|
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 |
 |
