Agriculture Reference
In-Depth Information
-11°C, with younger, smaller seedlings being killed at the higher end of this range.
Overwinter growth is typical of regions famous for their 'early season' production
of bulb onions - for example, the Rio Grande valley of Texas, USA.
At higher latitudes, longer, colder winters make autumn sowing impracti-
cable except in maritime areas, and the crop must be grown from spring sowing or
planting. The higher the latitude the shorter the growing season and, ultimately,
there may be insufficient length of season for the small-seeded, rather slow-
growing onion crop to make an adequate leaf area and to bulb before the end of the
growing season. In such regions bulb onions may be grown from transplants
sown in glasshouses in the late winter, thereby extending the growing season, or
they may be grown from sets produced in the previous year. By using a set with a
dry weight of about 0.2 g, rather than a seed with a dry weight of 0.003 g, the size
of the plant at emergence is correspondingly greater, and the time required after
emergence to make sufficient leaf area to produce a large bulb is shortened.
The timing of the late summer or autumn sowing can be critical. In the UK,
sowings in early August will produce many bolters, whereas sowing after early
September produces plants that are too small to survive the winter (see Fig. 6.9).
The same principle can be seen in New Mexico, USA except that, at this lower
Fig. 6.9.
Percentages of plants bolting and bulb yields of onion cv. 'Senshyu Semi-
globe Yellow' grown from a succession of August sowings in a year with a warm
autumn (1973) and in a year with a cool autumn (1974) at Wellesbourne, UK.
