Effect of Trunk Injection of Uniconazole on the Growth of Norway Maple and Green Ash Over Four Years

Materials and Methods

Norway maple and green ash trees, growing in Toronto and London, Ontario respectively, were trimmed flat across the top to a height of 6.1 m and 6.7 m, respectively, in June, 1988. Trunkdiameters (dbh) had been measured and the trees were ranked by diameter, with odd trees being injected and even trees not, or vice versa, to ensure the treated and control groups contained trees of similar trunk diameter. Six maple were injected with uniconazole (0.1 g active ingredient (a.i.) per cm trunk diameter) 5 days after trimming and twelve ash (0.2 g a.i. per cm) 21 days after trimming. Stock solutions were 20 g/L for the ash and 10 g/L for the maple. The latter was prepared by diluting the 20 g/L solution 1:1 vol:vol with isopropyl alcohol. Injection holes were made at approximately 15 cm intervals around the trunk about 15 to 20 cm above soil level. Holes, typically 5 to 6 cm in depth, were made with a Makita drill with a 5.5 mm (7/32 in) brad point wood bit, at a slight downward angle, about 5° to 10° from the horizontal, and at a 30° to 45° angle to the plane of the trunk to intercept the outer sapwood. Injections were performed with T.I.S. Stallion 75 injectors using an initial pressure of 621 KPa. After injection the holes were sealed with a silicone sealant to prevent pathogen entry. Control trees were trimmed and not injected. Such controls were chosen because, for a utility, it has to be demonstrated that the complete treatment (trimming plus drilling and pressure injection of uniconazole in a carrier) offers advantages over the present vegetation control method (trimming alone). Tree height and trunk diameter were measured each fall from 1988 to 1991, and each spring and fall the injection holes were checked for bleeding and the bark above and around the holes examined for splits and cracks. In the fall of 1991 it was discovered that three of the ash trees (two treated and one control) had been removed by a local forestry crew and thus no 1991 height or trunk diameter data could be collected. In addition the top of one tree was lost in an ice storm and so the height could not be determined but the trunk diameter could.

Results and Discussion

Injection times for the maple were fast (mean of 8.4 minutes per hole for a total of 29 holes, range of 3.4 to 19.4 minutes) and the ash slow (mean 25.8 minutes per hole for 32 holes, range 2.8 to 71.8 minutes). Watson (22) reported average injection times of 7.3 minutes for Norway maple and 28.2 minutes for white ash. In a recent study conducted at Newmarket, Ontario, injection times for green ash were reported to be short (about 5 minutes), although these trees were injected in the fall (17). Injections performed from July through November were reported to be quicker than the rest of the year for a variety of species (22). In addition, injection times were shorter in diffuse porous species than ring porous species. Norway maple and green ash are diffuse and ring porous species, respectively.

Seven of the 29 holes in the maple bled at some time during 1988. In June 1989, 14 holes were bleeding, and by the spring of 1990 only two. Bleeding in Norway maple has been reported previously (6). No bark splits were observed in the maple, although the bark around two holes was cracked - with one crack the result of the bark being lifted at injection. Bark splitting was found above 16% of injection holes in Norway maple in an earlier study (15). No holes in the green ash bled and no bark splits or cracks were observed in the present study.

The effects of uniconazole on tree growth are presented in Table 1. For both tree species in each of the 1989, 1990 and 1991 growing seasons there was a significant reduction in the increase in trunk diameter as a result of uniconazole application. For the four growing seasons combined (1988-1991) treatment with uniconazole resulted in a reduction of the increase in trunk diameter for both species. No significant effect on trunkdiameter was seen in 1988. The trees were injected in late June 1988 when growth had already started. Hence values for increase in trunk diameter and height for untreated trees are lower for 1988 than subsequent years (Table 1). The data for increase in trunk diameter also were calculated as percent inhibition (Table 2). Levels of inhibition were consistent for the 1989 through 1991 growing seasons for both tree species, with no indication of a decline of inhibition. This suggests a longevity of effect which may carry over into 1992 and beyond. Lack of inhibition in the first year of treatment has been reported before (3) and is probably the result of application of the growth regulator after the growth flush has started and the delay in the product getting from the point of application to the growing points (3).

The data for increase in tree height were less consistent. In the green ash there was little inhibition in 1988 and then about 50% inhibition in both 1989 and 1990, apparently declining to 24% in 1991 (Table 2). The height data for Norway maple were inconsistent however, with no significant inhibition over the four growing seasons (Tables 1 and 2). The number of Norway maple trees used in the experiment was small (12 - all that were available) and a lack of consistency of response between trees led to this result. One treated tree grew 3.5 m during the experiment, while all other treated trees grew 1.5 m or less. As discussed above, increase in tree height is a conservative measure of growth. However it may still be a better measure than sprout length given the range in length of the sprout population.

There have been previous reports of the inhibition of growth in woody species following the trunk injection of uniconazole (3,19) but such studies were short-term. In determining the effectiveness of growth regulators from a utility perspective it might be useful to examine the trimming exercise itself, by measuring the time taken or cost to trim treated trees, the frequency of trimming, and the weight of material removed at trimming. The latter has been determined for fruit trees by two groups (16,23). In addition it has been reported that after the inhibitory effects of paclobutrazol, also a triazole, diminish there is accelerated sprout growth (5). The consequences of this to long-term tree health are unknown. Because of the problems associated with trunk injection, namely the bark splitting and cambial damage reported by some applicators (14), the efficacy of growth regulators implanted in capsule form should be investigated. Such a method would avoid the use of large volumes of carrier, such as methanol or isopropyl alcohol, applied under pressure.