Having thinned the clusters, it's time to fatten the berries

A few weeks ago, during bloom, we took steps to manage fruit set of our 'Crimson Seedless' table grapes, hoping to reduce set just enough to provide ample room for the berries to achieve their full size potential without becoming too crowded. 

 Whoops!  There will not enough berries on this cluster.

  This cluster is well thinned, with plenty of berries, and room for them to grow.

Now that our crop has been set, we are focusing our efforts on practices that maximize berry size.  The application of certain plant growth regulators including gibberellic acid (GA3), and forchlorfenuron, a synthetic cytokinin, promote the division and expansion of cells in the berries, and thereby improve berry size.  Girdling vine trunks, that is removing a thin ring of tissue from around the trunk, can also improve fruit size.  Girdling temporarily disrupts the phloem, the vascular tissue in which sugars are transported from leaves, making more sugar available to the developing fruits. 

A fresh girdle, made just below the one from last year. Girdling temporarily disrupts the phloem, causing sugars made by the leaves to accumulate in vine parts above the girdle. The extra sugar promotes the growth of young berries, the purpose of girdling, but the trunk also responds, as evidenced by its wider girth above last year's girdling scar. Ants are also capitalizing on the opportunity.