Agriculture Reference
In-Depth Information
Direct sowing
The crop can be established more cheaply than transplanting by direct sowing
into beds in the spring. The viability and vigour of leek seed is highly variable,
and high-quality seed is important for direct sowing (see Chapter 4). The
majority of leek seed sold in the UK is now primed using the 'drum' method (see
'Seed Priming', above). Besides being cheaper, direct sowing tends to result in
crops with less dirt in the leaf axils and with fewer bent pseudostems, but the
length of blanched sheath tends to be shorter than with transplanted crops,
and direct-sown crops are more prone to bulbiness (Williams, 1973).
Plant density
The optimum plant density for leeks depends on the size grade required at
harvest, the date of planting or sowing - which influences the potential yield
(see Figs 4.50 and 4.51) - and the intended harvest date. Mean width and
length increase as the crop grows, and increase as plant density decreases. For
leeks of 20 mm minimum diameter and 150 mm minimum length, a planting
density of about 30 plants/m
2
is optimal for early production, e.g. for late-June
harvesting following a late March or early April transplanting of mid-January-
sown modules.
Trials in the UK with leeks sown directly into beds at the end of March,
which were thinned to densities of 20, 42, 84 and 126/m
2
, showed that
percentage survival decreased with time and as density increased. Mortality
was almost 50% by mid-January from the two highest-planting densities, but
only 10% at the lowest density. Yields reached a maximum in about mid-
November, and the highest yields of marketable leeks (> 12.5 mm diameter)
occurred with 50-60 plants/m
2
at harvest. At this density a yield of 44 t/ha of
trimmed leeks was produced from a total crop fresh yield of 107 t/ha. Yields
were lower from both a mid-September and a mid-January harvest, with
marketable yields at 29 and 34 t/ha, respectively. Optimum densities for the
early and the late harvest, at about 40 plants/m
2
, were slightly lower than for
the November harvest, when the crop was at its peak yield (Williams, 1973).
The higher the plant density, the higher the proportion of undersized,
unmarketable leeks but, up to an optimum density, this is outweighed by the
larger total number of plants from high densities. However, to produce large
leeks, densities of 15-25 plants/m
2
are used. Leeks grown at a high plant
density appear more elongated than those grown at low density, i.e. the
pseudostems have a higher length:breadth ratio. Also, the degree of blanching
increases with density, especially for plants from the centre of beds.
