BACKGROUND: Ludwigia hyssopifolia (G. Don) Exell, one of the problem weeds in some rice-producing countries, was studied to determine its allelopathic potential based on the effects of aqueous extracts of its tissues (leaves, roots and stem) on seedling growth of selected weeds and rice. The major phenolic compound of its leaves was also isolated and characterized. RESULTS: L. hyssopifolia aqueous leaf extract showed significant inhibition of shoot growth and biomass accumulation of weeds (Amaranthus spinosus L., Dactyloctenium aegyptium L., Cyperus iria L.) while maintaining less adverse effects on rice (crop) compared to other aqueous extracts of roots and stem. Phytochemical screening showed that phenols, tannins, flavonoids, terpenoids, saponins and coumarins are found in its leaf aqueous extract. The Folin-Ciocalteu method revealed that its leaves contain 26.66 ± 0.30 mg GAE g −1 leaf. The extract was then acid-hydrolyzed to liberate the phenolics (25 mg phenolics g −1 leaf). The major compound was isolated via preparative thin-layer chromatography using formic acid-ethyl acetate-n-hexane (0.05:4:6) solvent system. It had maximum UV absorption at 272 nm while its Fourier transform infrared spectrum revealed phenol, carboxylic acid and ether functionalities. This also had similar chromatographic mobility when run together with syringic acid in two-dimensional paper chromatography and thin-layer chromatography. CONCLUSIONS: L. hyssopifolia has potential allelopathic activity and its leaf aqueous extract showed the highest phytotoxic activity (P ≤ 0.05) indicating its potential as a bioherbicide. The most probable identity of the major phenolic compound is syringic acid.