Agriculture Reference
In-Depth Information
requirements for hand-weeding varying from 100 to 600 h/ha have been
recorded for weed control in drilled onion and leek crops, depending on the
weed density present (Melander
et al.
, 2005). The requirement for 150-200
h/ha for mechanical and hand-weeding for organic bulb onions in the UK
accounts for 15-30% of variable production costs (DEFRA, 2002), whereas
pesticide applications as a whole represent about 15% of the variable costs of
conventional, drilled bulb onions (Nix, 2006).
Because of the high cost of labour for hand-weeding, even if it is available, it
is only economically feasible to grow drilled onion or leeks organically where
weed numbers are low. A seedbed prepared several weeks before sowing allows
the surface flush of weeds brought to the surface by prior cultivations to emerge
so that they can be destroyed by flame-weeding or harrowing the top 1-2 cm
before sowing the crop - this is termed a 'stale seedbed'. Stale seedbeds require
crop drilling to be delayed, making them problematical when early spring sowing
is necessary for bulb onion production. After emergence, weeds between the crop
rows can be destroyed by steerage hoes, flaming or brush-weeders, but intra-row
weeds require hand-weeding. In experiments in Denmark, a combination of stale
seedbeds and inter-row weeding lowered the time requirement for hand-weeding
of drilled leeks to about 50 h compared with 350-400 h for untreated plots
(Melander and Rasmussen, 2001). The time needed for hand-weeding was
linearly related to the number of intra-row weeds (see Fig. 5.6).
Intra-row weeds in leeks can be partially controlled by smothering, using
machines that brush or ridge soil into the crop row; this also aids in blanching
the leek pseudostems. Weed control is easier in bulb onions grown from multi-
seeded modules (see Chapter 6), since these can be planted at a spacing that
allows long-handled hoeing within the row. Weed control is generally simpler in
crops grown from sets or transplants than in drilled crops because large, more
robust plants are more quickly established that can withstand some damage
from weed control machinery, and their shorter growth period leaves less time
for weeds to get established. In temperate regions weed emergence normally
peaks in the spring, after which fewer weeds appear. If the weeds from the spring
flush of emergence are destroyed, those germinating later have less effect on
onion bulb yield. This is another advantage of transplants as, since they can be
planted later than a direct-sown crop, the spring flush of weeds can be destroyed
before transplanting. Leeks also are commonly grown from spring and summer
transplants, and therefore miss the worst time of year for weed emergence.
In order to achieve conditions of low weed pressure for vegetable allium
production it is important to integrate appropriate weed management into the
wider context of the cropping system (Melander
et al.
, 2005). Diversified crop
sequences, where winter and summer crops alternate and where both spring-
and autumn-sown crops are grown, can prevent the build-up of a competitive
weed community adapted to a particular rotation or mode of cropping. One
promising cropping system is to alternate spring-sown barley with a vegetable
row crop (Melander
et al.
, 2005). The highly competitive barley crop is grown
