Branch Pruning Wound Closure

Materials and Methods

The trees used in this study were located in the Street Tree Collection of the Morton Arboretum near Lisle, Illinois. The trees were planted in 1958 with 15m (50 ft) between trees in a row and 15m between rows. The trees ranged from 12 to 20 m tall in 1986. Ten cultivars within seven genera were chosen (Table 1). Four branches on each of five trees of each cultivar were selected for pruning. The branches’ diameters were measured with a steel tape approximately 8 cm from the trunk. Two branches on each tree were pruned beyond the branch shoulder using the “Shigo” method (6), and two branches were pruned through the branch shoulder using the conventional method (4) on September 22, 1986. The width of xylem exposed on each wound was measured on April 27, 1987, and again in mid-October of 1987, 1988, 1989, and 1990. The amounts of closure each year on each species were analyzed using Student’s test for paired samples.

Results

The average branch diameters and the average widths of wounds are given in Table 1. The widths of the wounds from conventional cuts were significantly larger than those associated with the Shigo cuts. The width of xylem exposed after a Shigo cut was sometimes even less than the branch diameter (e.g., 94 percent of branch diameter for Liquidambar and 96 percent for Fraxinus). With conventional cuts, the wound width was often 45 percent larger than the branch diameter (e.g., 151 percent of branch diameter for Acer (1) and 147 percent for Acer (2) and Gleditsia).

The average closures in the widths of wounds for each of the four growing seasons are given in Table 2. Closure of the Shigo cuts was slight or nonexistent in 1987 for all 10 cultivars and averaged 3.1 mm. By contrast, only on Quercus was the closure of conventional cuts limited in 1987, and closure of conventional cuts averaged 8.4 mm. With exception of Quercus in 1987 and 1990, the conventional cut wounds closed more each season on each cultivar than did the Shigo cut wounds.

At the end of the 1990 season, the conventional wounds were smaller than the Shigo wounds on Acer (1), Acer (2), Acer (3), Liquidambar, Platanus, and Ulmus; they were essentially the same size on Acer (4); and they were larger than the Shigo wounds on Fraxinus, Gleditsia, and Quercus (Fig. 1). Of the 100 wounds of each type on all species, 17 Shigo cuts and 12 conventional cuts were closed after four seasons.

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Figure 1.

The widths of wounds resulting from pruning of branches by the Shigo and conventional methods and the resulting widths during four years of wound closure.

Discussion

The specifications for a Shigo cut as used in this test were those recommended in 1984. The angle of the final cut was equal to that formed by the branch bark ridge and the vertical. Since then, Dr. Alex L. Shigo has stated, “Proper pruning of a living branch is a cut as close as possible to the branch collar. There is no set angle for a proper cut” (7, page 29). Had the 1989 specifications been used in this study, the results may have been different. Both Shigo and I include the warning—do not leave branch stubs!

The decision that must be made by the arborist or tree owner is whether to make the final cut either through or outside of the branch shoulder. A conventional cut through the shoulder will result in a more aesthetically pleasing and faster growing callus. A cut outside of the shoulder or collar will possibly reduce discloration and decay in the stem or trunk from which the branch was removed. Both objectives are desirable, but only one is obtainable.