With the passing of bloom, table grape growers are turning their attention from managing fruit set to maximizing berry size. One practice that increases berry size is girdling, removing a thin strip of bark from around the vine trunk. The girdle disrupts the flow of carbohydrates from the vine's canopy to its roots; the young berries grow fat on the extra sugar that would otherwise have been consumed by the roots.
Special double-bladed knives are used to make the girdles. Keeping the blades sharp makes the task go much easier. Some knives also have a single blade on one end of the handle. I prefer such knives as the single blade is a handy tool for removing loose bark that interferes with girdling. However, an experienced colleague prefers to remove the loose bark by hand.
Some trunks, especially on old vines, develop grooves and folds from which it is hard to remove the bark strip with a double-bladed knife.
If the bark is not removed from these places, the effectiveness of the girdle is diminished.
Within five or ten minutes, any remaining phloem (sugar-transporting tissue of the bark) will become brown colored. If any such tissue is seen, it should be scraped away. However, the girdles should not cut any deeper into the trunk than is necessary to remove the phloem, as deep cuts into the wood can disrupt water supply to the shoots, which may injure or even kill the vine. Proper girdles, applied at the right time, can increase berry size by 10% to 30%.