The production of flowers by onions ("bolting") reduces bulb yields. Consequently, control of flowering is of interest to onion growers who produce marketable bulbs as well as to those producing seed.
In the research described here, three sizes of sets (12.5mm, 17.5mm and 22.5mm diameter) of two cultivars ('Hygro' and 'Delta') were planted in the field every 15 days from 5 December. Onion sets planted on 5 December gave high levels of bolting accompanied by low bulb yields, especially when large sets were planted.
Small sets produced very little bolting even when planted early. The yield of seed per seed-head rose with increasing size of set at planting, especially when planted early.
By contrast, the highest bulb yield came from planting small sets of 'Hygro' and this was increased further by delaying planting. With 'Delta', however, the highest bulb yields came from medium-sized sets.
Overall, medium-sized sets planted as late as 5 March should produce high bulb yields under UK conditions.
Onion seed could also be produced commercially in the UK if large sets were planted as early as 5 December.
Unfortunately, however, fungicides would probably have to be employed to combat fungal diseases of the seed stalks in summer.
Effect of Set Size and Planting Time on the Incidence of Bolting, Bulbing and Seed Yield in Two Onion Cultivars by Khokhar (2008). Journal of Horticultural Science & Biotechnology 83 (4): 481-487. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.