Combining Herbicides and Fertilizers to Enhance Control of Leafy Spurge (Euphorbia virgata Wald & Kit)
Taillon, T. 2025.
Abstract
Across the Northern Great Plains, leafy spurge [Euphorbia virgata Wald & Kit (previously
known as Euphorbia esula L.)], is a problematic invasive plant that contributes to the
degradation of native ecosystems through displacement of indigenous vegetation. Controlling
leafy spurge is extremely challenging, often requiring multiple herbicide applications for
sustained suppression. Below ground, leafy spurge associates strongly with arbuscular
mycorrhizal fungi (AMF) in a symbiotic plant-soil relationship that typically benefits both the
host plant and the fungi by facilitating nutrient and resource exchange. However, in a nutrient
rich environment, mycorrhizal associations can be altered such that fertilizers diminish the
advantages of AMF, causing the relationship to become harmful to the host plant through
depletion of essential fats and sugars. Consequently, the strategic application of fertilizer may be
a tool in enhancing control of leafy spurge, effectively transforming the mycorrhizal association
from beneficial to parasitic. When combined with herbicide treatments, this approach holds
promise for addressing leafy spurge invasion more effectively and sustainably.
Research plots were established in leafy spurge invaded native prairie across four
locations in Saskatchewan. Fertilizer and herbicide treatments were applied in June 2022, with
fertilizer reapplied in 2023. Herbicide treatments significantly reduced leafy spurge biomass and
cover in year one, while in year two only Tordon and Navius significantly reduced leafy spurge.
However, these herbicides also decreased species richness through native forb loss resulting in a
shift in the plant community toward grass dominance. Select fertilizer treatments, specifically
micronutrient and nitrogen, decreased leafy spurge cover but only at the plot level suggesting
scale dependent effects. Herbicide and fertilizer effects on AMF were complex, where certain
fertilizers, particularly micronutrient and phosphorus, decreased AMF colonization, but only
when applied with select herbicides - 2,4-D combined with micronutrients and Tordon combined
with phosphorus - suggesting that it is possible to manage AMF through use of fertilizers.
Herbicide and fertilizer effects on forage quality were similarly dependent on specific treatment
combinations, with crude protein and fiber content all responding to some combination of
treatment. Given the complexity of these effects, more research is needed before
recommendations can be made to producers.
