Managing weeds in broccoli, cauliflower and cabbage

Craig Henderson, Department of Primary Industries and Fisheries, Queensland.

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Introduction

Depending on the establishment method, brassica vegetables, particularly the taller, quicker-maturing crops such as broccoli and cauliflower, can be relatively competitive against weeds. Nevertheless, effective weed management is an essential requirement for reliably achieving high-quality brassica produce.

As with other pests such as insects and diseases, brassica producers must take a long-term view of weed management, and not simply rely on reactionary herbicide solutions. Economic and environmental considerations mean we must adopt an integrated, multi-tactic approach to managing weeds in brassicas.

Herbicide options vary between brassica vegetable crops, so refer to herbicide labels, databases such as DPI&F Infopest, or seek expert advice, don't assume a product is legal to use. Broccoli, cauliflower and cabbage have the most registered herbicides, with fewer products available for Chinese cabbage or more exotic Asian vegetables (e.g. pak choy). Minor use permits for herbicides in brassica vegetables are regularly reviewed and updated so consult databases or advisory services for the latest updates.

Site selection and preparation

Weed management starts well before paddocks are prepared for the current crop. Although a range of pre-emergence herbicides is registered for the major brassica vegetables, it is still difficult to grow brassicas in land with large weed seedbanks. Pre-planting management is a key to success. Ideally, brassica vegetables should be grown in land with at least a two year history of effective weed control. This can be achieved by selective crop rotation and cover cropping. Where large populations of troublesome weeds such as nut grass (Cyperus rotundus) exist, it may be impossible to economically grow brassicas without a pre-plant weed management strategy.

Pre-planting management

Weed management starts with planting into a weed-free seedbed. Three possible options for achieving this are:

• Option 1. Form beds well before planting, then pre-irrigate to germinate an initial flush of weeds. These weeds are killed by spraying with a knockdown herbicide (using glyphosate, paraquat, or diquat, depending on weed species present). Alternatively, weeds can be killed with a very shallow cultivation. Avoid moderate to deep cultivations because they will cause more weeds to germinate in the crop. For organic farming, flaming, steam or hot water treatment are alternatives to knockdown herbicides.

• Option 2. Form beds just before planting, with a final cultivation to prepare the seedbed and kill any emerged weeds. This is not the preferred option in paddocks with substantial weed burdens.

• Option 3. Form beds before planting, then fumigate, for example with metham (refer to fumigant labels for specific rates). This option has the additional benefits of controlling some diseases, nematodes and insect pests, depending on the rates used. The long-term future for most broad-spectrum fumigants is unclear.

Management immediately before planting and early in-crop

Broadleaf weeds

Broadleaf species (Table 1) are the most significant weeds in brassica production. Of concern are plants from the Brassicaceae family, all of which are related to the cultivated crops. There are also other significant weeds in brassica crops that are not related to the crop.

Table 1.   Broadleaf weeds of brassica vegetables

Brassica weeds		Other weeds
Common name	Scientific name	Common name	Scientific name	Common name	Scientific name
wild radish	Raphanus raphanistrum	fat hen	Chenopodium album	blackberry nightshade	Solanum nigrum
wild turnip	Brassica tournefortii	small-flowered mallow	Malva parviflora	thornapples	Datura spp.
turnip weed	Rapistrum rugosum	nettles	Urtica spp.	apple of Peru	Nicandra physalodes
shepherd's purse	Capsella bursa-pastoris	deadnettle	Lamium amplexicaule	knotweed (wireweed)	Polygonum aviculare
London rocket	Sisymbrium irio	pigweed	Portulaca oleracea	potato weed	Galinsoga parviflora
lesser swinecress	Coronopus didymus	fumitories	Fumaria spp.	common sowthistle	Sonchus oleraceus
		amaranthus	Amaranthus spp.	cobbler's peg	Bidens pilosa

Herbicide strategies

Six different active ingredients are registered for pre-emergence weed management in vegetable brassicas. Several brand names are registered for most of these active compounds, so for simplicity the active compound name is used throughout the text.

These herbicides must be sprayed just before or just after planting, before weeds have emerged (see details for individual products). Oxyfluorfen, and to a lesser extent pendimethalin, may kill recently emerged seedlings of some weed species, but this should not be relied upon as a regular strategy. None of the herbicides will kill established weeds. The only selective herbicide for controlling emerged broadleaf weeds in vegetable brassica crops is a temporary permit for clopyralid in cauliflower in Western Australia (APVMA Per 9169).

In the following discussions, no specific details are given on application rates or legal crops, as product formulations comprise the same actives at different concentrations, leading to different registered product application rates and crop uses.

Your choice of which herbicide to use will depend on:

1.  planting method

2.  weed species likely to be a problem

3.  relative costs of application

4.  following crops in the rotation

5.  types of herbicides used in previous crops.

To avoid the build-up of resistant weed spectrums, it is important not to continuously use herbicides with the same modes of action. Note that not all herbicides are registered for all brassica vegetables, so it is very important to refer to crop labels to determine legal uses.

Transplanted brassicas

Oxyfluorfen (e.g. Goal^® and several others) is a Group G herbicide, which kills emerging and very small seedlings. This herbicide should be applied 4 to 7 days before transplanting, with higher water volumes where young weed seedlings are present. Note that oxyfluorfen is unlikely to kill emerged grass seedlings. For pre-emergence activity, oxyfluorfen needs to be activated by irrigation or rainfall, but does not require immediate incorporation by water. Once the herbicide has been applied minimise soil disturbance, caused by planting and other operations, to maintain the chemical barrier to weed emergence.

Of the available registered herbicides, oxyfluorfen kills the widest spectrum of important broadleaf weeds and is certainly cost effective. It can occasionally cause burning of seedling crop leaves, particularly in hot, humid conditions, or where lower leaves are continually in contact with the soil. Any damage is usually transitory and does not affect crop performance.

The herbicide is very toxic to aquatic organisms and should not be used where there is a risk of contaminating waterways. Always refer to the instructions on the herbicide label. No significant residues will be present in the harvested product when this herbicide is properly used.

Metolachlor or S-metolachlor (various trade names) are Group K herbicides, with multiple sites of action, and hence a low risk of stimulating herbicide resistance. They are mainly absorbed by weed shoots as they emerge through the soil surface, so must be present as the weed germinates. Spray immediately after planting into a weed free situation. Within 24 hours apply sufficient irrigation to wet the soil to a depth of 3 to 4 cm. Minimise soil disturbance after application.

Metolachlor and S-metolachlor will effectively control many grass and broadleaf weeds, such as potato weed, pigweed, some amaranthus and deadnettle. They are not very effective against fat hen, small-flowered mallow, or brassica weeds such as turnip weed or wild radish. Where these latter weeds are known to be a problem use an alternative product. They can cause stunting and chlorosis of crop seedlings, particularly in sandy or red alluvial soils. Growers will need to tailor the rates of application to suit their particular environment and soil type. Refer to the herbicide labels for more information. Metolachlor and S-metolachlor can be cost-effective rotational herbicides with oxyfluorfen in a long-term strategy.

Propachlor (Ramrod^®) is also a Group K herbicide, with a similar mode of action to metolachlor. Application practices are also the same and this product controls a similar broadleaf and grass weed spectrum. It is probably less effective against Solanaceae (e.g. thornapples, nightshades) and more effective against fat hen and mallows than metolachlor.

Propachlor is less likely to cause crop damage than metolachlor, however it is significantly more expensive and has high application rates (up to 12 L/ha). Propachlor is registered for use in Chinese cabbage and Brussels sprouts. Propachlor can also be sprayed in a tank mix with pendimethalin before transplanting, or separately after transplanting (with pendimethalin sprayed before transplanting). This twin application gives improved control of brassica weeds, fat hen, sowthistle and knotweed compared to propachlor alone. Refer to either label for details.

Pendimethalin (e.g. Stomp^® and several others) is a Group D herbicide that inhibits tubulin development in emerging shoots and particularly roots. Pendimethalin controls many broadleaf and grass weeds and is particularly active against fat hen, and against brassica weeds when combined with propachlor. Pendimethalin is not effective against most Asteraceae weeds (e.g. potato weed) or Solanaceae weeds. Pendimethalin must be applied before transplanting and incorporated by 12 to 25 mm of irrigation within 24 hours. As with the other pre-emergence herbicides, minimise soil disturbance after application.

Brassica vegetables are only marginally tolerant of pendimethalin and can be significantly damaged in adverse conditions such as waterlogging, very cold weather or overspray. New users should be cautious, confirming the best rates for their particular circumstances, before spraying large areas. Pendimethalin residues may break down sufficiently slowly in some soils that enough herbicide remains to affect following crops. Producers need to consider its effect on rotation crops when deciding whether to use this product.

Direct-sown brassicas

Producers may grow brassica vegetables by direct sowing rather than using transplants. Weed management in direct-sown crops is more difficult than in transplanted crops for several reasons. Firstly, the period between planting and crop canopy closure is up to five weeks longer than with transplants, so weeds have a much greater opportunity to emerge and compete with the crop. Secondly, the herbicide options for use in direct sowing, and the spectrum of weeds they control, is restricted when compared with transplanted crops. Thirdly, weeds that emerge with at the same time as a direct sown crop are more difficult to remove by cultivation as there is no differential in root system anchorage between the crop and weeds.

Successful pre-cropping weed management is therefore even more important in direct-sown brassica vegetables than transplanted crops.

Chlorthal-dimethyl (e.g. Dacthal^® and others) is a Group D herbicide that also inhibits tubulin formation and controls a range of broadleaf and grass weeds. Weed shoots mainly absorb chlorthal-dimethyl as they emerge through the soil surface, so the chemical must be present as weeds germinate. Spray chlorthal-dimethyl into a weed free situation immediately after planting, then within 24 hours apply sufficient irrigation to wet the soil to a depth of 3 to 4 cm. Minimise soil disturbance after application. This herbicide is applied at high rates (up to 15 kg/ha) and is relatively expensive. An option is spraying in narrow strips over the row and use other practices, such as inter-row cultivation, to manage weeds in the unsprayed areas. Experience has shown that chlorthal-dimethyl suppresses, rather than controls, many weeds that commonly occur in brassica vegetables.

Trifluralin (various trade names and formulations) is closely related to pendimethalin and is also a Group D herbicide that inhibits tubulin development in emerging shoots and particularly roots. Trifluralin is mainly active against grasses, although it does manage broadleaf weeds such as pigweed, some amaranthus and knotweed. Trifluralin is registered for application before planting and must be immediately (i.e. within 2 to 6 hours) incorporated into the soil by 25 to 50 mm of irrigation or cultivation. As with the other pre-emergence herbicides, minimise soil disturbance after application. Trifluralin residues can break down sufficiently slowly in some soils that enough herbicide remains to affect following crops. Producers need to consider its effect on rotation crops when deciding whether to use this product.

Both chlorthal-dimethyl and trifluralin can also be used in transplanted vegetables. However, the other pre-emergence herbicides mentioned in the transplanting section are probably more effective in most transplanting situations.

In-crop cultural practices

Broadleaf weeds appearing in brassica vegetable crops can only be killed by cultivation or hand weeding. If pre-emergence herbicides have been applied, cultural operations should still minimise disturbance of the chemical barrier and stimulation of further weed emergence. There is an optimum time window when cultural operations are most effective. If done too early, the full benefits of the pre-emergence herbicides are lost as there is still time for further weed germinations before the brassica crop canopy closes. If done too late, weeds may be too large to kill with shallow cultivation, and there may be insufficient space to cultivate between rows without damaging the brassicas. The optimum timing for these operations varies with environmental conditions, weed species present, brassica cultivar and soil type. For example, if a late summer planting of broccoli in the Granite Belt needed cultivation, it would best be done about four weeks after transplanting.

Tall, leafy brassicas such as broccoli and cauliflower are relatively competitive against weeds (particularly low growing weeds such as deadnettle), and can shade out late-emerging plants. Thus the focus is on early weed control, and killing tall-growing species that can break through the crop canopy, such as fat hen, sowthistle and to a lesser extent small-flowered mallow. Although Chinese cabbage is less competitive, (because of its upright growing habit), it is also relatively quick maturing, so extended weed management is not required. Other cabbages can be more problematic, as they are the slowest maturing major brassica vegetable, and not as effective at shading the soil as broccoli or cauliflower. Cabbage may require a second hand weeding toward the end of the cropping period to kill late-emerging weeds and prevent seed set.

Grass weeds

Grass weeds are seldom a problem in brassica production. Most grasses will be controlled by the same pre and post-planting cultural practices used to manage broadleaf weeds. All the pre-emergence herbicides previously mentioned control many grass species before emergence.

The herbicides fluazifop-p-butyl (e.g. Fusilade^® and others), sethoxydim (e.g. Sertin^® and others), quizalofop (various trade names) and clethodim (e.g. Select^® and others) are registered for post-emergence grass control in some brassica vegetables. If a post-emergence herbicide spray to manage grass weeds is necessary, the choice of chemical depends mainly on:

(i) grass species to be killed

(ii) registration status for the particular brassica vegetable crop

(iii) relative costs of application 

(iv) applicable withholding period (see Table 2).

Each product controls a slightly different weed spectrum (check labels for individual species). Be aware that some grasses are not controlled by these post-emergence herbicides and that other species are becoming resistant to them. All four products are Group A herbicides, to which several grasses (e.g. ryegrass) have or are developing resistance.

Table 2. Registration status and withholding period for post-emergence grass herbicides in the main brassica vegetables

Herbicide	Broccoli	Cauliflower	Cabbage
fluazifop	42 days	42 days	42 days
sethoxydim	42 days	42 days	42 days
quizalofop	not registered	14 days	63 days
clethodim	not registered	not registered	7 days

Late in-crop and post-harvest weed management

Whilst late-emerging weeds will not affect brassica yields they should still be managed. Species such as potato weed, fat hen and small-flowered mallow can grow dramatically in the last few weeks before harvest and interfere with cutting and packing processes. In an integrated weed management program, it is important to minimise the number of weeds setting seed. It may be economically sensible to devote labour resources to selective hand-weeding in the weeks before harvesting, particularly if weed seed-set can be prevented. It is important to destroy weeds in the paddock as soon as the crop has been harvested, don't let them set seeds.

Further information

• A general reference 'Australian Weed Management Systems' (B. Sindel Ed, publisher RG and FJ Richardson, Melbourne), includes a detailed general chapter on 'Vegetable Weed Management Systems' pp 355-72, by CWL Henderson and A Bishop. This chapter describes integrated weed management options, which are applicable to brassica vegetable production.
  
  
• The Weed Society of Queensland is an organisation involved in the management of weeds in Queensland. For further information write to The Secretary, Weed Society of Queensland, GPO Box 1607, Brisbane Qld, 4001.

Other publications which may assist in weed identification and management include:

• Auld, BA and Medd, RW (2002). 'Weeds - an illustrated botanical guide to the weeds of Australia'. Inkata Press, Melbourne.
  
  
• Banks, A et al (1990). 'Pesticide Application Manual', 2^nd edition. Queensland Department of Primary Industries, Information Series QI89003
  
  
• Dowling, RM and McKenzie, RA (1993). 'Poisonous plants - a field guide'. Queensland Department of Primary Industries, Information Series QI92035
  
  
• Friend, E (1983). 'Queensland weed seeds'. Queensland Department of Primary Industries, Information Series QM81013S
  
  
• Kleinschmidt, H Holland, A and Simpson, P (1996). 'Suburban weeds', 3^rd edition. Queensland Department of Primary Industries, Information Series QI96089
  
  
• Lazarides, M, Cowley, K and Hohnen, P (1997). 'CSIRO handbook of Australian weeds'. CSIRO Publishing, Collingwood.
  
  
• Wilson, BJ, Hawton, D and Duff, AA (1995). 'Crop weeds of northern Australia'. Queensland Department of Primary Industries, Information Series QI95017

DPI&F information and services

• To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Information Centre on 13 25 23 for the cost of a local call, from 8 am to 6 pm Monday to Friday (excluding public holidays). E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.

• Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.

Industry links

Industry links is a page of links to various sites of interest to horticultural growers.

Last updated 17 March 2008