Weed management in lettuce

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

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Introduction

Weed management is a significant proportion (about 20%) of pre-harvest variable costs in lettuce production. As with other pests such as insects and diseases, it is becoming increasingly evident that lettuce producers must take a longer-term view of weed management, as there are no simple, quick-fix solutions. Chemical options for managing weeds in lettuce are few. Economic and environmental considerations mean we must adopt integrated, multi-faceted approaches to lettuce production.

 

Figure 1. Potato weed (Galinsoga parviflora), a key weed of lettuce

Site selection and preparation

Weed management starts well before paddocks are prepared for the current crop. It is extremely difficult to grow lettuce in land with large weed seedbanks, pre-planting management is the key to success. Ideally, lettuce 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 there are large populations of troublesome species such as nut grass (Cyperus rotundus) or potato weed (Galinsoga parviflora), it may be impossible to economically grow lettuce without a preparatory weed strategy.

Pre-planting management

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

Option 1. Form beds well before planting, then irrigate to germinate an initial flush of weeds. Kill these weeds by spraying with a knockdown herbicide (use glyphosate, paraquat or diquat, depending on weed species present). Alternatively, kill weeds 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 treatments 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 numbers of weeds.

Option 3. Form beds before planting, then fumigate, with, for example, 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.

Management immediately before planting and early in-crop

Broadleaf weeds

Broadleaf species are the most significant weed problems in lettuce production. Of particular concern are plants in the Asteraceae (daisy-type) family, which are closely related to lettuce. Significant Asteraceae weeds are potato weed (Galinsoga parviflora), common sowthistle (Sonchus oleraceus), cobbler's peg (Bidens pilosa) and prickly lettuce (Lactuca serriola).

Herbicide strategies
Apart from a specific Minor Use Permit in Western Australia (APVMA Per 6920), three herbicides are currently permitted for broadleaf weed management in lettuce. These herbicides must be applied just before, at, or immediately after planting, before weeds have emerged. They will not kill established plants. There are no selective herbicides for controlling emerged broadleaf weeds in the lettuce crop. Choosing which herbicide to use will depend on:

• weed species likely to be a problem,
  
  
• following crops in the rotation,
  
  
• types of herbicides used in previous crops.

To avoid the build-up of resistant weeds, it is important not to continuously use herbicides with the same modes of action.

Propyzamide (BurstÒ, Clearway, Edge, KerbÒ, PropyzamideÒ and Royale), a Group K herbicide, will control a range of broadleaves and grasses, but is ineffective against important Asteraceae weeds (Table 1). It is sprayed at up to 4.5 kg/ha immediately after direct-seeding or transplanting, and incorporated with 10 mm to 15 mm of irrigation.

Pendimethalin (ArgoÒ, ChargerÒ, CycloneÒ, FistÒ, GazelleÒ, Panida GrandeÒ, PendiÒ, PendimethalinÒ, Pendimethex, PennantÒ, RifleÒ, StompÒ) a Group D herbicide, will also kill several broadleaf and grass species, with some activity against several Asteraceae species, including common sowthistle (Table 1). Pendimethalin can only be used on transplanted lettuce, and must be applied before transplanting. The formulations are sprayed at 1 L/ha to 4 L/ha (depending on formulation concentration and weed spectrum), between two and seven days before transplanting, and incorporated with 10 mm to 25 mm of irrigation. Take care on sandy soils, as the highest rate may cause crop stunting and yield reductions. Because of residual activity, sensitive crops (e.g. beets) should not be sown immediately following a lettuce crop treated with pendimethalin. Refer to the label for re-cropping intervals.

Propachlor (RamrodÒ), another Group K herbicide, can be applied directly after seeding or transplanting, and then irrigated in (see AVPMA permit 5776). The biggest advantage of propachlor is the improved control of Galinsoga , compared to the other registered herbicides. Although the permit rate is up to 12 L/ha, Queensland experimental and producer experience is that in some circumstances, significant crop damage could occur at that maximum application rate. Growers are strongly advised to only spray small areas at first, to determine what propachlor rates suit their soils, climate and varieties. A moderate rate of propachlor after transplanting, following a moderate rate of pendimethalin before transplanting, has been found to give a good spread of activity against a wide range of weeds.

It is essential to minimise the time between final cultivation (or knockdown herbicide application) and spraying propyzamide, pendimethalin or propachlor. The longer the delay (i.e. more than 2 to 3 days), the more likely it is that new weeds will germinate and not be controlled by the pre-emergent herbicides. To maintain an effective herbicide blanket, it is also important that cultural operations after herbicides are applied, e.g. transplanting, minimise soil surface disturbance.

In-crop cultural practices
Broadleaf weeds in the lettuce crop 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, to avoid stimulating 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 weed germination before the 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 lettuce. The optimum timing for these operations varies with environmental conditions, weed species present, lettuce cultivar and soil type. For example if a late summer planting of lettuce in the Granite Belt needed cultivation it would best be done about three weeks after transplanting.

Table 1. Effectiveness of propyzamide and pendimethalin herbicides on weeds⁺

Weed species		Propyzamide	Pendimethalin	Propachlor
Common name	Scientific name	(e.g. Kerb®)	(e.g. Stomp®)	(e.g. Ramrod®)
Potato weed	Galinsoga parviflora	minor suppression*	minor suppression*	control
Common sowthistle	Sonchus oleraceus	ineffective	suppression	suppression
Prickly lettuce	Lactuca serriola	ineffective	control	minor suppression
Amaranthus	Amaranthus spp.	control	ineffective	suppression
Pigweed	Portulaca oleracea	control	control	suppression
Chickweed	Stellaria media	control	control	control
Fat hen	Chenopodium album	-	control	control
Green fat hen	Chenopodium murale	-	control	control
Lesser swinecress	Coronopus didymus	-	suppression	minor suppression
Indian hedge mustard	Sisymbrium orientale	-	suppression	-
Shepherd's purse	Capsella bursa-pastoris	control	suppression	control
Turnip weed	Rapistrum rugosum	-	suppression	-
Wild radish	Raphanus raphanistrum	-	suppression	-
Deadnettle	Lamium amplexicaule	-	control	control
Nettles	Urtica spp.	control	suppression	control
Knotweed	Polygonum aviculare	control	control	-
Blackberry nightshade	Solanum nigrum	control	suppression	ineffective
Bladder ketmia	Hibiscus trionum	-	control	ineffective
Fumitory	Fumaria spp.	-	suppression	-
Slender celery	Apium leptophyllum	-	ineffective	ineffective

⁺ Check the date of publication of this DPI&F Note, as this Table should be regularly updated to reflect new registrations and approvals
* The extent of suppression is not commercially significant

Grass weeds

Grass weeds are seldom a problem in lettuce production. Most grasses will be controlled by the same pre and post-planting cultural practices used to manage broadleaf weeds. The pre-emergence herbicides pendimethalin propyzamide and propachlor control many grass species before emergence.

The herbicides FusalideÒ (fluazifop-p butyl), SertinÒ (sethoxydim) and SelectÒ (clethodim) are registered for post-emergence grass control in lettuce. If a post-emergence herbicide spray is necessary to manage grass weeds the choice of chemical depends mainly on grass species to be killed. Each product controls a slightly different weed spectrum (check labels for specific species). FusiladeÒ , SertinÒ and SelectÒ must not be applied closer than 28 days before lettuce are harvested.

Be aware that some grasses are not controlled by these post-emergence herbicides and other species are becoming resistant. All three products are Group A herbicides, to which several grasses (e.g. ryegrass) are developing resistance.

Late in-crop and post-harvest weed management

Whilst late-emerging weeds will not affect lettuce yields they should still be managed. Species such as potato weed 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 populations of weeds setting seed. It may make economic sense to selectively hand-weed in the weeks before harvesting, particularly if weed seed-set can be prevented. Destroy weeds in the paddock once lettuce have been harvested, don't let them set seeds.

Further information

The Lettuce Agrilink kit provides comprehensive up-to-date, practical information from planting, crop nutrition and protection, through to marketing. Agrilink kits are available for a range of crops. For further information on the Agrilink series, see the list of contacts on the Agrilink website. 

Current national information on agricultural chemicals registered for use on various weeds is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld. 4001, or email infopest@dpi.qld.gov.au for further information. Always refer to herbicide labels before application.

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, B.A. and Medd, R.W. (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, R.M. and McKenzie, R.A. (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, B.J., Hawton, D. and Duff, A.A. (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