Transplanting Management for Improving Marketable Root Ratio in Sweetpotato Cultivars with Contrasting Sink-Forming Characteristics

Improving the proportion of marketable storage roots (≥50 g) is increasingly important in sweetpotato (Ipomoea batatas (L.) Lam.); yet, few studies have integrated cultivar morphology with transplanting management to explain variation in the marketable root ratio. In 2023–2024, we combined cultivation period (120 vs. 140 days after transplanting), transplanting method (horizontal vs. vertical) and seedling node number (6- vs. 8-node cuttings) in two cultivars differing in storage root-forming characteristics, ‘Sodammi’ and ‘Hopungmi’. Extending cultivation raised the marketable root weight ratio from 65.1% to 75.9% in ‘Sodammi’ and from 86.9% to 92.6% in ‘Hopungmi’, and marketable yield by 40% and 39%, respectively. Vertical planting reduced storage root number in both cultivars, although its effect in ‘Sodammi’ was relatively limited, and 6-node cuttings reduced it in ‘Sodammi’. In ‘Sodammi’, the ratio ranged from 57.7% (120 days, horizontal, 8-node) to 80.5% (140 days, vertical, 6-node). Storage root number was negatively correlated with the marketable root weight ratio in ‘Sodammi’ (r = −0.68) but not in ‘Hopungmi’ (r = −0.06). Limiting buried nodes appears most useful in cultivars forming many small storage roots, and transplanting practices should match each cultivar’s storage root-forming characteristics.