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ICEBERG LETTUCE PRODUCTION IN

CALIFORNIA


THOMAS TURINI, UC Cooperative Extension Farm Advisor, Fresno County;

MICHAEL CAHN, UC Cooperative Extension Farm Advisor, Monterey, Santa

Cruz, and San Benito Counties; MARITA CANTWELL, Postharvest Extension


Specialist, UC Davis; LOUISE JACKSON, Professor and Cooperative Exten-

sion Specialist, Department of Vegetable Crops, University of California, Davis;

STEVE KOIKE, University of California Cooperative Extension Farm Advisor,

Monterey County; ERIC NATWICK, UC Cooperative Extension Farm Advisor,

Imperial County; RICHARD SMITH, UC Cooperative Extension Farm Advisor,


Monterey, Santa Cruz, and San Benito Counties; KRISHNA SUBBARAO, Plant

Pathology Extension Specialist, UC Davis; ETAFERAHU TAKELE, UC Coopera-


tive Extension Area Farm Advisor, Southern California Counties.


PRODUCTION AREAS AND SEASONS

The major production areas for iceberg (crisphead)

lettuce (Lactuca sativa) in California are the Central

Coast (Monterey, San Luis Obispo, San Benito, Contra

Costa, and Santa Clara Counties), the southern coast

(Santa Barbara and Ventura Counties), the Central Valley

(Fresno and Kings Counties), and the southern deserts

(Imperial and Riverside Counties). Production is highest

in Monterey County, followed by Imperial County.


Planting to harvest takes 70 to 80 days for

midsummer plantings and as long as 130 days for

late-fall or winter plantings. In the southern deserts,

iceberg lettuce is planted from mid-September to mid-

November for harvest from early December to early

March. In the lettuce-growing areas of the Central Coast,

where temperatures are fairly uniform year-round,

lettuce is planted from late December to mid-August

for harvest from early April to November. Southern

coastal plantings are made from November to August

for harvest from April to December. In the Central

Valley, iceberg lettuce is planted from mid-August to

early September for harvest from late October through

November. Spring plantings are made in November for

harvest in late March and April.


ICEBERG LETTUCE ACREAGE AND

VALUE


Year Acreage

Average yield

(tons/acre)


Gross

value/acre


2007 119,000 19.9 $7,810


2008 116,000 19.9 $7,562


2009 114,000 20.7 $7,915


Source: California Agricultural Resource Directory 2010

(Sacramento: California Department of Food and Agriculture,

2009).


CLIMATIC REQUIREMENTS

Lettuce is a cool-season crop with distinct

temperature requirements. The optimal growing

temperatures are 73ºF (23ºC) during the day and 45ºF

(7ºC) at night. Most California growing regions have

daytime temperatures from 63º to 83ºF (17º to 28ºC)

and night temperatures from 37º to 53ºF (3º to 12ºC).

High temperatures may cause bolting, bitterness,

poor head formation, and tipburn. At temperatures

near freezing, young plants are not damaged, but

growth is slow. Freezing can damage the outer leaves

of mature lettuce, leading to decay in handling and

storage.


VARIETIES AND PLANTING

TECHNIQUES

Lettuce varieties are adapted to specific planting

periods (slots) in the Southern California deserts.

Planting a variety out of slot will result in

nonheading, puffiness, and unacceptable core

lengths or bolting. Moderately high temperatures

can occur in early spring. As the season progresses,

temperatures change from extremely hot days to

cooler days and freezing nights.


Varieties commonly planted in Southern

California include Accolade, Annie, Beacon, Bubba,

Cibola, Coolguard, Coyote, Crusader, Del Rio, Del

Oro, Desert Queen, Desert Spring, Desert Storm,

Diamondback, Gabilan, Gran Max, Heatmaster,

Honcho II, Jackal, Javelina, Jupiter, Kofa, Lighthouse,

Mid Queen, Prestige, Raider, Sahara, Silverado, Sun

Devil, Valley Queen, Winterhaven, Dominguez 67,

Red Coach 74, and Wellton.


On the Central Coast, resistance to downy mildew

and corky root diseases are important considerations

for variety selection. Current varieties are the Salinas

types: Bay View, Cannery Row, Durango, El Dorado,


Vegetable

Production Series


vric.ucdavis.edu


UC Vegetable Research

& Information Center


University of California

Agriculture and Natural Resources


http://anrcatalog.ucdavis.edu • Publication 7215


http://anrcatalog.ucdavis.edu
http://vric.ucdavis.edu/


Hallmark, Laguna Fresca, Legacy, Legend, Liberty,

Pybas 251, Sharpshooter, Silverado, Sniper, Sureshot,

Target, Telluride, Tiber, and Trojan.


In the Central Valley, the most common fall

varieties include Crusader, Diamondback, Raider,

Sidewinder, and Sun Devil. Spring-planted varieties

include Annie, Bubba, Desert Spring, Desert Storm,

Diamond, Grizzly, Headmaster, Lighthouse, Mohawk,

Navaho, Red Coach 74, Valley Queen, and Winter

King.


Most iceberg lettuce is planted using pelleted

seed and a precision planter; very little lettuce is

transplanted in California. Seed are planted 1.75

to 3 inches (4.4 to 7.5 cm) apart in rows on 40-inch

(102-cm) beds. At a 2-inch (5-cm) spacing there

will be 157,000 seed per acre (388,000 seed/ha).

Lettuce is thinned to a spacing of 10 to 12 inches

(25.4-30.5 cm). The cost of seed varies with variety,

coating, spacing, seed enhancement, and priming

(osmoconditioning) treatments. Nonprimed, natural

lettuce seed may be susceptible to thermodormancy

when ambient temperatures are above 90ºF (32ºC)

for an extended period. Priming allows the seed to

overcome thermodormancy and germinate at higher

temperatures. Primed seed is commonly used in low

desert production areas. Thermodormancy can also

be broken by starting the initial irrigation in the late

afternoon so the seed can imbibe water and germinate

during the cooler hours of the night.


SOILS

Iceberg lettuce grows best in silt loams and sandy

soils in the southern deserts. Lighter-textured

soils provide better drainage during cold weather

and warm up more readily. In the Central Coast

and Central Valley, lettuce can be grown on heavy

clay soils as long as there is good soil structure and

adequate drainage. Lettuce is moderately salt sensitive:

electrical conductivity of the soil extract in excess of 2.5

dS/m may reduce seed germination and/or growth.

Irrigation water with salinity levels of less 1 dS/m are

considered the most suitable for lettuce.


IRRIGATION

In the southern deserts, most growers use sprinklers

for the first 5 to 7 days or until the seedlings emerge.

The field is then furrow irrigated for the remainder

of the season. In the southern deserts, 3 acre-feet

(3,700 m3) of water per acre is typically used to grow

a lettuce crop. The majority of the water is applied

in the last 30 days before harvest. Care must be

taken not to oversaturate the beds when growing

early-season lettuce—excess moisture favors the

development of bottom rot. Gated pipe is also used

to deliver water, especially near harvest. Gated pipe

allows uniform application of water down furrows


and maintains a dry head basin so that harvest

equipment can turn around on dry soil.


Most Central Coast fields are preirrigated with

about 2 to 4 inches (5 to 10 cm) of water, depending

on initial conditions, to soften soil for listing and

seedbed preparation. Both seeded and transplanted

lettuce are sprinkle-irrigated frequently (every 2 to

3 days) until seedlings emerge or are established

(usually 6 to 10 days). Both hand-move and solid-set

sprinklers are used for stand establishment. After

emergence the crop is irrigated less frequently until

thinning about 2 to 3 weeks after seeding. Water is

usually applied to soften the soil before thinning

with hoes and applied again after side-dressing of

fertilizer. Sprinklers are continued after thinning and

side-dressing on most of the acreage, but furrow

irrigation is still used in some areas. In regions with

strong winds, a well-managed furrow system may

provide better irrigation distribution uniformity than

sprinklers. Depending on soil type and terrain, fields

may be sprinkled to maturity with hand-move, linear-

move, or permanently buried sprinkler systems. In

late summer or fall when corky root disease can be

a problem, sprinkler irrigation is often used because

the plants’ root systems are degraded. As the crop

approaches maturity, excess water and fertilizer

causes heads to become large and puffy, reducing

their value. The volume of applied water is typically

1.5 to 2 acre-feet per acre (750 to 1,000 m3/ha) for a

sprinkler-irrigated lettuce crop and 2 to 2.5 acre-feet

per acre (1,000 to 1,250 m3/ha) for a furrow irrigated

crop on the central coast. Drip-irrigated lettuce

requires approximately 1 to 1.5 acre-feet of water per

acre (500 to 750 m3/ha).


Surface-placed drip has become a major method

of irrigating lettuce on the Central Coast. Drip

irrigation was used on 30% of the vegetable acreage

in the Salinas Valley in 2006. Surface drip is usually

installed after the first cultivation and sidedressing,

and it permits growers to water frequently during

the phase of rapid vegetative growth. One drip line

is installed between 2 plant rows on 40-inch (1-m)

beds, or 3 drip lines are installed between 6 or 5

plant rows on 80-inch (2-m) beds. The drip lines are

typically retrieved before harvesting and reused for

subsequent crops. Drip can potentially distribute

water more uniformly than furrow or sprinklers and

has helped growers attain uniform growth in fields

with variable soil textures by maintaining similar

soil moisture levels in all areas of the field. Drip

can be managed to minimize leaching of nitrate-

nitrogen (NO3-N) by fertigating weekly with low

rates of fertilizer and applying less water more

frequently than can be achieved with sprinkler and

furrow systems. A subset of growers bury drip tape

to a depth from 2 to 3 inches (2.5 to 7.6 cm) before

planting. Drip tape is rarely buried deeper than 3


2 • Iceburg Lettuce Production in California


inches for lettuce. Some growers use the shallowly

buried drip tape for full-season irrigation, including

for germination. High bicarbonate, manganese,

or iron levels in groundwater in some areas of

the Central Coast can plug drip emitters. Acid is

periodically injected to remove bicarbonate and iron

precipitates.


The combination of soil moisture monitoring

and weather-based irrigation scheduling can

help determine the water needs of lettuce. Water

use is highest during the 20 days of the crop when

vegetative growth is high. Soil moisture tensions

are typically targeted for less than 25 to 30 cbars

(25 to 30 kPa) during this period. The water

extraction of lettuce can be estimated using reference

evapotranspiration data adjusted with a crop

coefficient, which is closely related to the percentage

of ground covered by the canopy. At a maximum

canopy cover of 85%, the crop coefficient is nearly 1.0.

Because evaporation represents a majority of the water

loss during the early stages of growth, a crop coefficient

between 0.3 and 0.7 should be used for overhead

sprinklers, depending on the irrigation frequency, until

the canopy is greater than 30% cover. The California

Irrigation Management Information System (CIMIS,
http://wwwcimis.water.ca.gov), coordinated by the

California Department of Water Resources, provides

daily estimates of reference evapotranspiration for

most production regions of California.


FERTILIZATION

Soils in the central and south coast regions can have

elevated levels of nitrate-nitrogen and phosphorus,

which can cause elevated levels of these nutrients in

runoff; this makes it difficult for growers to comply

with water quality standards established by local

regional water quality control boards. As a result,

application of these nutrients needs to be carefully

managed.


Phosphorus fertilization should be applied based

on the soil test level of bicarbonate-extractable

phosphorus. Levels above 60 ppm are adequate for

lettuce growth; for soils below this level, especially in

the winter, preplant applications of 40 to 80 pounds

per acre (45 to 90 kg/ha) of P2O5 or at-planting

applications of 20 pounds per acre (22 kg/ha) of P2O5

are recommended. The need for potassium can also be

determined from soil tests; soils with greater than 150

ppm of ammonium acetate–exchangeable potassium

have sufficient quantities of potassium for the crop.

Potassium fertilization presents no environmental risk,

and many growers routinely apply potassium even in

fields with high levels of exchangeable soil potassium.

While fertilizing to replace potassium removal with

the harvested crop, approximately 120 pounds per acre

(134 kg/ha), is appropriate to maintain soil fertility,


fertilization rates above that level are economically

wasteful. Zinc fertilization is recommended if the

DTPA extractable soil level is less than 1.5 ppm. Zinc

fertilization is commonly practiced on the Central

Coast due to high soil phosphorus levels, which

reduces zinc uptake by plants.


Fall application of nitrogen is not recommended

due to the risk of nitrate-nitrogen leaching beyond

the root zone by the winter rains. Small quantities

of nitrogen, 20 pounds per acre (22 kg/ha), are

applied preplant or at planting. At thinning, 50 to

80 pounds per acre (56 to 90 kg/ha) of nitrogen is

sidedressed into the beds. One or more additional

sidedressings are common, typically several weeks

apart. Seasonal nitrogen application to the first lettuce

crop of the year on the Central Coast range from 150

to 180 pounds per acre (168 to 202 kg/ha) of nitrogen.

Due to residual nitrogen from prior crops and

mineralization of nitrogen from soil organic matter

the fertilization rates for the second crop of lettuce

typically range from 100 to 150 pounds per acre (112

to 168 kg/ha) of nitrogen. The sidedress nitrogen

requirement can be estimated by pre-sidedress soil

nitrate testing (PSNT). Soil nitrate levels greater than

20 ppm in the top 12 inches (30 cm) are adequate

for crop growth; the test can be repeated later in the

season to ensure continuing nitrogen sufficiency. A

small quantity of nitrogen, 10 to 15 pounds per acre

(11 to 17 kg/ha), is often applied 7 to 10 days prior

to harvest to assure that the crop color and growth

rate are acceptable. In drip-irrigated fields nitrogen

can be applied through the drip system as well.

Typically, drip systems are more efficient at managing

water and delivering nitrogen fertilizer, and therefore

fertilizer application rates are often 20 to 30% lower

than in conventionally irrigated fields.


In the southern deserts and the Central Valley,

where soil test phosphorus is usually lower than

on the Central coast, growers apply preplant P2O5

at rates as high as 250 pounds per acre (280 kg/ha).

Nitrogen can be applied preplant at rates as high as

50 pounds per acre (56 kg/ha); it is sidedressed just

after thinning and during later growth. Early, warm-

season lettuce has a shorter growing season than a

crop grown in January and February, and it usually

receives less nitrogen fertilization. A seasonal total of

approximately 150 pounds of nitrogen per acre (168

kg/ha) is typical for early-season crops, while 200

to 250 pounds per acre (224 to 280 kg/ha) is applied

during cold weather.


Lettuce is sensitive to high levels of ammonium in

the soil. Ammonium toxicity typically occurs in the

early spring (March-April) when soils are cool and

the transformation from ammonium to nitrate is slow.

Injured roots may have the tip of the root browned

off and may also develop a hollow, reddish, brown

cavity on the inside of the upper part of the root.


3 • Iceburg Lettuce Production in California


http://wwwcimis.water.ca.gov


4 • Iceburg Lettuce Production in California


Ammonium toxicity can also occur later in the season

(June-July) on heavier soils.


Due to food safety concerns manures are not used

in lettuce production; composted manures and yard

wastes are used by some growers. Application rates

are typically 4 tons per acre (9 t/ha) and are primarily

applied to maintain good soil structure.


INTEGRATED PEST MANAGEMENT

Contact the UC Davis IPM Web site at http://www.

ipm.ucdavis.edu or your local UCCE Farm Advisor

for current pest management information (see the UC

IPM Pest Management Guidelines for Lettuce, http://
www.ipm.ucdavis.edu/PMG/selectnewpest.lettuce.

html, ANR Communication Services Publication 3450,

October 2010).


Weed management. Several herbicides are used

for lettuce weed control. Consult your UCCE Farm

Advisor or the UC IPM Web site for more details on

the best approach to controlling weeds under your

conditions. Preemergence herbicides are typically

applied in a band 5 to 6 inches (12.7 to 15.2 cm) wide

over the seedlines after planting prior to the first

irrigation. Cultural weed control methods used by

both conventional and organic growers include the

use of preirrigation followed by shallow cultivation.

Burndown herbicides or propane flaming also

provides significant weed control and can be repeated

to provide additional control. Other important

cultural weed control tools include mechanical

cultivation and control of weeds before they go to

seed both in and around the field. Lettuce is thinned

and weeded approximately 30 days following

planting. An additional hand weeding is carried out 2

to 3 weeks later.


Insect identification and control. The most

important insect pests of lettuce in California are

aphids, leafminers, caterpillars, thrips, and whiteflies.

Pest problems vary according to the growing region

and time of year.


The lettuce aphid (Nasonovia ribisnigri) became

established on the Central Coast in 1998 and has since

become the most important insect pest of lettuce in

that area. Lettuce aphid infests the inner leaves of

the lettuce head, making it unmarketable. Foxglove

aphid (Aulocorthum solani) also infests the inner

leaves of lettuce. Green peach aphid (Myzus persicae)

and potato aphid (Macrosiphum euphorbiae) can be

significant pests of Central Coast lettuce, although

they tend to build up on the outer leaves, making

them easier to treat with insecticides. Since lettuce

aphid and foxglove aphid become protected within

the lettuce head as more leaves develop, detection

and treatment of incipient populations is essential.

Lettuce aphid and foxglove aphid may not be


adequately controlled with neonicotinoid insecticide

treatments via soil injection or foliar sprays. Aphid

pests of lettuce have many natural enemies, including

fungal pathogens that are common during cool, wet

spring weather. Parasitic wasps help suppress aphids

species that colonize outer leaves of lettuce. Syrphid

fly larvae and other aphid predators such as ladybird

beetles can help suppress aphid species that infest

inner leaves. Predation of lettuce aphid by syrphid

fly larvae is essential for organic production of leaf

lettuce on the Central Coast. A variety of Nasonovia-

resistant romaine lettuce called Nirvanus is available,

but it is not resistant to other species of aphid.


The primary damage from leafminers is caused

by the larvae, which form tunnels between the upper

and lower leaf surface, feeding on the mesophyll

tissue. In addition, female leafminer flies puncture

leaf surfaces with their ovipositor in order to extract

fluid on which to feed, and causing stippling

damage in the process. Leafminer larvae are highly

susceptible to parasitism by parasitic wasps,

especially those in the genus Diglyphus. Parasitic

wasps can help suppress leafminer populations if

insecticides do not interfere with their activity. In

coastal areas, the pea leafminer (Liriomyza langei)

is the most common leafminer. The serpentine

leafminer (Liriomyza trifolii) is the prevalent species

in the southern region, and the vegetable leafminer

(Liriomyza sativae) is also found in coastal areas.

Insecticid

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