Allelopathy plays an important role in plant–plant interactions through the release of bioactive compounds that influence the germination and growth of neighbouring plants. The present study comparatively evaluated the allelopathic activity and total phenolic content of aqueous leaf extracts of Cassia fistula L. and Caesalpinia pulcherrima (L.) Sw. using Phaseolus vulgaris L. as the test species. Fresh leaf extracts were prepared in distilled water to obtain 50% and 100% concentrations and assessed through laboratory germination bioassays. Germination percentage, radicle length and radicle growth inhibition were recorded on the seventh day after treatment. Total phenolic content of the aqueous extracts was determined using the Folin–Ciocalteu colorimetric method, and the data were analysed by one-way analysis of variance (ANOVA) followed by Tukey's Honestly Significant Difference (HSD) test at P < 0.05. Cassia fistula significantly inhibited seed germination at both extract concentrations, whereas C. pulcherrima exhibited only a concentration-dependent but statistically non-significant reduction in germination. In contrast, radicle elongation was more sensitive to allelopathic stress than seed germination, with the 100% aqueous extract of C. pulcherrima producing greater inhibition of radicle growth than the corresponding extract of C. fistula. Total phenolic content of the aqueous extract of C. pulcherrima was approximately three-fold higher than that of C. fistula. However, the greater phenolic content was associated with stronger inhibition of radicle elongation but not with greater inhibition of seed germination, indicating that total phenolic content alone does not fully explain the observed allelopathic effects. The findings demonstrate that both species possess allelopathic potential and provide a basis for future studies aimed at identifying the bioactive allelochemicals responsible for their phytotoxic activity and evaluating their potential applications in sustainable weed management.