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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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