374 Handbook of herbs and spices
3.5 m high, the best results were obtained with 4 ¥ 1.7 m polyethylene tunnels
(Benoit and Ceustermans 1978).
Trials with leeks (cv. Santina) showed that a white polyethylene soil mulch resulted
in higher yields than a black one, but did not result in any improvement compared
with bare ground. Bolting in spring curtails the period of marketability of over-
wintered leek crops. There is then a gap of about three months before spring planted
crops reach marketable size. Early harvests can be advanced by using transparent
crop covers. Trials have shown that mulches of polyethylene film with 500 ¥ 1 cm
diameter perforations/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 seven
weeks after transplanting. In European 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 crops.
22.3.11 Blanching
Blanching is done by covering the plants to a certain height with soil to improve the
quality of the crop. For this purpose, plants are sunk up to their centre leaves in
trenches or pits that are heavily manured to earth up soil as they grow. Care should
be taken not to earth up soil early when the plants are young.
22.3.12 Weed control
There are no registered chemicals for weed control. Alternatives that can be useful
are stale-seedbed technique pre-planting, selecting fields with a low weed population
(crop rotation), and using row spacing that can be easily cultivated. If the size of the
crop warrants, special row crop tillage equipment is a good acquisition (Randy Baker,
1991). In two-year experiments carried out in Bulgaria, the best results in leek seedling
production were obtained with Ramrod at 7 kg/ha, applied pre-emergence, and Afalon
at 1 kg/ha sprayed post-emergence at the 2–3 leaf stage. Afalon alone was sufficient
to control annual dicotyledonous weeds. Prometryne at 3 kg/ha applied post-
transplanting, destroyed both dicotyledonous and monocotyledonous weeds. Satisfactory
results were obtained with Afalon at 2 kg/ha plus Butisan at 10 l/ha, the former being
effective mainly against the dicotyledons and monocotyledons. All these herbicides
were well tolerated by the leeks (Velev and Ivanov, 1973).
22.3.13 Intercropping
Baumann et al. (2000) reported that intercropping of leek and celery in a row-by-row
replacement design considerably shortened the critical period for weed control in the
intercrop, compared with the leek pure stand. The relative soil cover of weeds that
emerged at the end of the critical period was reduced by 41% in the intercrop. In
another experiment, the biomass of Senecio vulgaris, which was planted 20 days
after crop establishment, was reduced by 58% in the intercrop and the number of
seedlings which emerged as offspring was reduced by 98%, all reductions compared
with the pure stand of leek. The relative yield total of the intercrop exceeded that of