372 Handbook of herbs and spices
countries, leek plants are transplanted after a nursery period of about 12 weeks. This
method permits the rejection of weak-growing plants. However, in the UK direct
drilling is more frequent.
Leeks are always started from seed and need a fairly long growing season to reach
a marketable size. In cool climates, leeks are usually planted as early in the spring as
possible, and are frequently transplanted from hotbeds or cold frames into the open
as soon as the soil becomes warm. Such early plantings are often ready by late
summer; successively later plantings may be harvested into the autumn, winter and
early spring. These later plantings are direct-seeded or grown from transplants. When
the false stems are to be blanched, the leeks are transplanted into trenches, where the
soil can be gradually banked against them. Leeks are sufficiently cold-resistant that
even in cool climates harvesting can continue throughout the winter, when many
other greens are off the market. Unlike onions or garlic, leeks have no definite
maturity dates and, as markets usually accept a wide range of sheath sizes, single
plantings may be harvested over a considerable period (Wurr et al., 1999; Jones and
Mann, 1963).
22.3.5 Seedling raising
Seed germination depends upon temperature, with 18–22 ∞C being the optimum. In
India, seeds are sown during August to October in the nursery beds, and seedlings are
ready to plant when they attain a height of 15 cm. About 5–7 kg of seeds are sufficient
to raise seedlings for planting one hectare of land. A method of magnetic separation
of leek seeds of low germination from commercial seed lots was described by Krishnan
and Barlage (1986).
22.3.6 Seed quality and priming
Adequate seed cleaning and grading and following this, the selection of large and
uniform seedlings at transplanting for improved crop uniformity in leek is very
important. A further improvement in germination performance and field uniformity
can be achieved by seed priming, in which controlled hydration of seeds permits
pregermination metabolic events to take place without radicle emergence. The process
engineering of leek seeds was developed, comprising osmotic priming, washing,
fluidized-bed drying (heated air is blown up from underneath through a layer of seeds
to promote rapid drying while they are floating in the air) and film coating; this has
been proven feasible (Bujalski et al., 1991). The superiority of the processed seeds is
usually reflected in improved germination, rapid and uniform emergence in the field
and improved early plant growth compared with untreated seed.
22.3.7 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 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.
However, to produce large leeks, densities of 20–25 plants m–2 are used. Leeks grown
at a high plant density appear more elongated than those grown at low density, i.e.,