Herbicide Toxicity Testing with Non-Target Boreal Plants: The Sensitivity of Achillea millefolium L. and Chamerion angustifolium L. to Triclopyr and Imazapyr

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)₅₀ = 1443.8 g a.i. ha⁻¹) and was lethal to C. angustifolium at the lowest dose tested (1210.9 g a.i. ha⁻¹). Both species demonstrated extremely high sensitivity to imazapyr foliar spray: IC_50s = 8.29 g a.i. ha⁻¹ and 4.82 g a.i. ha⁻¹ (<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 IC₅₀ estimates between 2–20 µg g⁻¹). For imazapyr, IC₅₀ estimates could not be calculated as there was >75% inhibition of endpoints at the lowest doses of ~2 µg g⁻¹. 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.