A Comparison of Non-Chemical Alternatives for Reducing Creeping Thistle (Cirsium arvense) Abundance in a Conservation Grassland

Abstract

Semi-natural grasslands harbour important biodiversity, yet managing them for nature conservation can involve toxic agrochemicals. This study compares treatment efficacy of two non-chemical methods of reducing creeping thistle (Cirsium arvense) abundance to the efficacy of a narrow spectrum herbicide containing triclopyr and clopyralid. We used six replicates of six 100 m² plots (n = 36) over 3 years to compare mechanical topping (cutting), manual pulling and no management to spot-spraying with Grazon Pro. Two complete thistle counts were carried out per year to quantify change in abundance. We also tested for temporal trends across 4 years, evaluated edge effects, and characterised spatial distribution. Leaving creeping thistles unmanaged yielded similar counts within years and a marked increase across years (effect size = 0.26; p-value = 0.001). The efficacy of topping compared poorly to spraying (effect size = −0.33; p-value < 0.001), and no temporal trend was apparent. Conversely, manual pulling was marginally more effective than spraying (effect size = 0.15; p-value = 0.08), with a consistently negative temporal trend (effect size = −0.34; p-value < 0.001). No temporal trend was detected in sprayed plots. Edge effects were negative in topped plots and positive in sprayed and pulled plots. Spatial distribution was random where thistles were left unmanaged (aggregation index R = 0.95), uniform in sprayed plots (R = 1.27), and clumped where thistles were pulled or topped (R = 0.69 and 0.89, respectively). These findings signify that manual pulling of creeping thistle is slightly more effective at reducing abundance than spraying, and is the only method that we tested to produce a consistent abundance reduction over time. We recommend that manual pulling replace spot spraying with herbicides on conservation grasslands to avoid the negative impacts of herbicide on biodiversity.