The benefits of girdling grapevines were documented more than 250 years ago through a fortuitous accident. A donkey had been tied to the trunk of a grapevine. It was observed that this vine matured fruit earlier and of higher quality than fruit on surrounding vines. Inspection of the vine revealed that the donkey's rope had worn away the bark and a portion of the trunk just below the bark. Girdling has the same effect by cutting through the phloem tissues below the bark so that the downward (basal) flow of sugars and other compounds produced in the leaves is blocked. Therefore, these substances become deposited in and increase the size of berries and other tissues above the girdle. Girdling is a powerful tool for the table grape grower because it may dramatically increase berry weight and advance fruit maturity, but girdling may also weaken the trunk and root tissues of grapevines in a temperate climate. Experiments a century ago in New York indicated that repeated annual trunk girdling, especially when it was done midseason as the fruit began to ripen, would significantly reduce vine vigor. More recent experiments in a temperate-climate vineyard indicate that vines can tolerate repeated annual trunk girdling when girdling is per formed at fruit set and when girdling widths do not exceed 1/8 inch. Other girdling variations, including cane and knife girdling, can also be sustainable practices in temperate-climate vineyards.
If girdling is performed at the start of or during bloom it will promote an increased set of berries as well as increased berry size. Increased berry set is often not desirable because it leads to excess cluster compactness with the prospect of fruit cracking and fruit rot. The berry-sizing benefit of girdling often diminishes the later it is performed after fruit set. Therefore, the optimum time for girdling to increase berry size but not berry set is typically immediately after fruitset when berries are 4 to 6 mm in diameter (Fig. 22).
Three types of grapevine girdling are cane, trunk and knife (a variation of trunk girdling). Cane girdling is easily performed with a specialized tool for this purpose (Fig. 25). A several-year experiment with the Himrod variety indicated that if cane girdles were made between the second and third nodes on canes (Fig. 25) — i.e., so that the two shoots at the base of the cane were below the girdle and were not influenced by it — it was a sustainable practice in a temperate-climate vineyard. The shoots below the girdle are defruited and they become a reliable source of fruiting canes for the next year. After a girdle is made (Fig. 26), plant cells on the edge of this girdling cut
begin to multiply, and they appear as a whitish mass of tissue called a callus. Over a several-week period, the callus growth on both sides of the girdle cut gradually unite, then phloem conducting tissue redevelops, and ultimately the girdled area becomes as strong and functional as a normal part of the vine. That's the ideal situation (Fig. 27). Unfortunately, complete cal-lusing and repair of the girdle do not always occur. Varieties vary greatly in their response to cane girdling. For example, cane girdles on Himrod and Lakemont callus well, while those on Concord and
Vanessa do not. Moreover, the smaller the diameter of the cane being girdled, the less likely it will callus well. Therefore, cane girdle varieties with an unknown response on a limited scale in the first year. Prune to retain canes with a diameter of about 3/8 inch. Leave renewal spurs. Place the cane girdles so the basal two nodes on each fruiting cane are not influenced by the girdle so there will be a reservoir of healthy canes the following year regardless of how well the cane girdles callus.
Vines almost always callus trunk girdles well. For example, even the varieties Concord and Vanessa, which do not callus cane girdles well, do callus trunk girdles well (Fig. 28). To minimize the risk of long-term stunting of vine size in response to girdling, 1/8-inch-wide trunk girdles are recommended in temperate-climate vineyards.
Knife girdling, a variation of trunk girdling, simply scores a ring entirely around the trunk with a knife cut but without removing any tissue. These cuts quickly and reliably form callus on the Vanessa variety and promote berry size comparable to 1/8-inch-wide trunk girdles. Knife girdling at the Southwest Michigan
Research and Extension Center has been accomplished with a serrated-edge linoleum knife (Fig. 29), which has been considerably easier to use than a trunk girdling tool. When trunk girdling, the exfoliating bark on the trunk should be removed before applying the girdle. More skill is required to use a trunk girdling tool efficiently than a cane girdling tool. A backyard viticulturist can apply a few trial cane (Fig. 26) or trunk girdles with a pocket knife by making two ringing cuts around a cane or trunk about 1/8 inch apart and then removing the ring of tissue between those cuts (Fig. 26). However, if more than a couple of vines are to be girdled, specialized girdling tools make this task easy and fast (Fig. 25). See Appendix B for sources of girdling tools.
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