Critical Period of Weed Competition in Onion (Allium cepa L.) in Jordan

The effect of different weed competition and weed-free periods on growth and yield of onion and weeds was investigated in two field experiments carried out at the University of Jordan Campus, Al- Jubeiha and at Jordan University Research Station located at the central Jordan Valley, during 1996/1997 growing season. Results showed that the longer the period that onion kept weed free or the earlier those weeds were removed from the crop after transplanting, the higher the onion growth and bulb yield obtained. The highest yield was obtained from the weed-free plots for the entire growing season. While no yield was obtained from weed-infested crop for the whole growing season at the Jordan Valley, bulb yield reduction was 86% in the same treatment at the University Campus experiment. Bulb yield was not significantly changed at 14 to 49 days of weed-infested crop at both locations. While none of the weed-free periods gave similar yield to that of the weed-free control in the Jordan Valley, weed-clean crop for only three weeks after transplanting at the University Campus location significantly increased bulb yield over the weed-infested control. In the Jordan Valley, weed competition for 14 days after transplanting significantly reduced bulb dry yield compared with the weed-free control. Results showed that weed growth and development was much higher than that of onion plants especially under irrigation. Based on the dry bulb yield obtained, critical period of weed competition was at 50 days after onion transplanting at both locations. This however, depends on the competing weed species, their densities and growing of onion under irrigation or rainfed conditions