What causes an Onion to bulb?
Bulbing in onions primarily controlled by photoperiod – day length – as the days become longer, plants begin to bulb – some cultivars required shorter days to bulb than others. Hours needed to begin bulbing used to classify cultivars: • 8-12 hours – short-day • 13-14 hours – intermediate-day • 15+ hours – long-day Southern states grow short-day onions. As the northern latitude increases, the day length requirement for bulbing increases. During the summer months, northern latitudes will have longer day-lengths than southern latitudes. Need to prevent bulbing in order to get large plant otherwise small plant, small bulb. Long-day cultivars at northern latitudes grow longer before bulbing than short-day cultivars. Conversely, Long-day cultivars grow in southern latitudes never reach critical day-length for bulbing so grows a large plant with out bulbing. (For a better understanding check-out The Onion’s Story.
