Agriculture Reference
In-Depth Information
In The Netherlands, the main set-producing country for Europe, seed is
sown in early April at a rate of about 10-12 g/m
2
in rows 15-20 cm apart
using seed drills with coulters (part of the seed drill) that spread the seed into a
band 7-10 cm wide.
Sets have the potential to transmit diseases and pests, and for this reason
The Netherlands government runs an inspection regime to ensure that land
used for set production is free of the stem and bulb eelworm,
Ditylenchus dipsaci
and onion white rot,
Sclerotium cepivorum
(see Chapter 5). The crop is grown
with a low level of N fertilizer - normally just 40 kg/ha or less - to encourage
'hard growth'. At harvest, which is earlier than a normal bulb crop because
the high density promotes early bulb initiation and foliage fall-down, the leaves
are mown off, the crop is undercut, windrowed (air-dried) and then picked up
and transferred to a store for rapid drying in a draught at 20-25°C. Thereafter,
they are ventilated with unheated air until fully dry. Sets can be stored in bulk
to a depth of up to 3 m. The sets of the widely grown, bolting-resistant cvs
'Stutgarter' and 'Sturon' are stored through the winter at 3-4°C.
To ensure that sets from other, normally seed-grown cultivars do not bolt
following planting out, high temperature storage at about 28°C is desirable
from November to March. Large sets, > 20 mm in diameter, may need warm
storage until April to prevent bolting. In general the smaller the set, the less is
the likelihood of bolting. High-temperature storage also increases vigour,
delays maturity and increases yield. Relative humidity of 60-70% is necessary
to prevent excessive moisture loss during high-temperature storage but, in
spite of this, 10-25% weight loss occurs.
A novel technique has been developed for producing 'mini-sets' only 8 mm
in diameter, which utilizes surplus glasshouse capacity following the UK
transplant-raising season. Seeds are sown in early June into cellular trays, as
for transplant production, with five to six seedlings per module and left in the
glasshouse to produce ripe mini-sets by August. After winter storage at 27°C, a
high yield was produced from spring planting. Furthermore, these sets are so
small that they can be sown with an ordinary seed drill adjusted for a large-
seeded crop like beans.
In warm regions like Israel and Pakistan, sets produced from a spring
sowing are replanted in the late summer to produce a quickly maturing bulb
crop for harvesting in December. In Israel, seeds of cv. 'Bet alpha' are sown in
February and harvested as sets in April. The sets are planted in July or early
August and bulbs are harvested in December. During this growing season
photoperiods decline from 14 to about 10 h. Cultivars with a slightly longer
day-length requirement for bulbing than 'Bet alpha' revert to leaf growth and
fail to complete bulbing when grown in this way (Corgan and Kedar, 1990).
The more rapid bulbing of sets compared with seedlings of the same cultivar
(Fig. 4.33) may also be important here.
At high latitudes with a late spring and a short growing season, the rapid
production of a bulb crop is possible by using sets. In Norway, large sets are
used to produce early bulb crops (Vik, 1974).
