Trunk Injection of Plant Growth Regulators to Control Tree Regrowth

Materials and Methods

In 1977, four sites (Philadelphia, Pennsylvania; Delaware and Lorain, Ohio; and Hayward, California) were selected to conduct injection studies. Table 1 summarized the species, chemical concentrations, and range of dbh (diameter at breastheight) used at these locations. At Philadelphia and Hayward, three concentrations of each chemical were selected, whereas at Delaware and Lorain only two concentrations were utilized to evaluate the effects on sprout growth. Twenty trees per treatment were used at the Delaware and Lorain sites; 10 trees per treatment were used at the Philadelphia and Hayward locations. Aqueous solutions of dikegulac and MH were injected into tree trunks under pressure of 14 kg/cm² using the injection equipment described by Brown (3).

The details on statistical design, the volume of chemical used, and treatment method have been described elsewhere (5). Briefly, equidistant holes of 0.54 cm diameter were drilled tangentially around the tree trunk to intersect outer xylem vessels. The injectors were inserted into these holes and chemicals were injected under pressure. The trees were topped before treatment and 3 limb stubs were selected at random and tagged for determining growth data following treatment. In the fall, after leaf abscission, all sprout regrowth originating within 25cm (ash and sycamore) or 50cm (maple and oak) from the cut ends of preselected limbs was measured. These data were collected every year from 1977 through 1979 to determine the time when repruning might be required. Before the leaves turned color in the fall, foliar index ratings (Fl) for each tree were recorded. The data were statistically analyzed to compare growth and Fl of untreated controls with that of chemically treated trees.

Results and Discussion

Sprout regrowth data for 1977 have shown that higher concentrations of dikegulac and MH significantly reduced regrowth enough in some species so that mechanical pruning could be delayed for up to 2 years (5). The data collected in the fall of 1979 show that even after three growing seasons, a significant reduction in growth of silver maple was detectable at two concentrations (15.0 and 75.0 g/liters) of dikegulac (Table 2). At the highest concentration, sprout number was also significantly lower than that of the control. In the case of red oak at Lorain, OH, dikegulac was again found to be effective in retarding tree regrowth although sprout number was not affected. The higher concentration (187.5 g/liter) of dikegulac used for red oak was more effective than the lower one (75 g/liter) in reducing regrowth. However, the higher dose caused an unacceptable level of foliar damage on treated trees as indicated by an Fl of 2.7.

In the case of treatments with MH, the data indicate that red oak had overcome inhibitory growth effects of extremely high concentrations of chemical (281.25 g/liter) after three seasons. In the case of silver maple, however, MH at 112.5 g/liter significantly retarded tree regrowth although sprout numbers were not affected. This suggests that the concentrations required to control tree regrowth over a long period of time is based upon species sensitivity to the chemical.

The data collected in the fall of 1977 showed that significant growth reduction was obtained with all three concentrations of dikegulac (3.0, 9.0, and 15.0 g/liter) and MH (13.3, 22.5, 67.5 g/liter) injected into sycamore located at Philadelphia, PA (5). In 1979, at the end of the third growing season, no significant growth differences were detectable on trees treated with either chemical (Table 3). Although statistical analysis indicated that sycamore had overcome the inhibitory effects of chemicals, utility personnel visually observing the treated trees indicated that although controls could have been pruned in 1978, the trimming of trees treated with higher concentrations of chemicals may be delayed until 1980 or even 1981. This would suggest that chemical treatment can extend the trimming cycle up to two years, resulting in extensive savings.

Similarly, in the case of shamel ash, the 1979 results showed that there was no significant difference in growth between treated and untreated trees (Table 3), although two years earlier in 1977 dikegulac and MH were found to be effective in reducing growth (5). Still, from an economic point of view, chemical treatment was a success since the mechanical pruning of injected trees had been delayed by at least one year.

These studies show that, depending on species, appropriate concentrations of dikegulac and MH may significantly reduce the regrowth of pruned trees and consequently extend the trimming cycle by at least a year and probably more. This represents extensive potential savings for the utility industry which is confronted with inflated costs at this time.