Weed control methods commonly used for organic crops, such as hand-weeding and intensive tillage, are not suitable for erosion-prone fields and dry climates in many cotton-growing regions in the U.S. Organic cotton production was partially simulated in the Rio Grande Valley of South Texas. Nitrogen-fixing black oat and hairy vetch winter cover crops were planted in plots in a 1-ha field in Fall 2004. Additional plots were left fallow and fertilized non-organically. Plots were sampled using transects of 1-m² squares. In winter sampling, pigweeds (Palmer amaranth, Amaranthus palmeri) and purslane (Portulaca oleracea) were observed in 30% of plots. Purslane ground coverage was 3 to 6-fold higher in cover crop plots than on fallow plots, and pigweed showed the same trend. Cover crops were tilled and non-Bt cotton was planted in March 2005. Six weeks after planting, the proportion of total plant cover consisting of weeds was 2-fold greater in plots that had contained winter cover crops than in fallow plots. Winter cover cropping did not affect cotton height. Weeds were controlled using tillage. In no-till cotton plots in separate fields, weedy cover was 3-fold greater than in tilled plots in the organic production field, but cotton height was not affected by young pigweed, purslane and 10 other weed species. To simulate weed competition and organic weed control under no-till conditions, weeds and cotton were planted together in pots in a greenhouse and exposed to foliar applications of vinegar (9% acetic acid content at full strength). At 1.5 weeks post-germination, pigweed, purslane, and sunflower (Helianthus annuus) showed 100% mortality within 72 of application of full-strength and half-strength solution. Vinegar killed 77% and 28% of cotton seedlings at these doses, respectively. Vinegar diluted 10-fold killed 50% or more of seedlings of each of the three weed species, while causing leaf damage but no mortality in cotton. Damaged plants attained heights and leaf counts similar to controls two weeks after being sprayed. Older weeds (sprayed 3.5 and 5.5 weeks after germination) were substantially more resistant to vinegar, with mortality (12% pigweed, 68% purslane, 90% sunflower) occurring only at full-strength. Cotton showed no mortality and recovered from damage caused by full-strength vinegar in a manner similar to that of 1.5 week-old seedlings exposed to the 10-fold dilution. In 1-m² field plots, vinegar caused 30% mortality in young 10 to 15-leaf vegetative pigweed, but killed only 8% of mature plants with large flowering spikes, while causing 20% mortality in 2 to 4-leaf cotton seedlings. Winter cover crops may shelter weeds and contribute to weed development in organic/sustainable cotton production. Vinegar is an effective organic weed control tool against pigweed, purslane and sunflower in edge areas of tilled fields or throughout no-till fields.