Unlike repeated knife girdling, a plastic tie does no damage to the tree. It is applied in the winter and removed after harvest to allow the tree to recover.
A Georgia researcher is testing plastic cable ties as an alternative to knife girdling of peach trees for improving the size and sweetness of the fruit.
Knife girdling is used on peaches to manage fruit size and advance harvest, but it has drawbacks, too, Dr. Kathryn Taylor of the University of Georgia, noted during the International Fruit Tree Association’s annual conference. It can result in split pits (because of too much sugar in the fruit) and damage trees, making them more vulnerable to attack by pests and diseases.
Taylor’s technique, using cable ties to restrict sap flow, gives the benefits of girdling, but without causing long-term damage to the tree.
Starting in 2002, Taylor experimented using cable ties—the kind used to bind computer cables—which she bought at a hardware store. The ties were a sixteenth of an inch wide. The tests were done on fourth- and fifth-leaf Redglobe trees on Guardian rootstocks planted on an 18- by 18-foot spacing and trained to an open-center system. Four different girdling treatments were used on scaffolds of the trees. On one scaffold, one cable tie was applied during the winter. On another scaffold, two ties were applied. On a third scaffold, girdling was done with a knife just before pit hardening in April. And on the fourth, no girdling was done. After harvest, she compared fruit size, color, and Brix level. The cables were removed after harvest to allow the tree to recover and were reapplied the following winter.
In the first year, the girdling and cable treatments gave similar improvements in fruit size and yield. But in the second year, the two-cable treatment gave much higher yields than the knife girdling, while results from the one-cable treatment were similar to the knife girdling. The cable and girdle treatments resulted in similar increases in fruit size. However, during a rainy period, sap ran out of the trees that had been knife girdled, which was a problem, since it can result in carbohydrate losses.
In 2004, she repeated the trials, comparing a half-inch cable tie on the scaffolds with knife girdling or no treatment. Fruit size, yield, and Brix were improved with the cable treatment, though the knife produced the best results of all.
But in 2005, on the same trees, the cable tie gave better results than the knife treatment in terms of size, yield, and Brix. Return bloom was also improved by the cable treatment, whereas knife girdling reduced return bloom. Harvest was advanced by five days by the cable. Taylor noted that earlier ripening means growers can get their fruit to market earlier and usually increase their profits.
In the knife-girdled trees, sucrose and starch levels in the bark were lower than in the trees with the cable or no treatment, indicating lower reserves in the tree. Taylor said it appears that the tree becomes stressed by repeated girdling treatments.
“We’ve actually measured that it’s injuring the tree and that cable ties can give us improved fruit size, and yield, and Brix without causing that injury,” she said.
However, in trials with the early maturing varieties Juneprince and Springprince, cable ties did not improve fruit size or yield.
Taylor has also worked with growers to test the technique in commercial orchards. In a grower trial in 2004, on Rubyprince peaches, a half-inch cable tied around the tree trunk in January was compared with standard girdling done in April, and no treatment.
Yields were 2.7 bushels per tree, with 72 percent packout for the cable tie treatment, compared with 2.8 bushels per tree and 70 percent packout for the knife treatment, and 1.1 bushels per tree with 66.5 percent packouts for the control. The percentage of fruit larger than 2.75 inches was 39 percent for the cable tie, 40 percent for the knife girdling, and 26 percent for the control.
In similar trials with Blazeprince, there were no significant differences in yields or fruit size, but both the cable and the girdling increased the percentage of packout, which was 83 percent for the cable tie, 79 percent for the knife girdling, and 74 percent for the control.
Cost of cables was 25 cents per year if they were used just once—the same cost as for knife girdling—though the cables can be reused to reduce the cost.
The grower reported that the cable approach might be most useful for varieties that are difficult to size and for young bearing trees while the bark is smooth and undamaged. It can be difficult to get good contact on trees with rough bark.
Taylor sees no reason why the technique couldn’t be used on other types of tree fruits. Some orchardists in Nova Scotia have been trying it out on apples, she said.