied to the vines when 30% to 80% of the flowers have opened, and multiple applications may be needed in some cases.  The proper application timing and dose depends on a variety of factors including the cultivar, environmental conditions at bloom, and cultural practices such as type of trellis and spray equipment used.  The use of GA₃ can reduce return fruitfulness of certain seedless cultivars, and many recently released cultivars require less GA₃ than traditional cultivars to achieve adequate thinning.  Seeded grapes, including wine grapes, may also respond to GA₃  thinning sprays, but seeds are a natural source of GA₃, and seeded varieties are more apt to respond to such sprays with unwanted side effects including reduced shoot and cluster counts the following spring, and increased proportions of tiny “shot” berries that detract from the appearance of table grape clusters. Before applying GA₃, or any other agricultural chemical, be sure to review the label and ensure the instructions are followed.