Quantitative analysis of factors affecting bulb yield in terms of dry matter production across different planting dates and cultivars in spring-sown onions

•The yield of fresh and dry bulbs was lesser on later planting dates than earlier.•The yield was the greatest in the later maturing type.•Bulb yield is correlated with dry matter yield and cumulative light interception.•This was due to longer growth period, larger total leaf number and leaf area index.•Choice of cultivar and planting date are vital to reach production potential. Onion (Allium cepa. L.) is one of the most widely produced and consumed vegetable crops in the world. Bulb size is an important factor in onion production with optimal bulb development being the priority objective. In this study, we investigated the effects of three transplanting times, from mid-April to mid-May, on the growth and bulb yield of three commercially grown onion cultivars: ‘Turbo,’ ‘Momiji No. 3,’ and ‘Gaia,’ using a split-plot experimental design in which the planting date was treated as the main factor and the cultivar as the split factor over two years in spring-sown onions in Japan. In addition, a quantitative analysis of the factors influencing onion yield in terms of dry matter production was carried out, rather than focusing solely on the effects of daylength and temperature. Later planting dates resulted in lesser yield of fresh and dry bulbs than earlier dates. In terms of the same planting date, the yield was the largest in the later maturing type, ‘Gaia’, and the smallest in the earlier maturing type, ‘Turbo’. The yield of fresh and dry bulbs strongly correlated with the total dry matter yield, and the total dry weight was significantly influenced by the cumulative light interception, regardless of the cultivar or planting date. Cumulative light interception is, therefore, considered to strongly influence bulb yield. The earlier planting dates and later maturity type cultivars resulted in larger cumulative light interception and higher bulb yield owing to longer growth period, and greater total leaf number and leaf area index (LAI). The total leaf number and maximum LAI significantly correlated with the accumulated temperature from transplanting to the onset of bulbing, when leaf initiation was terminated across all planting dates and cultivars. Therefore, it was crucial to maximize the duration from planting to the initiation of bulb development to increase the bulb weight and yield with early planting using long-day type cultivars without issues such as the bacterial rot of onions due to high temperature and humidity at harvest in the case of spring-sown o