Effect of set-size and planting time on the incidence of bolting, bulbing, and seed yield in two onion cultivars

Sets of two onion (Allium cepa L.) cultivars ('Hygro' and 'Delta'), graded into three size classes (12.5, 17.5 and 22.5 mm in diameter) were planted at seven 15-d intervals from 5 December 1995 to 5th March 1996. Flowering and seed production were influenced by set-size. Small sets (12.5 mm in diameter) showed little or no bolting, while the bolting percentage increased with increasing set-size, following a linear relationship. The seed yield per umbel increased with increasing set-size from 12.5 mm to 22.5 mm diameter. Following initiation, inflorescence development (i.e., emergence and floret opening) occurred earlier in plants grown from larger onion sets than from smaller sets. Bolting percentage tended to decrease with late planting of sets (5 March). Maximum seed yield per umbel was observed from the earliest planting (5 December). Delaying planting until 5 March had an enhanced effect on bulb yield. Large sets, being prone to bolting, produced smaller bulb yields than medium or small onion sets. In the earliest planting (5 December), bulbs matured earlier, while the time to bulb maturity was delayed in later plantings.