Timing the removal of weeds from drilled salad onions to prevent crop losses

Hand-weeding experiments were made over a 5 year period to determine the optimum time for weed removal in salad onions sown at different times of the year. Weed that remained from sowing until harvest, reduced onion fresh weight by up to 96% compared with the weed-free controls. In experiments made in the first 2 years, yield data from onion plots left weedy or kept weed-free for different periods indicated that a single, carefully timed weeding could be sufficient to prevent crop losses. However, the optimum weeding time varied from 21 to 56 days after 50% crop emergence. In subsequent experiments with salad onions sown on four different dates in each of 3 years, single and multiple weedings made within the defined optimum period did not consistently prevent crop losses.