Agriculture Reference
In-Depth Information
latitude, the relevant range of sowing dates is later: (i) early September, giving a
high level of bolting and low yields; (ii) mid- to late September, giving little
bolting and maximum yields; with (iii) mid-October giving no bolting but lower
yields (Corgan and Kedar, 1990).
The optimum sowing date varies somewhat from year to year. In the UK, if
the late summer and autumn are warm, growth may be rapid and the plants
may reach the critical size needed to induce flowers (see Chapter 4) by late
autumn from a mid-August sowing, as in 1973 (see Fig. 6.9). Consequently,
such plants will be fully vernalized during the winter and a high percentage
will bolt. In a season with a cool autumn, like 1974 (see Fig. 6.9), this will not
occur, since plants sown in mid-August will not reach the critical size needed
for vernalization to begin until the spring.
Because it is critical that autumn-sown crops reach a size sufficient to
withstand the winter, it is essential to provide seedbed irrigation as well as the
correct sowing date. Seed sown into dry soil will not germinate until moistened
and, without irrigation, the start of growth will therefore depend on the
vagaries of rainfall. Figure 6.10 shows various stages in the growth of a typical
overwintered bulb onion crop grown in four-row beds.
The dates for spring sowings are limited, for different reasons. There is no
point in sowing until soil temperatures have warmed sufficiently to permit
germination and emergence. However, if sowing is left too late, the crop will not
reach sufficient size for bulbing to be completed before temperatures and day-
lengths decline in late summer sufficiently to cause bulbing to revert to leaf
blade growth, thereby producing 'thick-necked' plants (see Fig. 6.11).
In The Netherlands, experience has shown there is little point in sowing
onions before early April; in the UK, sowings are best done in late March. Once
onion seeds have germinated they begin to lose viability if exposed to longer
than 2 days at temperatures < -2°C (Gray and Steckel, 1983). The probability of
such damaging temperatures occurring declines as the season progresses and,
at Wellesbourne, UK, it is only three per 100 sowings made in April, whereas in
February it is 13 per 100. Even if seeds are not killed directly by low tempera-
tures, the lower the temperature the slower the emergence rate and the longer
time the seedbed has to deteriorate before seedlings emerge. In particular, a hard
'cap' or crust may develop, which is impenetrable to onion seedlings. Therefore
both direct and indirect effects of low temperature can reduce the percentage
emergence and make sowing too early in the spring a mistake.
Method of planting: direct sowing, transplanting or sets
Direct sowing
Sowing seeds directly into the soil where the crop is to be grown is potentially the
most economical method of raising an onion crop, particularly where the
availability of labour for transplanting is limited, its cost is high or where the
