LETTUCE

PRODUCTION GUIDELINE
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 LETTUCE

1. INTRODUCTION
Lactuca sativa, commonly known as lettuce, is a member of the Asteraceae family, grown as a leaf
vegetable.

1.1 HISTORY AND BACKGROUND

The origins of lettuce can be traced to the Mediterranean basin from where it spread to the rest of the
world. It has been cultivated as a food source for thousands of years and is very popular as a salad
ingredient all over the world. Lettuce is a very nutritious food source and generally the darker the leaf
the more nutritional value it has. It is a very good source of vitamins A, K, C, folate, manganese and
chromium.

2. ADAPTABILITY

2.1 CLIMATIC REQUIREMENTS

Essentially a cool weather crop, lettuce germinates at a temperature range of between 15 and 20ºC.
Germination may be compromised at temperatures above 25 ºC. The ideal temperature range for
growth is also between 15 and 20 ºC. It is more challenging to grow lettuce in summer, as the
disease pressure is higher than in winter and the hot conditions may induce bolting in certain
cultivars. However Starke Ayres has a range of cultivars adapted for the various seasonal slots

2.2 SOIL REQUIREMENTS

Well drained loamy soils with a pH of 6-6.8 are recommended, with a rooting depth of between 450 –
600mm.

2.3 PRODUCT TYPES

There are 4 main types of lettuce:
CRISPHEAD – The most popular type of lettuce grown in South Africa. Its leaves are tightly
packed together producing a firm head with crispy leaves.
BUTTERHEAD –These types of lettuce produce loose heads, with a butter texture with soft waxy
leaves and a mild flavour.
LOOSELEAF - This type of lettuce does not form heads, the leaves are joined at the stem.
Varieties include: oak leaf, lolla rosa, lolla bionda, Multi-leaf and green leaf.
ROMAINE OR COS - This type of lettuce has a more upright or oval head shape with dark outer
leaves with a crispy texture. Considered to be more nutritious than Crisphead lettuce.

3. CULTIVATION PRACTICES
3.1 SOIL PREPARATION
The soil should be prepared thoroughly and deeply before planting. The soil (if necessary) should first
be ripped and then ploughed and disced.

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 3.2 PLANTING PERIODS
Cultivars are available for various planting/harvesting slots throughout the year. Starke Ayres have
an extensive range. More information regarding the cultivar range can be obtained from the Starke
Ayres website, www.starkeayres.co.za or contact your nearest sales representative for the latest
information or for assistance in cultivar recommendations.

3.3 SEEDLING PRODUCTION

Generally seedlings are made by sowing seeds in seed trays (200 cavity trays). Seeds should be
sown at a depth of between 0.6cm and 1.3 cm. Germination may be compromised at temperatures
above 25 °C. The optimal temperature for germination is 20 °C.

Lettuce is essentially a temperate crop and is sensitive to temperature fluctuations. This is especially
important for the germination of lettuce seed which may experience thermodormancy in hot summer
conditions. Lettuce seed may enter a state of dormancy and consequently fail to germinate at high
temperatures. Dormancy occurs when seed is exposed to high temperatures while it imbibes
(absorbs) water. Thermodormancy is a condition that may be induced at temperatures above 25 °C.
Some varieties are more susceptible to thermodormancy than others. For example, triple red lettuce
varieties seem to be more affected by this phenomenon than other varieties.

There are several interventions that have been initiated to circumvent thermodormancy and include
the following:

1. Priming. Seed priming is probably one of the most effective ways to overcome thermodormancy.
- On request, Starke Ayres supplies primed and pelleted seed of certain cultivars.
- Primed seed must at all times be kept in a refrigerator and used immediately after purchase
as primed seed has a very limited shelf life.
2. Germinating in a germination room at correct optimal temperatures can also overcome the problem
of thermodormancy.
3. Pre-watering soil – the soil cools as the water evaporates.
4. Plant early in the morning as soil temperatures will be lowest at this time.
5. If possible, keep the soil "dark damp" for about the first 6 hours.
6. Pre-chill trays or water with chilled water.

3.4 PLANT POPULATION AND SPACING

For Crisphead cultivars a population of approximately 45 000 – 60 000 plants per hectare is
recommended. Higher plant densities result in smaller the head size. A larger head size usually
commands a higher price.

Speciality lettuces can be grown at much higher densities than Crisphead cultivars. For open field a
population of between 70 000- 80 000 plants per hectare is recommended and for hydroponics a
population of 80 000-100 000+ plants.

3.4.1 TRANSPLANTING SEEDLINGS

Seedlings should be transplanted between 4 - 6 weeks after sowing. Seedlings must be transplanted
at the correct depth in a little planting hole that has been made in the ground prior to planting. If the
seedlings are forced into the ground, without a hole being prepared for them to be inserted into, the
root system will be damaged and the plant will experience stress resulting in poor yield. Once the
seedling is placed inside the hole the area should be firmed so that sufficient contact is made
between the seedling and the soil.

When planting seedlings a choice can be made between the square method and the staggered
method of planting. The staggered method is more advantageous as there is less competition

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 between plants compared to the square method where plants are directly opposite each other,
thus maximizing competition.

4. FERTILISATION
The soil is a resource that needs to be managed and monitored meticulously. It is essential that
a grower have soil samples of the intended growing area analysed by an accredited laboratory to
determine the nutrient status of the soil. Based on soil analysis results a fertilisation programme
can then be developed. This programme is obviously specific for the type of soil that was
sampled and subsequently analysed. The grower should do this exercise every season or every
time a new crop is planted on the land.

In addition to having soil samples tested the grower should also have the quality of his water
analysed because water quality can have a direct effect on the growth of the plant. For example,
irrigation water with high calcium levels can increase the soil pH.
Fertilisation guideline (Grower must first have soil analysis done)
• 110 kg nitrogen (N), 14 kg phosphorus (P) and 190 kg potassium (K) per ha
• Care should be taken not to over-apply nitrogen

Side dressing of fertilisers at the correct times after transplanting is done with common fertilisers
such as LAN (Limestone Ammonium Nitrate). The placing of the fertilisers should be as close to
the plant as possible to ensure that the young seedling utilises the Nitrogen efficiently to produce
a framework that would contribute to large head development. (Side dress at approximately 4
weeks).

As mentioned previously a soil analysis should be done for the area that is to be used for plant-
ing. This analysis would also reveal the pH of the soil. Generally vegetables require a slightly
acidic environment in which to grow (a pH of between 6 and 6.8). Continued use of fertilisers on
a piece of land generally results in acidic soils. If the pH drops below 5 the uptake of important
minerals such as calcium, magnesium, potassium, molybdenum, phosphates and sulphur will be
compromised. A very acidic soil could also result in the disease club root forming in cabbage.

The addition of lime can correct land that has low pH values. The correct type of lime needs to be
applied. Basically the two types of lime used are calcitic or dolomitic. The ratio between calcium
and magnesium in the soil would determine the choice of lime to be used.

5. IRRIGATION
Soil moisture is a very important factor, therefore it is essential that the plants are given the
correct irrigation so as to avoid stress which will compromise yield. Frequent light irrigations for
lettuce is recommended. Weekly water requirements are 25-30mm.

6. WEED CONTROL
Weed control is extremely important, and the land under cultivation should be cleared of weeds
before planting by using the recommended herbicides. Weeds create competition and compro-
mise yield.

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 DISEASES & PESTS
Major Diseases
• Botrytis
• Downy Mildew-Bremia
• Fusarium wilt
• Powdery Mildew
• Lettuce Big Vein Associated Virus
• Lettuce Mosaic Virus
• Corky Root
• Pythium
• Rhizoctonia
• Sclerotinia
• Erwinia
• Tip Burn

Major Pests
• Aphids-Nasonovia
• Cut worm
• White Fly
• Thrips
• American Bollworm

HARVESTING AND MARKETING

Crisp head lettuce is harvested when the heads are firm. When harvesting, care must be taken
not to damage the wrapper leaves. The loose-leaf types are harvested when the leaves have
acquired the required size. A highly perishable crop, lettuce needs to be harvested early in the
day when temperatures are still cool. After harvesting store in a cool dry place and refrigerate if
possible. Avoid harvesting when plants are wet or after rains as wet foliage is more conducive to
rotting. Crisphead lettuce are marketed loose and packed in units of 12 in a carton for the fresh
market or as pre packs in a punnet. Loose leaf lettuce are often cut and further processed as
part of a prepack salad mix.

INDEMNITY
All technical advice and/or production guidelines given by STARKE AYRES or any of its personnel with reference to the use of its
products, is based on the company’s best judgement. However, it must be expressly understood that STARKE AYRES does not
assume responsibility for any advice given or for the results obtained.

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