Physiology of Flowering
Flowering of onions is of great economical importance during the two growing seasons. When grown for bulbs, bolting plants — those which flower during the first season — are of very poor quality. The flower stalk of the onion, as in all Alliums, is an apical extension of the stem, but without histological differentiation into nodes and intern odes. Following induction and initiation, the differentiated central axis starts protruding and the scape is formed. The flower stalks of most commercially available cultivars reach a final length of 1 to 2 m, but flowering may also occur, though very rarely, within the fleshy scales of the bulb. The buds are grouped in small cymes of five to ten flowers each; the umbel is entirely enveloped by a spathe. This thin sheathing bract splits open when flowering begins. Temperature, day length and a level of nitrogen fertilizer seem to be the important environmental factors affecting flowering of the bulb onion.