Vernalization is an important step for floral initiation in onion (Allium cepa), but our understanding of the minimum vernalization time for long-day storage onions to gain floral competence is limited. A series of time course experiments were conducted over 4 years to determine the effects of vernalization time on sprouting, scape emergence, and flowering in ‘Cortland’, ‘Sherman’, and CUDH2107. We found an endodormancy period is present in the bulbs of these cultivars that lasts for 8 to 10 weeks. In addition, these three accessions achieve optimum uniform scape emergence after chilling for 14 weeks at 10 °C. On average, when bulbs were chilled for 14 weeks, it took 96.8 ± 15.6 days from planting to flowering in all accessions evaluated. As storage duration under vernalizing temperatures increases, the time to sprouting, scape emergence, and flowering decrease. Furthermore, the variance in time from sprouting to scape emergence was greatly reduced between 10 and 12 weeks of storage for ‘Cortland’ and CUDH2107, and between 12 and 14 weeks for ‘Sherman’. After 10 weeks of chilling, each additional week of storage resulted in an average decrease in time to flowering of 4.1%. We also observed large percentages of bulbs flowering without receiving any vernalization. This observation supports our finding that the relationship between vernalization and flowering in long-day storage onion is facultative rather than obligate. These findings help define the relationship between dormancy, vernalization, and flowering in long-day storage onion.

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Long-day storage onion bulbs (Allium cepa) undergo a period of endodormancy that begins before harvest and lasts for several weeks, depending on the genotype. Onion seed production relies on cold treatment to break endodormancy and vernalize the bulbs. When bulbs are planted shortly after harvest, endodormancy results in delayed growth and, in turn, slower flowering and seed production. Through this work we sought to explore the use of hydrogen peroxide as a treatment to break dormancy in onion bulbs. Endodormant bulbs of two long-day cultivars, Cortland and Sherman, were treated with hydrogen peroxide solutions at various concentrations in a series of experiments over a 3-year period and were monitored for root and leaf (sprout) development. We found a 2- to 4-hour exogenous treatment of 20% (weight by volume) hydrogen peroxide to be highly effective at initiating uniform root growth in endodormant bulbs. When compared with a purified water control, the 20% treatment resulted in a 61.3% average reduction in the time to rooting in 2016. We also observed improved uniformity in rooting time between ‘Cortland’ and ‘Sherman’ in all 3 years of this work. We propose this novel method as a tool for breeders, researchers, and seed producers seeking rapid, uniform endodormancy release in onion bulbs to hasten seed production.

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