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WHY DO ONIONS BOLT?

Grower Solutions Magazine
Lefroy Valley 

April 2002

Onions and other commercially cultivated alliums are biennial plants, which means that it usually takes them two growing seasons to go from seed to seed. Normally the onion seed is sown in year one, during which time the plant forms a bulb. At the end of year one, the plant goes into a dormant phase, losing most of its leaf and root mass. In year two, when regrowth recommences, shoots and roots redevelop and eventually produce the flower stalk and finally the seed. Cold temperature and day length are the two critical factors that initiate flower stalk development. In the case of fresh market onion growers, the process is stopped at the end of year one. The bulbs are lifted from the field, cured and sold over the subsequent months, before the plant commences regrowth. As with other aspects of mother nature, things do not always go exactly to plan.

 

Given a certain set of environmental conditions, onions can be tricked into believing they have gone through two growing cycles during their first year. Instead of finishing with a well-cured bulb, ready for the market, a seed stalk can develop prematurely, causing onions to be unmarketable. During the summer of 2001/02, many areas in Southern Australia experienced cooler than average temperatures. In many onion crops, subsequently there was a higher than average bolting percentage. In Werribee, nights in November were 4ºC cooler than in 2000 and in December, the days were 5ºC cooler, massive temperature drops from the year before. Onions sown in August would have grown to a sufficient size by November and December to believe that the next winter was already upon them and that it was time to set flower stalks. The reason why later sown onions have a reduced bolting percentage or no bolting at all is that while the weather was cold in November and December, the plants were not at the critical size in their development to accept the cold as a signal to begin flower stalk initiation.

Limiting bolting: While it is impossible tocontrol the weather, sowing at the correct time for the variety in question is the most important factor to limit premature bolting. Over-fertilising can also contribute to bolting - if onions are too vigorous, too early in their development, bolting can result.

by Bruno Tigani

Grower


 
 
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