Amaranthus lividus is the most frequently reported troublesome weed in the production of Capsicum annuum in some regions because it is an aggressive invader, difficult to control, and reduces yield significantly. The effects of A. lividus on the growth of C. annuum ‘Baklouti’ were evaluated under greenhouse conditions. Three experiments were conducted to determine the effect of A. lividus on the biomass accumulation of C. annuum seedlings for 40 days. In an additive experiment, to one C. annuum seedling and 1, 2, 3 or 4 A. lividus seedlings were grown per pot. The second experiment was established to assess C. annuum intracompetition using from one to six plants per pot as the density. In a replacement experiment, C. annuum and A. lividus seedlings were transplanted into pots with different proportions of both plants (1/5, 2/4, 3/3, 4/2, 5/1). Competition by A. lividus reduced C. annuum plant dry weight by as much as 93%. However, C. annuum had little effect on A. lividus, reducing dry weight by 31.3% at a 5:1 ratio of C. annuum: A. lividus. Relative yield analysis between C. annuum and A. lividus demonstrated the competitive advantage of C. annuum over A. lividus. The relative crowding coefficient of both plants changed significantly in the presence of the other plant, at any ratio. The aggressivity of C. annuum was higher at its lower proportion and C. annuum was more aggressive towards itself than towards A. lividus when its density increased.
