Agriculture Reference
In-Depth Information
Plastic mulches for early cropping
Bolting in spring curtails the period of marketability of overwintered leek crops.
In the UK there is then a gap of about 3 months before spring-planted crops
reach marketable size. Storage of harvested leeks can help maintain supplies
during this gap (see Chapter 7), but also early harvests can be advanced by
using transparent crop covers. Trials show that mulches of polyethylene film
with 500 perforations of 1 cm diameter/m
2
, or with non-woven polypropylene
fabrics, can advance harvests. The films are laid over the crop at transplanting,
which is usually in late March or early April, following a January sowing in
glasshouses. As the crop grows these light films are raised by the foliage, and
'float' on top of the canopy of leaves. Rainfall penetrates the perforations. Mean
temperatures are raised by 1-2°C under these mulches, resulting in faster
growth. The mulches are removed about 7 weeks after transplanting. This is
particularly important if the weather becomes hot.
For early production it is important to use quick-growing, long-pseudo-
stemmed cultivars, which rapidly reach marketable length. Densities of about
30 plants/m
2
are optimal, and herbicide and base fertilizer treatments are
similar to those for open-grown crops. Two N top-dressings using 50-100
kg/ha N should be applied to maintain an unrestricted growth. Leeks are
slightly elongated by growing under such mulches, and bolting can be slightly
delayed. In UK conditions marketable yields of 14 t/ha were achieved by late
June, and 31-40 t/ha by late July, with an advancement of 7-9 days over the
unprotected crop. Imports from southern Europe also help fill the supply gap in
May and June in northern Europe.
Leek seed production
The quality of leek seed - as shown by its germinability, rate of germination and
emergence percentage in field conditions - is highly variable. In a survey of 23
commercial seedlots, Gray and Steckel (1986) reported that 50% of them had
below 80% germination and several were as low as 50-60%. Without high-
quality seed, it is impossible to establish the required plant population in
directly drilled crops. Also, the high costs of modular transplant production
make it important that empty modules, caused by non-viable seeds, are few.
These considerations have motivated research into the factors influencing seed
quality in leeks and into methods for producing high-quality seed.
Traditional production methods
Traditionally, leek seed is produced from stecklings (shoots with most of the
foliage trimmed off ) planted in the autumn at 5-17 plants/m
2
, which flower
outside in the following year. Seed heads are cut in the autumn and dried in
stores, ensuring that aeration is sufficient to prevent rotting. After drying, the
