In commercial table grape production, berry size is of considerable importance due to the strong market demand for large, high quality fruit. Vines are commonly girdled at fruit set (bloom plus 10-14 days) to increase berry size and at veraison (color break or berry softening) to enhance fruit color and maturation.
The practice of girdling removes the bark, phloem and cambium from around the trunk or cane. Botanically, the phloem is live tissue 1-2 mm thick and lies below several layers of dead bark. The cambium is few cell layers thick and resides just between the xylem (water conducting tissue) and phloem. Its function is to renew both the xylem and phloem tissue.
Physiologically, the phloem is responsible for the movement of carbohydrates (sugars and starches) produced by photosynthesizing leaves to developing organs (including the fruit and roots). Removal of these tissues prevents the translocation of carbohydrates to the root system thus making more available for fruit growth until the girdle heals.
Prior research on Flame Seedless showed that berry size may also increase due to improved vine water status of girdled vines in comparison to non-girdled vines. Results from this study indicate that transpiration (i.e. vine water use) is decreased, making the vines less sensitive to soil-water deficits. Vines are especially susceptible to water stress at fruit set. Deficits during this time can decrease both cell division and elongation of the developing berries.
In addition, the reduction in fruit size at this time cannot be reversed with supplemental irrigation. Therefore, any increase in vine water status will maximize fruit size and quality.
Double-bladed girdling knives are manufactured in three widths, one-eighth inch, three-sixteenth inch, and one-fourth inch. Previous experimental work showed that there was no significance among knife widths provided that the girdle was complete.
To ensure that the benefits from girdling are attained, it is essential that all of the phloem tissue is removed. It's best to check for completeness approximately 20 minutes after the girdle is made. A proper girdle is characterized by the appearance of an all white, fibrous xylem ring. Any browning of the ring indicates an incomplete girdle and the remaining tissue must be cleaned out. If there is even a one-eighth inch of phloem tissue left, the girdle's benefits are lost. Finally, care should be taken that the girdle is not made too deep, as it can damage the xylem and weaken the vine. Girdles will typically heal in four weeks.
Some varieties are double girdled, or girdled once at fruit set for berry size and then a second time at veraison. Although no additional increase in berry size is achieved from the veraison girdle, it can hasten maturity and fruit coloration so as to advance harvest dates and reach market early. However, double girdling is thought to be debilitating and should not be made on heavily-cropped vines or repeated on the same vines year after year.
To easily achieve the double girdle, girdles made at fruit set are reopened by removing the newly-formed callus tissue with a smaller width-girdling knife.
As with many cultural practices, vine response to girdling operations can differ among varieties. For example, a girdle made at fruit set to increase berry size may come at the expense of poor color development, as in the case of most red seedless varieties.