Qualification rates and post-treatment spring regrowth reveal divergent efficacy of ten Spartina alterniflora control methods: evidence from 848 field-managed patches.

Invasive plant management programmes are commonly evaluated using qualification rate-the proportion of treated patches that pass post-treatment inspection-as the primary measure of control effectiveness. In this study, the term refers to an administrative inspection metric rather than sustained ecological suppression. However, this metric captures only a single inspection window and may fail to reflect the subsequent spring regrowth intensity (plants ha-1), arising from in-situ regrowth from surviving rhizomes or seeds as well as recolonization from adjacent sources. Here, we conducted a large-scale assessment of ten control methods for Spartina alterniflora, a globally important invader of coastal wetlands, using data from 848 patches in a government-led eradication programme. For each patch, we quantified both autumn qualification outcome and spring regrowth intensity (plants ha-1) among patches that had passed inspection. Qualification rates differed significantly among methods (χ2 = 62.17, p < 0.001), ranging from 41.2% for uprooting & deep burial + manual follow-up to 97.7% for rotary tillage combined with waterlogging and manual removal. However, high qualification rates did not necessarily translate into lower spring regrowth. For example, mowing combined with rotary tillage and deep ploughing achieved a high qualification rate (93.9%) but showed a spring regrowth rate of 49.4% and a significantly higher probability of spring regrowth than herbicide treatment (Hurdle model odds ratio = 2.23, p = 0.022). By contrast, waterlogging-based methods achieved similarly high qualification rates (78.9%-96.1%) while maintaining the lowest spring regrowth rates (6.7%-10.2%) and spring regrowth intensities (median = 0 plants ha-1; mean = 6-29 plants ha-1). These findings show that qualification outcome does not necessarily indicate sustained cross-season control success. We therefore propose a dual-metric evaluation framework that combines initial qualification with post-treatment spring regrowth. Incorporating cross-season monitoring into official assessment protocols would provide a more robust basis for evaluating long-term eradication effectiveness.