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The exotic ornamental tree Senna spectabilis (DC.) H. S. Irwin and Barneby has invaded various Protected Areas of the Nilgiri Biosphere Reserve including the Wayanad Wildlife Sanctuary in India creating a significant threat to biodiversity. This study examined the allelopathic potential and heterotoxicity of invasive S. spectabilis leaf extract by bioassay under laboratory conditions and identified its allelochemicals using HRLC-ESI–MS (QTOF) analysis. The bioassay experiment with test plant species including Vigna radiata (mung bean), Cicer arietinum (chickpea), and Amaranthus cruentus (red amaranth) was conducted in the laboratory and the methanolic leaf extract found to exhibit the greatest inhibitory activity. The percentage of inhibition of plumule growth by methanolic leaf extract on V. radiata, C. arietinum, and A. cruentus were 82%, 86%, and 99%, respectively. Phenol and proline content were significantly increased in germinated seedlings treated with even a quarter strength of leaf extract. Further by HR-LCMS analysis fifteen important allelochemicals were identified including phenolic compounds, flavonoids, anthraquinone, benzoic acid derivatives, and cinnamic acids. A soil sterilization experiment was used to examine the role of soil microbes in the phytotoxicity of leaf leachate. No significant variations exhibited in the phytotoxicity. The full strength of leaf leachate inhibited the shoot length of B. bambos by 52% in the sterile and 48% in the non-sterile soil, whereas it was 74 and 68%, respectively, in V. radiata. Further studies in the process of allelochemical release and their mode of action may confer a more lucid knowledge concerning the invasion mechanisms of S. spectabilis through allelopathy, guiding to practical management strategy. [ABSTRACT FROM AUTHOR]