WEED MANAGEMENT IN HORTICULTURAL CROPS

Traditional vegetable-growing areas are usually situated adjacent to waterways, flood

plains, river deltas, marsh zones, and, if herbicides are used, their environmental impact and
usage conditions must be taken into account. Another aspect related to the complexity of
herbicide use is its soil persistence that can seriously affect the next crops in the rotation as a
result of soil residues or carryover. Vegetable rotations are very fast and intensive in many
places, and herbicide toxicity can affect the next crop if the cycle of the previous crop is short
enough.
We have to consider all these aspects, as well as consumer concerns on the probable
presence of pesticide residues in fruit, leaves and roots of these crops and the strict limitations
for marketing and export that can invalidate the hard labour and endurance of many workers.
Therefore, a careful use of herbicide is compulsory, and good field practices must be followed,
especially when recognition of a labelled production is desired. There is a great interest in the
integration of tilling practices with chemical control because of the reduction of the herbicide
impact and the cost of hand-labour.
SEED BEDS
Many vegetables are grown in seed beds to develop suitable seedlings for transplanting
in the field. Soils dedicated to seed beds are usually light, with good tilth, and fertilized to obtain
a good plant emergence. Seed beds are usually flood-irrigated and plastic-protected. Here we
add some possibilities for weed management.
STALE SEED BEDS
Stale (‘false’) seed beds are sometimes used for vegetables when other selective weed-
control practices are limited or unavailable. Basically, this technique consists of the following:
1. Preparation of a seedbed 2-3 weeks before planting to achieve maximum weed-seed
germination near the soil surface.
2. Planting the crop with minimum soil disturbance to avoid exposing new weed seed to
favourable germination conditions.
3. Treating the field with a non-residual herbicide to kill all germinated weeds just before or after
planting, but before crop emergence.
SOLARIZATION
Soil solarization is a broad-spectrum control method, simple, economically feasible and
environmentally friendly. It is an effective method for the control of many weeds. It does not
affect soil properties and usually produces higher yields (Campiglia et al. 2000). There are also
some disadvantages in its implementation. For example, previous irrigation is a requirement, (or
frequent and abundant rain) and the soil must be kept solarized (non-producing) for a period of
at least one month. Results are often variable, depending on weather conditions. Cold (high
latitude) or cloudy places are usually not suitable for implementing solarization. Some species
can tolerate solarization (e.g. deep rooted perennials: Sorghum halepense, Cyperus rotundus,
Equisetum spp. and also some big weed seeds such as legumes).
The soil must be clean, surface-levelled and wet, previously to being covered with a thin
(0,1-0,2 mm) transparent plastic sheet and very well sealed. The soil must be kept covered
during the warmer and sunnier months (30-45 days). Soil temperatures must reach above 40° C
to exert a good effect on weed seeds.
After solarization the plastic must be recovered, and the use of deep or mouldboard
tillage must be avoided. This system is more suitable for small areas of vegetables, but it has
been mechanized for extensive areas of tomatoes. Soil solarization is widely used under plastic
greenhouse conditions.
CHEMICAL CONTROL IN SEED BEDS
There are even less registered herbicides for seed beds than for planting crops.
Herbicide treatments under plastic cover are always hazardous and careful application should
be carried out. Under plastic, high levels of moisture and elevated temperature are common and
plants grow very gently. Selectivity could be easily lost and phytotoxicity symptoms may occur,
while sometimes they are just temporary. The effects are often erratic. The best way to deal with
it is to be prudent and make some trials before a general treatment.
Selective pre-emergence and early post-emergence herbicides for vegetable seedbeds
a) Pre-emergence
Herbicide Dose (kg a.i./ ha) Crop
Clomazone 0.18 - 0.27 Pepper, cucumber
DCPA 6.0 - 7.5 Onion, cole crops, lettuce
Metribuzin 0.15 - 0.5 Tomato
Napropamide 1.0 - 2.0 Tomato, pepper, eggplant
Pendimethalin 1.0 - 1.6 1.0 - 2.5 Onion, garlic Lettuce
Propachlor 5.2 - 6.5 Onion, cole crops
b) Post-emergence (crops with at least 3 leaves)
Clomazone 0.27 -0.36 Pepper
Ioxinil 0.36 Onion, garlic, leek
 Linuron 0.5 - 1.0 Asparagus, carrots
Metribuzin 0.075 - 0.150 Tomato
Oxifluorfen 0.18 - 0.24 Onion, garlic
Rimsulfuron 0.0075 -0.015 Tomato

DIRECT-SEEDED AND TRANSPLANTED CROPS

WEED IDENTIFICATION
Dicotyledons (most broad-leaf weeds) and monocotyledons (e.g. grasses) are the two
main plant types. Weed grouping has a significant impact on the potential for management. The
more closely related a weed is to the host crop, the harder it will be to manage.
Weed and crop family groupings (monocotyledons - 'M')

Family Weed examples Related crops

Apiaceae slender celery (Ciclospermum leptophyllum) celery, carrot, parsley
Australian carrot (Daucus glochidiatus)
Amaranthaceae amaranths (Amaranthus spp.) Chinese amaranthus
Asteraceae billygoat weed (Ageratum spp.) lettuce, artichokes
sowthistle (Sonchus oleraceus)
cobbler's pegs (Bidens pilosa)
fleabanes (Conyza spp.)
parthenium (Parthenium hysterophorus)
potato weed (Galinsoga parviflora)
Brassicaceae wild turnip (Brassica tournefortii) cabbage, cauliflower,
wild radish (Raphanus raphanistrum) broccoli, brussels
turnip weed (Rapistrum rugosum) sprouts, Chinese
shepherd's purse (Capsella bursa-pastoris) cabbage
peppercress (Lepidium spp.)
lesser swinecress (Coronopus didymus)
Chenopodiaceae fat hen (Chenopodium spp.) beetroot
Convulvulaceae bell vine (Ipomoea plebia) sweetpotato
bindweed (Convolvulus erubescens)
Euphorbiaceae caster oil plant (Riccinus communis) cassava
caustic creeper (Euphorbia drummondii)
 Fabaceae rattlepod (Crotalaria spp.) peas, beans
vetch (Vicia monantha)
medics (Medicago spp.)
Liliaceae (M) onion weed (Nothoscordum gracile) onion, garlic
Malvaceae small-flowered mallow (Malva parviflora) okra, rosella, cotton
sida (Sida spp.)
bladder ketmia (Hibiscus trionum)
anoda weed (Anoda cristata)
Solanaceae apple of Peru (Nicandra physalodes) tomato, potato,
nightshades (Solanum spp.) capsicum, eggplant
thornapples (Datura spp.)

CROP ROTATION
Crop rotation is the programmed succession of different crops during a period of time in
the same plot or field. It is a key control method to reduce weed infestation in vegetables. Crop
rotation was considered for a long time to be a basic practice for obtaining healthy crops and
good yields. At present, however, crop rotation is gaining interest and is of value in the context
of integrated crop management. Classically, crop rotations are applied as follows:
 Alternating crops with a different type of vegetation: leaf crops (lettuce, spinach, cole), root
crops (carrots, potatoes, radish), bulb crops (leeks, onion, garlic), fruit crops (squash,
pepper, melon).
 Alternating grass and dicots, such as maize and vegetables.
 Alternating different crop cycles: winter cereals and summer vegetables.
 Avoiding succeeding crops of the same family: Apiaceae (celery, carrots), Solanaceae
(potato, tomato).
 Alternating poor- (carrot, onion) and high-weed competitors (maize, potato).
 Avoiding problematic weeds in specific crops (e.g. Malvaceae in celery or carrots, parasitic
and perennials in general).
Examples of crop rotations are as follow (Zaragoza et al. 1994):
In temperate regions: Pepper - onion - winter cereal
Melon - beans - spinach - tomato
Tomato - cereal - fallow
Lettuce - tomato - cauliflower
 Potato - beans - cole - tomato- carrots
Melon - artichoke (x 2) - beans - red beet - wheat - cole
In tropical regions: Tomato - okra - green bean
Sweet potato - maize - mung bean
Introducing a fallow in the rotation is essential for the control difficult weeds (e.g. perennials),
cleaning the field with appropriate tillage or using a broad-spectrum herbicide. It is also
important to avoid the emission of weed seeds or other propagules.
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 rotovator 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
done with mouldboard 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.
Selective herbicides for weed control in vegetable crops

Herbicide Dose Treatment moment Crops

kg a.i./ha
Alachlor 2.4 Post 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 D.s. tomato, carrots, peas
Oxifluorfen 0.36-0.48 Pre/Post emergence Onion, garlic, cole crops
Oxifluorfen 0.24-0.48 Pre Plantation Tomato, pepper
Pendimethalin 1.32-1.65 Pre Plantation / pre- Artichoke, cole, lettuce, leek, pepper,
plant incorporated 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. Eg: a weevil for the aquatic weed salvinia, rust for
skeleton weed, and a caterpillar (Cactoblastis sp.) to control prickly pear.