Arboriculture & Urban Forestry 33(1): January 2007 59 Both formulations of imazapyr were very effective for the control of small and medium trees using stem injection. Crown reduction was between 95% and 98% with either the water-soluble (Habitat) or emulsifiable concentrate (Stalker) formulation of imazapyr 10 months after treatment (Table 2). Because imazapyr is known to translocate slowly, it was pos- tulated that a single winter season would not be sufficient to allow the herbicide to circulate throughout the tree and give adequate control. With saltcedar or tamarisk (Tamarix spp.), it was demonstrated that plants could not be cut until two full growing seasons after foliar treatment (Duncan and McDaniel 1998). However, with tree-of-heaven there was no difference between trees cut down at 4, 8, or 12 months after treatment and no difference between the two formulations of imazapyr by 21 months after treatment. At this evaluation timing, only 12.5% and 8.3% of the Stalker- and Habitat-treated plants, respectively, had stump sprouts. In addition, sprouts were about four times fewer in number and four times smaller than resprouts from the mechanically cut trees without an herbi- cide treatment. Stem injection treatment of tree clumps with half of the standard rate of Stalker gave 84% crown reduction 8 months after treatment and 99% reduction 21 months after treatment (Table 2). The quarter rate, however, gave only 82% crown reduction, which is was not considered to be satisfactory control. Our results indicate that imazapyr applied as a stem injec- tion treatment can be an effective method for tree-of-heaven control even with plants growing in clumps. In some cases, it also was observed that imazapyr appeared to move from treated stems to other adjacent trees, presumably through root grafts. Symptoms of imazapyr injury were evident in other untreated tree-of-heaven stems up to ≈5 m (16.5 ft) away from the treated stem. This phenomenon has been observed in other trees. A similar response was noted in black oak (Quer- cus kelloggii) after a stem injection imazapyr treatment (DiTomaso et al. 2004). Despite this response within tree-of- heaven plants, we observed no injury resulting from root grafting among other species. In our second study, we compared the effect of imazapyr, triclopyr, and glyphosate on tree-of-heaven using stem injec- tion, stump injection, cut stump, and basal bark treatments. As was the case with the University of California, Davis, site, mechanical cutting gave very poor control with ≈86% of the stumps producing new sprouts (Table 3). By 2003, 22 months after cutting, stumps averaged 6.0 new sprouts with a mean height of 2.4 m (7.9 ft). Although it is widely believed that herbicide should be applied to the stump immediately after cutting (Tjosvold and McHenry 1986), our results indicate that a delay between cutting and herbicide treatment in both the cut stump and the stump injection techniques did not significantly affect the percentage of trees with sprouts (Figure 1) or the vigor of the new sprouts (Figure 2). Applying the herbicide 1 hr after cutting gave the same result as an application immediately after cutting in the cut stump treatments. In the stump injec- tion technique, applying the herbicide 1 week after cutting also provided the same level of control as applying the stem injection immediately after cutting. Based on these results, data for different timing intervals were combined for each herbicide in the cut stump and stump injection treatments. Using a cut stump technique, imazapyr and triclopyr both provided excellent results. Although glyphosate results were statistically better than the cut controls, they did not give adequate tree-of-heaven control. By 2003, ≈41% of the glyphosate-treated trees resprouted and the resprouts were a similar height (2.5 m [8.3 ft]) as the untreated trees (2.4 m [7.9 ft]). Triclopyr produced slower results but was equal to Table 3. Results for cut-tree treatments in 2001 tree-of-heaven trialz Vigor mean over all treesy,x 2002 Cut stump Cut control Stump Cut control Glyphosate 2.5 b Imazapyr 0.0 c Triclopyr 1.0 bc 8.3 a Glyphosate 6.7 a injection Imazapyr 0.9 b Triclopyr 4.8 a 8.3 a 2003 4.1 b 0.6 c 0.5 c 7.4 a 6.6 ab 1.5 c 5.0 b 7.4 a . Means for sproutsv Percent of trees with sproutsw 2002 2003 33.5 b 3.1 c 21.0 bc 85.7 a 85.7 a 35.8 c 60.7 b 85.7 ab 40.8 b 9.8 c 6.7 c 85.7 a 92.9 a 32.1 c 62.5 b 85.7 ab Vigor Number Height (m) 2002 2003 2002 2003 2002 2003 7.5 9.9 3.9 2.9 1.0 2.5 0.5 6.3 0.0 2.8 — 0.7 4.3 7.5 1.8 1.5 0.4 0.6 9.7 8.7 7.3 6.0 1.1 2.4 8.0 7.1 5.5 4.2 0.8 1.7 2.1 4.0 6.1 6.3 0.2 0.7 7.9 7.7 3.3 2.4 0.4 1.2 9.7 8.7 7.3 6.0 1.1 2.4 zAll evaluations made in August 2002 and 2003. Values are means over all application timings. Within each treatment group, values followed by the same letter are not different at 0.05. Cut control values were used for separate comparisons with cut stump and stump injection treatments. yVigor is a visual evaluation scale where 0 dead, 10 healthy. xDifferences determined by analysis of variance and Student-Newman-Keuls test. wDifferences determined by logistic regression. vOnly for trees with resprouting. ©2007 International Society of Arboriculture
January 2007
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