Plant growth inhibitors identified in acidic extracts of flower buds from mature peach trees were p-coumaric acid, feruIic acid, an isomeric hydroxymethoxycinnamic acid, a dihydroxycinnamic acid having the properties of 3,5-dihydroxycinnamic acid, and benzoic acid. The inhibitors were isolated by preparative gas-liquid chromatography, tested for growth activity on the wheat coleoptile straight-growth bioassay, and identified by high-resolution mass spectrometry.