Weed Management in Horticultural Crops

MIXED CROPPING

Growing two or more crops at the same time and adjacent to one another is called mixed cropping, or intercropping. The advantages are a better use of space, light and other resources, a physical protection, a favourable thermal balance, better plant defence against some pests and fewer weed problems because the soil is better covered. Sometimes the results are less productive than cultivating just one crop alone. Some examples are:

• In temperate regions:
• lettuce + carrots;
• cole crops + leeks, onion, celery, tomato;
• maize + beans, soya.
• In tropical regions: this technique is very well adapted to the traditional agricultural system:
• maize + beans + squash,
• tomato + pigeon pea,
• sugar cane + onion, tomato.

Preventive measures

It is necessary to avoid the invasion of new species through the use of clean planting material and to prevent seed dispersal on the irrigation water, implements and machines. A written record of the weed situation in the fields is very useful. Another aspect is to impede perennial weed dispersal (or parasitic weeds) through the opportune use of treatments and tillage and the use of drainage tillage to prevent propagation of some species that need high moisture levels. (Phragmites spp., Equisetum spp., Juncus spp.) It is also necessary to scout the field edges to prevent invasions.

Land preparation and tillage

Suitable land preparation depends on a good knowledge of the weed species prevalent in the field. When annual weeds are predominant (Crucifers, Solanum, grass weeds) the objectives are unearthing and fragmentation. This must be achieved through shallow cultivation. If weeds have no dormant seeds (Bromus spp.), deep ploughing to bury the seeds will be advisable. If the seeds produced are dormant, this is not a good practice, because they will be viable again when they return to the soil surface after further cultivation.
When perennial weeds are present, adequate tools will depend on the types of rooting. Pivot roots (Rumex spp.) or bourgeon roots (Cirsium spp.) require fragmentation and this can be achieved by using a rotators or cultivator. Fragile rhizomes (Sorghum halepense) require dragging and exposure at the soil surface for their depletion, but flexible rhizomes (Cynodon dactylon) require dragging and removal from the field. This can be done with a cultivator or harrow.
Tubers (Cyperus rotundus) or bulbs (Oxalis spp.) require cutting when rhizomes are present and need to be dug up for exposure to adverse conditions (frost or drought). This can be done with mould board or disk ploughing. Chisel ploughing is useful for draining wet fields and reducing the infestation of deep-rooted hygrophilous perennials (Phragmites, Equisetum, Juncus).

Mulching material

The use of plastic mulching is very popular in many vegetable-growing areas. A non-transparent plastic is used to impede the transmission of photosynthetic radiation through the plastic to the weeds so that the development of weeds is then arrested.
Chemical weed control
The best approach to minimize inputs and to avoid any environmental problems is to apply herbicides in the crop row to a width of 10-30 cm. Many herbicides are effective in the control of perennial weeds. Sometimes a combination of two herbicides having a different weed-control spectrum may be used. Mixtures of different herbicide are possible to achieve better efficacy, but previous trials are necessary. Their foliar activity is enhanced by adding a non-ionic surfactant or adjuvant. The use of any herbicide in vegetables requires previous tests to verify its effectiveness in local conditions and selectivity to available crop cultivars.
In general pendimethalin 3.3 l/ha or Fluchloralin at 2 lit/ha or metolachlor 2 l/ha as pre-emergence herbicide is recommended for most of the vegetable crops, followed by one hand weeding 30 days after transplanting.

Table: Selective herbicides for weed control in vegetable crops:

Herbicide	Dose kg a.i./ha	Treatment moment	Crops
Alachlor	2.4	Pre emergence	Brassica crops, onion
Ethalfluralin	0.8-1.7	Pre Plantation	Tomato, pepper, beans, squash
Linuron	0.50-1.25	Pre emergence	Carrot, artichoke, asparagus, faba bean
Metribuzin	0.10-0.35	Pre/Post emergence	Tomato, carrots, peas
Oxyfluorfen	0.36-0.48	Pre/Post emergence	Onion, garlic, cole crops
Oxyfluorfen	0.24-0.48	Pre Plantation	Tomato, pepper
Pendimethalin	1.32-1.65	Pre Plantation	Artichoke, cole, lettuce, leek, pepper, tomato, onion, green peas
Rimsulfuron	7.5-15(g)	Post emergence	Tomato
Trifluralin	0.59-1.44	Pre-plant incorporated	Beans, carrots, celery, cole crops, artichoke, onion, pepper, tomato

Hand weeding:

Apart from chemical weeding, one hand weeding is done 30 days after transplanting.

Biological control:

Myco-herbicides are a preparation containing pathogenic spores applied as a spray with standard herbicide application equipment.