Allelopathy has garnered considerable attention, but the effects of different allelochemicals on invasive plants remain unclear. This study addressed the knowledge gap surrounding allelopathy and its impact on native and invasive plant species. We focused on the impact of caffeic acid and its derivatives on the growth and competitiveness of the native Lantana indica and the invasive plant Solidago canadensis. We selected three allelochemicals, caffeic acid, methyl caffeic acid, and ethyl caffeic acid, for evaluation at two concentrations (0.1 mM and 1.0 mM). Three planting methods were employed: (1) a single species of S. canadensis, (2) a single species of L. indica, and (3) a combination of S. canadensis and L. indica. In addition, a control group was also included. Results revealed that high concentrations (1 mM) of methyl caffeate (MC) and ethyl caffeate (EC) significantly reduced seed germination rate, seed germination index, and seed germination speed index of L. indica compared to a low concentration (0.1 mM). Plant height, stem diameter, biomass, and root length in the control group (CK) of S. canadensis were significantly higher than those in the treated groups. However, with increasing allelochemical concentration, L. indica’s relative competitiveness gradually decreased. These findings provide insights into the concentration-dependent effects of allelopathic compounds on the growth of L. indica and S. canadensis. By analyzing how these allelochemicals influence the growth and competitiveness of native and invasive plants, the study sheds light on the dynamics of allelochemical interactions between these species. This knowledge can be pivotal for understanding plant competition dynamics in ecosystems and could inform strategies to control invasive species or promote native plant growth.

Summary: Globalization has accelerated the establishment of non‐native plant species, increasing the prevalence of multispecies co‐invasions; yet interactions among co‐invaders remain poorly understood. We combined extensive field surveys of 3039 plots with a global meta‐analysis of 4415 observations from 122 experiments to quantify co‐invasion prevalence, its effects on non‐native plant performance, and the frequency of interaction types. Co‐invasion was common in the surveys, occurring in 40% of the plots. Meta‐analysis showed that co‐invasion reduced non‐native plant performance by 18% on average, indicating that invasional meltdown among plant species is not the norm. Most interactions were neutral (70%), whereas competition (18%) and facilitation (12%) were relatively uncommon. Thus, the widespread co‐occurrence of non‐native plants does not necessarily imply pervasive facilitation among co‐invaders. Crucially, the mean competitive outcome was context‐dependent. Interactions shifted from competition to facilitation with increasing non‐native richness and intensified competition with natives. Furthermore, phylogenetic distance among co‐invaders did not explain the interaction outcomes, whereas interactions among woody, nitrogen‐fixing species, or invaders with greater SLA produced mean neutral effects. These findings expand our understanding of the invasional meltdown hypothesis by showing that co‐invasion outcomes are context‐ and trait‐dependent and offer a predictive framework for assessing and mitigating multispecies invasion risks.