The interference caused by weeds is one of the factors that can affect the productivity of the cassava culture. This study aimed at evaluating the interference of different periods of control and coexistence of weeds with the cassava culture, White Starch cultivar, and also observes its effect on the growing of the plant, as well as in its production. The experiment was performed at Unioeste- Marechal Cândido Rondon, Paraná, in Eutrophic Oxisol. The culture was conducted during a vegetative cycle of 12 months, the planting occurred on October 3, 2001 and the harvest on October 4, 2002. The experiment was installed as a completely randomized block design with a factorial scheme 2x10, made up by growing periods of coexistence and control of the weed interference. After the harvest, the stand, the number of roots per plant, the amount of starch of the roots and the weight of the root production were determined. Results showed that the losses in the productivity of roots in the culture caused by the competition of weeds, in extreme situations, were quite significative, almost 100%. The period before the interference (PBI) was 60 days and the total period of interference prevention (TPIP) was 90 days, which indicates a critical period of interference (CPI) from 60 to 90 days after the planting.
