In the Yukon Territory, transmission rights‐of‐way (ROWs) are managed using brushing and mowing techniques alone. When cut, target species such as Populus tremuloides Michx. and Salix spp. grow rapidly shortening maintenance cycles. Long‐term vegetation control may be improved by integrating herbicide application. However, prior to implementation, the dissipation and toxicity of herbicides in northern latitudes needed to be assessed. The dissipation of Garlon XRT (triclopyr) and Arsenal Powerline (imazapyr) in soils was assessed at five ROW locations representative of the main ecoregion types where ROWs occur within the Yukon Territory. Soils from four sites were collected at 1, 30, and 365 d after treatment to determine persistence of herbicides for each of three application methods (backpack spraying, cut stump, and point injection). Increased sampling intervals were added to better determine the dissipation rate of each herbicide in Yukon Territory soils. Soil dissipation data were linked to a series of standardized toxicity tests, including three soil invertebrates (Enchytreaus crypticus, Folsmia candida, and Oppia nitens). Additionally, the dissipation of both herbicides from the target species Salix glauca L. was assessed at one site. Herbicide residues persisted in soils for longer than 365 d after treatment and longer than 30 d after treatment in S. glauca. However, concentrations were below the concentration that would affect 25% of the invertebrate species tested. Weight of evidence and toxic exposure ratios were used to characterize the risks associated with herbicide application in northern latitudes and provided both qualitative and quantitative means to communicate the results to the public. Core Ideas Herbicide use may improve vegetation management along rights‐of‐way in the Yukon. Triclopyr is a good alternative due to a mobile‐phase DT50 of 1 d after treatment. Imazapyr presented no unacceptable risk to the soil invertebrates tested. Risk characterization indicated triclopyr and imazapyr are acceptable in northern latitudes. Triclopyr presents less risk due to fast dissipation and low risk to soil invertebrates.

Terrestrial plant toxicity tests were conducted to determine the sensitivity of two boreal plants, yarrow (Achillea millefolium L.) and fireweed (Chamerion angustifolium L.), to the herbicides imazapyr and triclopyr. Both plants are common non-target species on northern powerline rights-of-way where the impacts of proposed herbicide applications are of concern. In the vegetative vigour test, triclopyr foliar spray caused extensive damage to A. millefolium at <50% of the maximum field application rate (inhibition concentration (IC) = 1443.8 g a.i. ha) and was lethal to C. angustifolium at the lowest dose tested (1210.9 g a.i. ha). Both species demonstrated extremely high sensitivity to imazapyr foliar spray: IC = 8.29 g a.i. ha and 4.82 g a.i. ha (<1.5% of the maximum field rate). The seedling emergence and seedling growth tests were conducted in the organic horizon of five boreal soils. Few differences in herbicide bioavailability between soils were detected. Triclopyr limited growth of A. millefolium, C. angustifolium and standard test species Calamagrostis canadensis at low levels (most IC estimates between 2-20 µg g). For imazapyr, IC estimates could not be calculated as there was >75% inhibition of endpoints at the lowest doses of ~2 µg g. A foliar application of triclopyr or imazapyr for woody species control would likely cause significant damage to boreal non-target plants. The high sensitivity of both species to herbicide residues in soil indicates long term impacts are dependent on herbicide degradation rates in northern conditions. A. millefolium performed well and is recommended for use in toxicity testing relevant to boreal regions.

Integrated vegetation management programs have successfully reduced the frequency and intensity of power line right-of-way management by promoting low-growing plant communities resistant to tree invasion. To examine whether these principles are transferable to northern ecosystems, researchers tested eight treatments at four sites in Yukon, Canada. Two herbicides, imazapyr and triclopyr, were applied by three methods, as well as a native grass seeding treatment and a mowing control. Vegetation cover was recorded prior to treatment and after one year along with herbicide damage assessments. Overall, treatments caused significant changes to vascular plant communities after one year. Short-term control of woody target species was greater in chemically-treated plots (66%–94%) than with mechanical methods (<55%). All treatments caused a minor reduction in non-target vegetation cover. In seeded plots, seedlings emerged but total non-target species cover was reduced by seedbed preparation. Triclopyr broadcast spray reduced non-target vegetation cover by <10%, but the common shrub, kinnickinnick (Arctostaphylos uva-ursi), was highly impacted. Selective application of triclopyr effectively controlled targets with minimal effects on non-target species. Imazapyr consistently caused more impacts to non-target plants than triclopyr. Both selective and non-selective imazapyr applications resulted in chlorosis, stunting, and deformity of shrubs and forbs one year after treatment. This suggests imazapyr can remain active in northern soils for at least 365 days as well as transfer to untreated plants. The range of sensitivities of boreal plant species to imazapyr and triclopyr and potential persistence in northern soils highlights the need for focused toxicity research in the North.