WEED MANAGEMENT IN ORGANIC ONION PRODUCTION: OPTIMIZING CULTIVATION TECHNIQUE AND MECHANICAL WEED CONTROL TO REDUCE HAND LABOUR

The challenges of growing organic onions (Allium cepa) are the strong weed competition (hand weeding as the main cost factor) and quality problems with the traditional cultivation system of planted onion sets (e.g., due to poor skin firmness and propagation material-borne diseases). Although weed control is easier in planted onion sets than with direct sowing; the quality in terms of firmness of skin and storability is lower. In our experiments, the advantages of planted seedlings were studied as a possible alternative. With the aim to reduce production costs and increase quality, three cultivation methods (direct sowing, onion sets, and seedlings) and two weed control methods (flame weeding and a commercial floating row cover) were compared in field trials over two years. For each method, the following factors were assessed: yield, time for hand weeding, weed control and the cultivation costs. The onions sown directly required between 272 and 756 h per ha of hand weeding. The method of planted seedlings reduced hand weeding to 65 and 79 h per ha, and onion sets even to 24 and 61 h per ha. Flame weeding was effective in set onions at a growth stage of 2 to 3 true leafs, where it reduced the labour hours per hectare by 30% compared to hand weeding. It was even more effective in directly-sown onions in crook stage (42-53% h/ha less). The floating row cover before and after direct sowing stimulated the weed growth before flame weeding but did not reduce the time for hand weeding afterwards. Flame weeding had only a small negative effect on the yield (a decrease of 3 to 6%, compared with hand weeding). The cultivation costs for the alternative method of planted seedlings (9 seedlings per m², with 5-7 seeds per pot) was as cost efficient as onion sets and directly-sown onions (sub treatment with flame weeding) and proved to be an interesting option for organic onion growers.