This research was carried out to evaluate the effects of weed coexistence periods (0, 14, 28, 42, 56, 70, 84 and 98 days after transplanting) on the productivity of four onion cultivars (Mercedez, Granex 33, Superex and Serrana). The experiment was installed in Jaboticabal - SP - Brazil, from April to October 2000, arranged in a randomized complete block design, with four replications, in a factorial design 4 x 8. The main weed populations at the end of the coexistence periods were Coronopus didymus, Amaranthus hybridus and Cyperus rotundus. The cultivars Mercedez (2.90 kg m-2) and Granex-33 (2.64 kg m-2) were the most productive, under the conditions of weed interference.Weed interference during the first 98 days reduced production weight around 95% and the average weight of bulbs was reduced by 91%. For all the cultivars, weed coexistence the during the first 42 days after transplanting did not reduce the total bulbal weight or the average weight by bulb, being defined as the period before interference (PBI). There was no difference in the PBI extension of the onion cultivars studied.
infesting community; cultivars