122 Ballard and Nowak: Timing of Cut-Stump Herbicide Applications mortality was high for all treatment plots, indicating that enough herbicide was applied and that the amount of herbi- cide applied did not affect the mortality rate. DISCUSSION Results from this study demonstrate a wide treatment window for cut-stump treatment of common hardwood trees on rights- of-way from April to November. Maximizing the percentage mortality throughout this range of treatment dates may be contingent upon matching herbicide mix to the date of appli- cation. With over 90% mortality, Garlon 4 + Stalker or Path- way could be used from April through November, but the Accord + Arsenal mix should be used only in the fall. Comparison with Other Studies Studies of cut-stump treatments to control hardwood trees were rather limited, but in those that have been published, the efficacy of treatments was generally lower than we observed. For example, the percentage mortality 2 years after cut-stump treatment with glyphosate, 2,4-D + picloram, and triclopyr ranged from 46% to 66% for applications made during the dormant season and 49% to 56% for growing-season appli- cations, relative to an untreated control (Zedaker et al. 1987). Percentage mortality after one growing season for February cut-stump applications of picloram + 2,4-D, 2,4-DP + 2,4-D, and triclopyr ranged from 53% to 94% in a study conducted in Arkansas, U.S. (Troth et al. 1986). The high percentage mortality that we observed may be partially attributed to the relatively young, small trees treated and the immediate application of herbicide—applications were made to the cut surface within minutes after felling. In other studies, herbicide applications were made within 30 to 90 min (Troth et al. 1986) after cutting, or applied immedi- ately after cutting to up to 4 hr later (Zedaker et al. 1987). The delayed applications may account for some of the differences in tree mortality. In addition, we used mixtures that were not used in the other studies (triclopyr + imazapyr and glypho- sate + imazapyr). Broader spectrum of control with herbicide mixtures may also account for high mortality. The use of tank mixtures has become more common in right-of-way vegeta- tion management (Sulak and Kielbaso 2000). More effective, broad-spectrum mixtures may allow for reduced concentra- tion of active ingredients in herbicide applications. Most of the herbicides used by Zedaker et al. (1987) and Troth et al. (1986) were undiluted, so active ingredient concentrations were more than double those used in our study. Despite using lower-concentration herbicide mixes in our study, percentage mortality was still very high. However, because the dose was not regulated in our study, concentration differences could be offset by higher application rates. Herbicide Mode of Action and Translocation Some of the observed differences among herbicide mixes and treatment dates may be related to the physiological processes ©2006 International Society of Arboriculture occurring in hardwood trees during different seasons and translocation and mode of action of the herbicides. All of the herbicides used in the study can be translocated in the phloem. Glyphosate, triclopyr, and 2,4-D are translocated pri- marily symplastically (i.e., in the phloem), and imazapyr and picloram are readily transported in both the xylem and phloem (Ahrens 1994). Transport of sugars in the phloem is from “source” to “sink.” For example, leaves serve as a source of photosynthate during the active growing season, so sugars are transported downward in the phloem from leaves to roots. Xylem flow is in the opposite direction. In the spring, the opposite occurs in the phloem: the roots are the source and developing leaves are the sink, so phloem trans- port is upward. For herbicides to kill hardwood trees with the cut-stump treatment, they must move to growing points in the plant, such as roots, buds on the stump and root collar, or roots of suckering species. This has been demonstrated for root suck- ering American beech (Fagus grandifolia) using stem injec- tion (Kochenderfer et al. 2004). By severing the stem, a source or sink, depending on the season, is removed. Imme- diate application of herbicide to cut stems of dogfennel (Eu- patorium capillifolium) demonstrated that xylem cavitation can initially pull herbicide inward, but after xylem flow halts due to the formation of air pockets, the herbicide likely dif- fuses laterally to sieve elements and is translocated via phloem to the roots (Wahlers et al. 1997). Because sap flow in the phloem in spring is primarily upward, it may be diffi- cult for translocation of phloem-mobile herbicides toward the roots, particularly those with very low Kow (e.g., glyphosate; Table 1). In the fall, movement of sugars is downward in the phloem to the root storage tissues, so herbicide transport to- ward the roots is more likely. All three herbicide mixtures used in this study included one herbicide that is translocated primarily in the phloem and one that is readily translocated in both xylem and phloem. Garlon 4 + Stalker (triclopyr and imazapyr) and Pathway (2,4-D and picloram) worked well at all three dates of application. The poorest-performing mixture was Accord + Arsenal (glypho- sate + imazapyr), except in the fall. In another study, late- summer treatments with glyphosate using the hack-and-squirt method were very effective at controlling beech root sucker- ing (Abrahamson 1983). Glyphosate has been found to per- form better in the fall compared to spring or summer in other perennial plants (e.g., Johnsongrass; Lourens and Bayer 1996), because sugar transport to the rhizomes was greater in the fall. In addition, the very low Kow values (Table 1) asso- ciated with glyphosate indicates that this herbicide has opti- mal permeability to enter and stay in the phloem (Grimm et al. 1995). Therefore, in the spring, when phloem transport is upward, glyphosate would be “pushed out” rather than pulled down to the roots from the cut surface. However, in the fall,
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