Trunk girdling is the removal of a thin strip of bark all the way around the trunk. It is done each year on an area of the trunk or cordons that was not girdled in previous years. The practice temporarily disrupts the downward flow of carbohydrates and hormones through the phloem (inner bark) and, when timed properly, increases the berry size of most seedless table grape varieties. It can make berries about 10 to 30 percent larger when done correctly.
The practice of girdling removes the bark, phloem and cambium from around the trunk or cane...Removal of these tissues prevents the translocation of carbohydrates to the root system thus making more available for fruit growth until the girdle heals....