Hand weeding continues to be one of the most laborious aspects of nursery
maintenance. Oxygenated monoterpene, an important group of secondary metabolite found in
essential oils, has a potential herbicidal activity that could be exploited as natural herbicide
whereas organic mulch could delay weed emergence. Thus, this study aimed to examine the
phytotoxicity of geraniol, an oxygenated monoterpene compound, in combination with
lemongrass leaf mulch against three common weeds, Eleusine indica (grass), Cyperus distans
(sedge), and Tridax procumbens (broadleaf). Greenhouse experiments were carried out by
treating 4.0 t/ha lemongrass leaf mulch with 7.5% (v/v) geraniol compound. The pretreated
mulch acted synergistically and inhibited the emergence and shoot growth of T. procumbens
completely. However, the pretreated mulch exhibited a moderate inhibitory effect on C. distans
emergence and growth. Geraniol-treated lemongrass leaf mulch acted synergistically and
inhibited the emergence of E. indica by 72%, but it acted antagonistically and caused a 45%
reduction of shoot biomass. The present findings suggest that geraniol-treated lemongrass leaf
mulch has potent herbicidal activity but its phytotoxic effect is species-dependent.