Horticultural Science and Technology 원예과학기술지 ISSN:1226-8763(Print) 2465-8588(Online)

While onion production is increasingly adopting mechanical transplanting, the problem of missing plants frequently occurs, resulting in the phenomenon of stand reduction, potentially affecting the yield and bulb size distribution. This study evaluated the effects of missing plant patterns and stand reduction on bulb growth and marketable yield of onion under mechanical transplanting conditions. Field experiments were conducted during the 2017/2018 growing season in southeastern Korea. Missing plant patterns were classified based on the number of missing plants in adjacent positions. Stand reduction levels ranged from approximately 9% to 38%. Bulb weight did not differ significantly among missing plant pattern treatments (p > 0.05), although numerical variations were observed among the treatments. Stand reduction significantly affected the bulb size distribution (p ≤ 0.05), primarily by decreasing the proportion of medium-sized bulbs while maintaining a high bulb yield. The marketable bulb yield showed a significant negative relationship with stand reduction (p < 0.05), decreasing from 94.6 Mg·ha^-1 in the control to 79.5 Mg·ha^-1 at 35-38% stand reduction. At moderate stand reduction levels (15-28%), the yield was maintained at 88–93% of the control and did not differ significantly (p > 0.05), indicating partial compensation through an increase in the bulb size. However, stand reduction exceeding 30% resulted in significant yield losses. These results indicate that moderate stand reduction under mechanical transplanting did not significantly reduce the marketable yield, despite reductions in the plant population. This finding suggests that onion production may tolerate a certain degree of stand reduction under mechanical transplanting conditions. A stand reduction level of up to approximately 20-28% may be acceptable without a substantial yield penalty, whereas maintaining stand reduction below 30% is critical for stable production.