The changes in herbage yield, essential oil yield and chemical composition of the essential oil of rose-scented geranium (Pelargonium sp.) cv. Bourbon were investigated in the presence and absence of weeds during dierent periods of crop growth. The crop, which was kept weed-free for the ®rst 30 or 60 days after planting, or which was not kept weed-free for the ®rst 90 or 120 days after planting, suered greater yield losses (herbage yield 40.6±58.3%, oil yield 46.9±65.0%) than the crop which was kept weed-free for the ®rst 90 or 120 days after planting or for the last 90 or 120 days before harvesting. Therefore, the rose-scented geranium crop should be kept weed-free, either during the ®rst 90 days after planting or during the last 90 days before harvesting, to obtain high herbage and essential oil yields. The presence of weeds during dierent periods of crop growth decreased the concentrations of linalol (20.8±33.3%), isomenthone (4.8±12.0%), citronellol (1.8±14.9%) and citronellyl formate (4.3±20.3%) and increased the contents of geraniol (8.8±22.7%) and geranyl formate (10.0±50.0%) in the essential oil. The percentages of rose oxide and 10-epi-geudesmol in the essential oil were not aected by either the presence or absence of weeds.