Growing lettuce
Common questions
Sue Heisswolf, Craig Henderson & Peter Deuter, Department of Primary Industries and Fisheries, Queensland.
This is an update of information from the 'Agrilink Lettuce Information Kit' and the 'Agrilink Lettuce Information Kit Update 1999' which provide information on all aspects of growing and marketing lettuce in Queensland.
On this page:
Introduction
This section contains the most commonly asked questions about growing lettuce. The answers are as brief as possible. Where this is difficult and more detail is required, we refer you to other sections of the 'Agrilink Lettuce Information Kit'.
Varieties
What's the best variety to plant?
Lettuce varieties are changing constantly, contact a commercial seedling nursery or seed supplier for up-to-date information.
Temperature fluctuations throughout the growing period have the greatest impact on a variety's performance. Select varieties with recent weather patterns in mind. Nutrient level and time of application can also have a big impact on lettuce quality.
Lettuce varieties are bred to perform under a distinct temperature range. A cool weather variety grown under warm conditions, for example, will tend to bolt.
Here are some points to remember when choosing varieties:
- Warm weather varieties will not produce quality heads once temperatures exceed 28°C.
- Winter varieties will tolerate mild frosts but no varieties will withstand heavy frosts.
How do I test new varieties?
Test new varieties on your farm against the standard varieties. Try two or three new varieties every time you plant by planting them beside one of your standard varieties so that you can compare them easily. Every farm is slightly different and lettuce is sensitive to weather changes. If you test varieties over several seasons, you will develop a variety plan for your farm. Try only small quantities at first.
What varieties should I plant for autumn or spring harvest?
Some commercial growers 'hedge their bets' by planting two varieties, so that at least half the crop will perform well if the weather is cooler or warmer than usual. Refer to the comments under the previous question. It is particularly important to choose the right variety in the Lockyer Valley or Darling Downs where the difference in temperature at that time of year can vary from day to day. It is difficult to choose a variety for autumn and spring harvest because there is a chance of a light frost in autumn or some hot days in early spring.
Planting
Should I direct-seed or plant seedlings?
The decision to direct-seed or use transplanted seedlings is based on two considerations: to save money or to save time.
If you decide on direct-seeding, you will not have to pay for cell-grown transplants. Cell-grown transplants cost about $55 per 1000 plus the cost of the seed. Direct-seeded crops are planted two to three weeks earlier, increasing the amount of water used and greatly increasing the risk of weed, disease and pest problems. Cell-grown transplants have a shorter cropping cycle and this reduces some of the risks. A shorter cropping cycle may also reduce reliance on herbicides.
It is difficult to establish a lettuce crop using direct-seeding techniques under hot conditions, and the risk of failure is higher. We recommend new growers use cell-grown transplants from a commercial seedling nursery, at least until they sort out their production system.
My lettuce hasn't come up. What's gone wrong?
There are many reasons why direct-seeded lettuce may not emerge. Do not blame the seed in the first instance, as seed quality is usually very high.
Lettuce seed can germinate poorly or fail to emerge for three reasons:
- Damping-off, which is caused by a range of fungal diseases, will prevent the seedling from emerging even though the seed might germinate.
- Hot dry conditions can force lettuce seed into high temperature dormancy. Seed will take longer to break this dormancy and then germinate normally but unevenly.
- A range of soil insects, including earwigs, can attack the germinating plant and prevent it from emerging.
Weeds
I've got weeds in my lettuce. How do I get rid of them?
There is no registered chemical to control broadleaf weeds in lettuce once they have emerged, so the only alternative is to hand chip to get rid of them. A Minor Use permit for killing some broadleaf weeds after emergence is current in NSW only. If grass weeds are a problem you can use the herbicides Fusilade®, Sertin® or Select® up to 28 days before harvest. The choice of chemical depends mainly on the grass species to be killed. Check the label for specific species.
The DPI&F note Weed management in lettuce provides more detailed information.
Is there a good herbicide to control weeds?
There are few herbicides you can use in lettuce. Except for the products that only control grasses, they must all be applied at or before planting. It is important to reduce the number of weed seeds in the soil. Do this by rotating crops and reducing the weed population before planting.
Herbicides with the active ingredients propyzamide and pendimethalin are registered for pre-emergence use on lettuce. Pendimethalin can only be used on transplants, not on direct-seeded lettuce. Herbicides control different weeds so the herbicide used depends on the weeds on your farm. Sowthistle and potato weed are the most difficult weeds to control.
How do I control sowthistle (milk thistle)?
Pendimethalin is probably the best herbicide to suppress sowthistle. It can only be used in transplanted lettuce. You will probably need to hand chip to remove sowthistle that still comes up. It is important to minimise the number of sowthistle seeds in your lettuce paddock, by controlling it in the previous crop and around your paddock so that seeds do not blow in.
Irrigation
How do I use tensiometers in lettuce?
Place the tip of one tensiometer about 15 cm below ground level and another one about 45 cm deep. The shallow tensiometer tells you when to water, which in cool conditions is when the gauge shows about 25 centibars (kPa). In warmer conditions water more frequently, that is when the shallow tensiometer reads 15 or 20 kPa. The deep tensiometer should normally read between 10 and 15 kPa. If it drops to less than 5 kPa after irrigation, you have put on too much water. Apply less water next time and monitor what is happening using the deep tensiometer.
It is best to have a tensiometer site for each planting. Tensiometers are useful for scheduling irrigations once the crop is established, ten days to two weeks after transplanting.
How often do I need to water lettuce?
Lettuce has a shallow root system and is highly sensitive to water stress and tipburn. For cool season production, water lettuce every four to six days once the crop is established. In warm conditions irrigate every three to five days on clay soils. If using overhead sprinkler systems you may need to irrigate every day on sandy soils.
You may need daily irrigations to establish transplants. If you are using drip systems, you may need to water more frequently. Tensiometers will help you make this decision once the crop is established.
Pests, diseases and disorders
How do I control tipburn?
You cannot cure tipburn but you can try to prevent it. Tipburn is mainly caused by calcium deficiency, but is associated with several other factors. These include hot weather, hot dry windy days, high nitrogen levels, water stress and problems that prevent lettuce taking up enough water (for example root rot). The right choice of variety will reduce the risk of tipburn.
Check that you have planted the right variety for the time of year, as varieties have different tolerances to tipburn and warm weather. Check that the crop is getting enough water. Test your soil to determine the calcium level. Increase soil calcium by applying lime, dolomite or gypsum, depending on the soil pH.
How do I control aphids and thrips to stop viruses spreading?
Both thrips and aphids spread from weeds, so controlling weeds around fields will reduce their spread and therefore virus diseases.
If necessary apply one of the pesticides registered for use on lettuce against these pests. Use a chemical that has the least negative affects on beneficial insects. The broad-spectrum pesticide dimethoate is used against thrips and aphids but some aphids have developed resistance to it.
Two species of thrips, melon thrips and western flower thrips, are resistant to a range of insecticides. If thrips are at economically damaging levels and spray failures occur, the pest needs to be professionally identified. Do not continue to use insecticides if they do not work.
How do I control rust (lettuce mosaic virus)?
Rust is a term used by some growers when referring to lettuce mosaic virus. It is a brown discolouration that is found on the lower parts of the lower leaves. These symptoms are often not seen until the crop is harvested.
To control rust, use virus indexed seed, maintain good farm hygiene by destroying old lettuce crops and weed hosts, encourage neighbours to control aphids and weed hosts, and control aphids in the seedling nursery.
What's causing the yellow brown spots on my leaves?
The most likely cause of yellow brown spots on leaves is downy mildew, the only major disease that lettuce growers may be able to control. The yellow brown spots appear on the upper surface of the lower leaves. Downy mildew fungal spores appear on the underside of the spots but are only seen under cool moist conditions. Leaves become infected up to five or six days before the symptoms appear.
To control downy mildew you need a regular spray program to apply a protectant fungicide before the symptoms appear. If downy mildew is already in the crop, use a systemic fungicide. Restrict use of systemic fungicides as much as possible to delay development of resistance to them. Some varieties of lettuce are less susceptible to this disease than others and you should contact your seed supplier for up-to-date information.
Why are my plants wilted and stunted?
Lettuce plants can be wilted and stunted for many reasons. Possible causes include necrotic yellows virus, corky root, ammonium toxicity, black root rot , lettuce big vein, Rhizoctonia infections and tomato spotted wilt. Insufficient water or fertiliser can also cause wilting or stunted growth.
Nutrition
What's a good hydroponic solution?
There is probably no real answer, but you need to consider a range of points when determining what hydroponic solution to use. Several nutrient solution recipes have been published by researchers and hydroponic societies and these could be the basis of a good hydroponic solution.
A hydroponic solution should be easy to make and if possible use fertiliser grade nutrients. These nutrients are usually the cheapest and most readily available.
If you are buying a nutrient solution 'off the shelf', make sure it has a detailed nutrient analysis. If you have production problems, your knowledge of the nutrient make-up will help you find an answer. Published nutrient solutions often have a variance available for summer and winter production so that you can adjust your management.
How much nitrogen should I put on my lettuce?
Variety, soil type and season influence the nitrogen requirements of lettuce. The best way to determine how much nitrogen to apply is to follow the recommendations of a soil analysis. Where a soil analysis is not used, 100 to 150 kg/ha of nitrogen is commonly used. Some growers produce very good quality lettuce using only 50 kg/ha of nitrogen. Pre-plant fertiliser is not always used.
High levels of nitrogen can be of concern for two reasons:
- Some varieties may not heart up properly with high nitrogen applications.
- High levels of nitrogen may endanger ground water quality, as excess nitrogen can be leached through the soil.
Discoloured lettuce heads
When I cut the head in half there are brown marks and streaks inside. What's causing this?
If you see brown marks and streaks inside the head, check if they also appear around the edges of the leaves, particularly the inner leaves. If they do, it could be tipburn.
Heliothis grub damage can look similar to tipburn, but instead of brown leaf edges you will find chewed leaves, pellets of grub excreta, and often the grub.
What's causing brown edges on the leaves?
The most likely cause of brown edges on leaves is hot weather, particularly if hot days follow cool days. Sclerotinia and varnish spot can cause similar symptoms.
Brown edges can also be caused by windburn, most likely on hot days, and by poor quality water (water high in either sodium or chlorides). If the outer leaves have brown edges and the lettuce are close to maturity, cut a few heads in half and check the inside. If you find some internal brown streaks, this condition is tipburn.
Using chemicals
How do I know how much chemical to use?
Always read the label. The label on the chemical container gives the registered rate of application for lettuce and the withholding period. The label also tells you how to use the chemical and how to mix and apply the product safely.
Do I need training in the safe use of chemicals?
In Queensland you cannot buy certain pesticides unless you have a current ChemCert accreditation, endosulfan can only be supplied to or used by an accredited person.
Most customers now see it as highly desirable for their growers to be able to demonstrate safe, responsible use of chemicals. One of the best ways to demonstrate this is to obtain a ChemCert accreditation.
How do I get spray accreditation?
You must attend a ChemCert Farm Chemical User Training course. Course participants complete an open book assessment at the end of the course and successful candidates are then issued with a statement of accreditation by ChemCert. This accreditation is valid for five years so participants are kept up to date with changes and developments.
ChemCert accreditation will assist in the documentation of a quality assurance (QA) program. It is highly desirable for growers to be able to demonstrate safe, responsible use of chemicals.
How do I store my chemicals?
Chemicals need to be stored in accordance with the AS2507-1998 standard. This does not mean you will have to spend a fortune on elaborate storage facilities. You do, however, need to be aware of several safety, environmental and food safety factors whenever you deal with chemicals. Further information on correct storage of chemicals is covered in the chemical user course.
How do I calibrate my spray equipment?
To work efficiently equipment needs to be calibrated. Document the results of this calibration as part of your quality management system. You will learn the correct method of calibration at the spray accreditation course.
Do I need to keep a diary of spraying records?
Yes you do! Records of chemical application are now one of the most important pieces of documentation you will need to be able to prove what you have done with chemicals. Most merchants and agents supplying the retail sector now expect you to keep spray records showing at least what was applied, how much, by whom, and when the application took place. This information is an essential part of a QA system.
What is an MRL?
MRL stands for Maximum Residue Limit, which is the maximum level of a chemical permitted to be present in a food. It is expressed as milligrams of active ingredient per kilogram (mg/kg) of the food.
Produce at the markets is randomly tested for chemical residue. Farmers have been prosecuted when chemical residues were above the MRL, or residues of non-registered chemicals were found.
What is a withholding period (WHP)?
The withholding period is the number of days that must pass between the last chemical application and harvest for human consumption, grazing by livestock and cutting for stock feed. For postharvest treatments it is the number of days from the postharvest treatment to consumption. The WHP will be on the pesticide label.
Some chemicals do not have withholding periods for livestock, if these chemicals are used, the crop must not be fed to livestock. The withholding period for livestock consumption may be greater than that for human consumption.
Residues should be below the MRL if the chemicals are used at the registered rate and frequency, and the withholding period is observed.
Environmental management
What is an EMS?
An Environmental Management System (EMS) is a systematic approach to managing the impacts that a business has on the environment. An EMS does not dictate levels of environmental performance, however it should enable a business to comply with legislative requirements concerning the environment. It should also build on existing activities such as industry best management practices, industry codes of practice and quality assurance schemes.
What type of EMS do I need?
We do not know when or what type of EMS will be required in the horticulture industry in the next few years. It will depend in part on the markets you are aiming to supply. As a minimum, you should become familiar with the Growcom Farmcare Code of Practice for fruit and vegetables. It was developed by Growcom to assist growers meet their general environmental duty of care under the Environmental Protection Act 1994.
The most widely recognised auditable international EMS standard is ISO 14001. This process standard requires a business to have an implemented, documented environmental policy in place which is available to the public. It also requires the business to have a documented environmental management system in place that is capable of being audited by an accredited third party. ISO 14001 does not prescribe specific levels of environmental performance but it does require a business to be able to identify and have access to all relevant environmental legislation.
In Europe, a protocol based on good agricultural practice called EurepGAP has been operating since the late 1990's. A significant number of Australian horticultural businesses are working towards EurepGAP in order to meet compliance deadlines stipulated by UK/European retailers and importers.
What is Enviroveg?
Enviroveg is a relatively new program from the Australian Vegetable and Potato Growers Federation (AUSVEG). Enviroveg is committed to encouraging vegetable growers throughout Australia to adopt and implement good environmental practices. It includes a self-assessment tool to help growers compare their current farming practices with the environmental best practice activities listed in the Enviroveg guidelines.
Farm safety
What are my workplace health and safety obligations?
Australian Agriculture has one of the highest rates of workplace accidents. Your obligation as an employer and farm manager are based on three principles:
. a concern for people and their well-being, including yourself and your family;
. sound business management;
. compliance with current workplace health and safety legislation.
The best way to meet your obligations is to take part in the Managing Farm Safety program developed by Farmsafe Australia. The Managing Farm Safety program is aimed at developing skills in risk management of farm safety - an approach that is consistent with the way other farm business risks are managed. The training course and resource package are based on real data about the major risks on Australian farms, including specific agricultural industries, and takes into account the requirements of current occupational health and safety legislation.
Harvesting
When is lettuce ready to harvest?
The time of harvest depends on the season and the variety you have planted. Ask your seedling nursery or the seed company how many days the variety normally takes from seeding or transplanting to harvest. If the weather is warmer than normal, your lettuce will be ready a few days earlier. In cold weather it might take six or seven days longer before the crop is ready to harvest.
About a week before your estimated harvest date, walk through your field and pick out a few lettuce that look ready to harvest. Cut them in half and check how closely leaves are packed in the head. If five or six heads out of ten are tightly packed inside, the crop is ready for harvest. If a few of the heads you have cut open have a core, that is they are starting to elongate, start harvesting immediately, before the crop runs to seed. Consider recent weather patterns as you may have chosen the wrong variety.
Ideally, the whole planting is cut in one pass. Two cuts several days apart, however, may be necessary to maximise head quality.
Why aren't the heads filling out or sizing up properly?
If the heads aren't filling out or sizing up properly you may have selected the wrong variety for that timeslot. Some winter varieties need cold weather to heart up properly and form a solid head. If lettuce is showing signs of splitting, bolting or stress, such as brown leaf margins or tipburn, the variety may be a cool weather variety planted out of season.
Too much nitrogen can cause 'fluffy' heads, because the heads are being grown too quickly and are not able to form a head before bolting. Stunted lettuce may look as though the heads are not filling out. If one area of the crop is better than another, you may have a problem with nutrition or irrigation.
To determine a possible cause, refer to the pictures and descriptions of stunted plant symptoms in Section 5, Problem solver of the lettuce kit.
Why has my lettuce split open or bolted?
Lettuce that is split open or bolted has started to flower. Either you chose the wrong variety for that time of year or the weather was a lot hotter than normal. You may also not be harvesting early enough.
If only some of the lettuce is bolting, the block may be variable in its maturity. You may be stressing the crop. Check your general management practices, irrigation, fertiliser and planting out.
Marketing
What level of quality assurance (QA) do I need?
You will need different levels of quality assurance (QA) depending on who you supply. If you supply direct to a retailer, in some instances you may need a full quality management system which is accredited by a third party, for example Freshcare or HACCP plans. If you supply to a processor you will need to meet their requirements.
Freshcare is a national, on-farm food safety program for the fresh produce industry and it may be all you need if you supply to a wholesaler, processor, packing shed or marketing group.
Money
Can I make money out of lettuce?
The market for lettuce is oversupplied and even experienced lettuce growers have had difficulty breaking even in recent years. Identify your market and use realistic costs to estimate returns before you decide to plant. The ability to supply lettuce of consistently high quality is essential and can be difficult to achieve for first-time growers.
Further information
The complete Agrilink Lettuce Information Kit, produced by the Agrilink Information Unit of the DPI&F, provides information on all aspects of growing and marketing lettuce in Queensland. The kit includes the following sections:
- Before you start
A checklist of things you need to know before you start growing the crop. - Common questions
The twenty or so most commonly asked questions about growing the crop. - Growing the crop
Our guide for establishing, producing and marketing the crop. - Key issues
Detailed information on the key decisions affecting the crop. - Problem solver
A picture series of the common problems and how to solve them. - Contacts and references
A list of industry organisations, product suppliers, and further reading. - Index
An A to Z index to help you find information quickly. - Your information
A place for you to store your special information. - Your information
A place for you to store your special information.
To contact Agrilink for further information on the Agrilink series, visit http://www.dpi.qld.gov.au/agrilink/4693.html.
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. The Business Information Centre is open 8 a.m. to 6 p.m. Monday to Friday (excluding public holidays); E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.
This Note is also published on the DPI&F's PrimeNotes CD-ROM.
Other horticultural information is available on the Horticulture home page.
Industry links
Industry links is a page of links to various sites of interest to horticultural growers.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last reviewed 26 August 2005
