Lettuce is produced on both mineral and muck (organic) soils.
Production practices and varieties are quite different for each soil type. This guide is directed to mineral
soil production unless indicated otherwise.
Four morphological types of lettuce dominate U.S. production, these are
crisphead, cos (or romaine), leaf, and butterhead. Two others, stem and Latin are rarely found,
although stem lettuce may be found in Oriental food stores.
The predominant lettuce grown in the U.S.A. is crisphead (iceberg or head lettuce)
which is best adapted for long distance shipment. "Batavian" varieties are sometimes classified as
loose-heading "crisphead" types. Leaf types are grown for local and regional markets
primarily, although, with proper cooling (vacuum cooling), packaging and refrigerated truck
transportation, these lettuces may also be distributed nationally.
Supermarkets are devoting increased space to cut lettuce and lettuce mixes of many types, some complete
with salad dressings. The supply of this profitable and popular product is dominated by a
handful of producers because of the large capital investment that must be made to produce this product
and meet the exacting demands of the produce industry. Although varieties and production practices are fairly
similar to conventionally marketed lettuces, several important differences exist in production, harvest
timing and handling, and the way the harvested product is processed and marketed. These differences will not
usually be discussed in this production guide.
Crisphead (Iceberg) types (approximately 60-75 days when direct seeded):
Transplant - Summertime, Ithaca, Fame, Salinas; PennLake (earliest plantings
only). New, for trial when downy mildew is expected: Target, Alpha (resistant to downy mildew
pathotypes I and III), and Bullseye, Top Gun, Patriot, Warrior (all tolerant to downy mildew
pathotypes I, II and III).
Direct Seeding - For harvest periods before July 15 and after September
1 and when downy mildew can be a problem: Alpha (resistant to downy mildew pathotypes I, III), Salinas,
Target. For trial when downy mildew is expected: Target, Alpha (resistant to downy mildew pathotypes I and III),
and Bullseye, Top Gun, Patriot, Warrior (all tolerant to downy mildew pathotypes I, II and III).
Others for trial: El Dorado, Etna (red crisphead), Fame, Mission, Pablo (a red-tinged crisphead).
For harvest between July 15 and Sep. 1: Summertime, Ithaca (both are
heat tolerant, both need special attention to irrigation and fertility (130-150 lb N/A) to produce
Batavian or French crisp or Summer crisp types (approximately 50-60 days,
"intermediate" between butterhead and crisphead):
Green: Nevada (resistant to downy mildew strains 1,2,3,4). For trial: Canasta, Cardinale (Summer Crisp), Loma.
Red: Sierra (heat tolerant).
Butterhead types (approximately 60 days)
Green: Balisto (dark green, slow bolting, BWY virus resistant), Divina (resistant to downy mildew strains 1,2,3,4), Butter King,
Dark Green Boston, Tania (downy mildew resistant), Augusta, Little Gem, Esmarelde, Optima.
Red: Sangria (resistant to downy mildew strains
1,2,3,4); Red Boston. For trial: Pybus Red Butter.
Bibb types, green (approximately 60 days):
Buttercrunch (heat tolerant), Little Gem (resistant to downy mildew strains 1,2,3,4), Salad Bibb, Summer Bibb. For trial:
Miniature bibb types: Tom Thumb.
Leaf types (approximately 50-60 days):
Green: Slobolt (non-bolting, BWY virus
resistant), Waldmann's Green (dark green, slow bolting), Salad Bowl, Royal Green, Grand Rapids.
For trial: Genecorp Green, Green Ice, Green Vision, Loriol (light green, slow bolting), Two Star.
Red: Garnet, Prize Head, Red Prize, New Red Fire (all are
slow-bolting and BWY virus resistant); Aragon Red, Deep Red, Redina, Red Sails, Royal Red.
Oak Leaf types (approximately 50-60 days)
Green: Krizet (bolting and BWY virus resistant).
Red: Raisa (dark red), Brunia (both resistant to bolting and
BWY virus); Red Salad Bowl.
Cos (Romaine) types (approximately 65 days)
Green: Olga, Parris Island Cos (bolting
resistant, vigorous growth), Green Towers, Valmaine (downy mildew
resistant). For Trial: Paris White, Corsair, Darkland, Rosalita.
Red: Majestic Red (slow bolting and resistant to BWY virus).
Stem Lettuce (approximately 85 days): Celtuce (for specialty Oriental
Mesclun mix greens:
"Mesclun" is the term applied to a blend of fresh, tender greens
combined for their variety of textures, flavors and colors grown and marketed
together. Leaves are harvested by cutting and plants are allowed to regrow. Ingredients
vary, consisting of half a dozen or more of any of the following lettuces and
Lettuces such as looseleaf, red leaf, oakleaf, romaine,
miners lettuce and others mixed with other greens such as: arugula, chicory, corn salad, dandelion, endive,
escarole, frisée, mizuna, mustard tips, radicchio, sorrel, spinach, tango, tat-soi, edible chrysanthemum, nasturtium leaves,
orach, parsley, travissio, kale, chard, mache, beet leaf, watercress, plantain, purslane and
herbs and flowers such as: basils, borage, chervil, chives, fennel,
salad burnet and blossoms of borage, calendula, nasturtium, violas, and violets.
Mechanical harvesters for mixed plantings of salad greens are now available. See the Harvesting
and Handling section, below.
Sandy peats and mucks, deep black sandy loams and loams are the most
suitable types of soil. Good moisture-holding capacity with good drainage is important, especially for
heading types. Soils that compact easily, or are compacted can adversely affect head lettuce
growth. For successful head lettuce production, soils should be managed to reduce
compaction as much as possible.
Germination occurs at as low as 40 F and may not occur at temperatures
of 90 F and over unless irrigation is used to cool the soil. Crop growth is usually good between 61
and 65 F. Lettuce is planted from April through mid-August.
Lettuce seed numbers approximately 400,000 per pound. Use only mosaic-indexed seed from a reliable seed source.
For direct field seeding 1/4 to 1/2 lb/acre is required when a precision
seeder is used with unpelleted seed. Pelleting greatly improves precision planting and reduces
thinning costs. Advances in priming and coatings can improve stand establishment under adverse
conditions. Consult your seed dealer about the availability of primed seed.
Use only fungicide-treated seed. Seed is available pelletized with
various types of coatings. Pelletized seeds are available in which the seed is vigorized, or conditioned,
so that it germinates rapidly even under high temperatures. Pelletizing facilitates precision
seeding in the field. The lighter coatings are preferred.
A soil test is the most accurate guide to fertilizer requirements. The
are general guidelines.
For the early crop, band 2 inches to the side and 2 inches below the
planting depth the following:
Nitrogen: 75-100 (N) lb/acre for heading types developed for California
production. Use 150-175 lb/A for Summertime and Ithaca grown on mineral soils, and for leaf lettuces
planted at 6-9 inches in-row spacings.
Phosphate: 150-200 (P2O5) lb/acre.
Potash: 60-200 (K2O) lb/acre
pH: Add lime if below 6.0
Slightly less N and P is required for the later
plantings of June and early July.
Head lettuce is most commonly direct seeded in the field. "Leaf" types are currently
mostly transplanted from greenhouse-grown plugs.
Present recommendations are to use coated seed. A single coated seed
placed every 2 to 3 inches, or two seeds spaced 1 inch apart every 12 inches, has worked very well.
Direct-seeded plants should be thinned when two or three true leaves have formed. Delaying thinning can
result in plants that are left to be disturbed or damaged, resulting in uneven harvest.
Raised beds are ideal for lettuce production. They help prevent damage
from soil compaction and flooding. This is especially important for the varieties Summertime and
Ithaca. Raised beds also improve air flow around the plants resulting in reduced disease incidence.
TRANSPLANT PRODUCTION AND TRANSPLANTING
For transplant production, one-quarter of a pound of seed will supply
sufficient seedlings to transplant one acre.
The earliest seedings are started in flats in greenhouses in early
February. The seedlings are then transferred into other flats or modular trays allowing 1.5 square inches
per plant and are planted out as soon as the fields can be prepared. Transplants put out early
will benefit from a starter solution high in phosphate. Spacing between rows is 14-18 inches with
11-13 inches between plants.
Lettuce requires frequent irrigations. As many as 8-10 irrigations and
10-12 inches of water per acre may be necessary depending on seasonal variation, variety and planting date.
Soil type does not affect the amount of total water needed, but does
dictate frequency of water application. Lighter soils need more frequent water applications, but less
water applied per application.
See also the OSU Irrigation Guide for Leafy Greens.
HARVESTING AND HANDLING
The University of California-Davis has a file on Minimal
Processing of Fresh Vegetables that discusses film wrapping and other topics.
Yields of crisphead lettuce are approximately 600-800 cartons per acre,
bibb and leaf lettuce approximately 800-1200 cartons per acre and romaine approximately 900-1000
cartons per acre (note container sizes and weights under PACKAGING, below).
Because lettuce is so fragile, it is handled as little as possible. Most fresh market lettuce is hand cut and trimmed, and
placed in cardboard cartons in the field. It is then trucked to a central
area for vacuum cooling. In a few areas it is not vacuum cooled, but
placed in a cooler for temporary holding until trucked to market. No
lettuce is washed before it gets to the store, but some may be hydrocooled
or hydro-vacuum cooled.
Cut lettuce, which is found in grocery stores in plastic bags "ready to
eat," is harvested in a different fashion. Crews hand cut and core the lettuce and place
it in bulk containers which are transported to a processing facility. There
it is cut and washed (in suitably cold water). It is then centrifuged to remove
excess water and is often mixed with other types of lettuce or greens, shredded
carrot and/or red cabbage. It may be treated with a chlorine-containing compound and/or an antioxidant or preservative during washing or before packaging.
It is then bagged in special plastic films that
maintain, internally, a certain ratio of atmospheric gases (N2, O2 and CO2)
that is different from ambient (lower in O2 and higher in CO2). The bags are
then placed in cartons for temporary cold storage or for immediate shipment to
Specialty leaf lettuces and other greens for bag mixes have usually been hand harvested, but
harvesters for this purpose are now available. Three are:
Lettuce and other leafy items must be kept clean,
and free of soil and mud. A stronger bitter taste and toughness develops if
harvest is delayed or if
crop is over- mature, and then the product becomes unmarketable.
Green Crop Harvester, made in England. Sole US distributors are C. and
K. Anderson, Fresh Herb Co., 4114 Oxford Rd, Longmont, CO. The cost $20,000 (1998 prices)
for a 4-foot wide model which hold the greens upright by chain-driven
sweeps and cuts the greens with a reciprocating knife (like a hedge trimmer).
A picture of the machine can be seen in Johnny's Select Seed '98 catalogue,
- Quick Cut harvester, an Italian, battery-powered, walk-behind machine with
a 39", 48" or 54"-wide head and a band-saw cutter. Cost is $11,000. Sold by
Ferrari Tractor CIE, PO Box 1045, Gridley, CA 95948; and
by David Washburn and Meg Anderson of Red Cardinal Farm, 9694 75th St.
North, Stillwater, MN 55082.
- Enha Pro, a human-powered machine designed by Norbert Hufnagl, Field of
Dreams, 117 Fredon Springdale Rd., Newton, NJ 07860.
Cost is $2,429 for a two-head unit and $2,966 for a three-head unit.
Lettuce is extremely perishable and needs to be handled delicately, and
marketed rapidly. Lettuce
may be held temporarily at 32 F and 90-95% relative humidity for several
Head lettuce is harvested when the heads are of good size (about 2
lbs), well formed and solid. If
the plants are wet with rain or dew the leaves are more brittle and break more
easily. Leave three
undamaged wrapper leaves on each head. Put 24 heads in rigid cardboard
containers in the field
and avoid bruising. Grade heads according to size, pack in cartons (vacuum
cooling is mandatory) for long shipments. Leaf, butterhead and cos types are cut,
trimmed and tied into compact bundles before placing in cartons.
STORAGE (Quoted or modified from USDA Ag. Handbook 66 and other sources)
Hold lettuce at 32 F and 98 to 100 % relative humidity. Lettuce should
be precooled to 34 F
soon after harvest and stored at 32F and 98 to 100 % relative humidity for
retention of quality
and shelf life. Precooling is commonly done by vacuum cooling because it is
more effective and
rapid that hydrocooling. Also, since most head lettuce is field packed in
vacuum cooling is more suitable. For vacuum cooling, containers and film
wraps should be
perforated or readily permeable to water vapor. To aid vacuum cooling, clean
water is sprinkled
on the heads of lettuce prior to carton closure if they are dry and warmer
than 75 F. Thorough
precooling is essential because mechanically refrigerated rail cars or trucks
do not have enough
cooling capacity to cool warm lettuce during transit.
Lettuce is highly perishable and deteriorates rapidly with increasing
temperature. The respiration
rate increases greatly storage life decreases concomitantly as the storage
over the temperature range from 32 to 75 F. Leaf lettuce respires at about
twice the rate of head
lettuce. At 32 F, head lettuce can be held in good condition for 2 to 3
weeks, the time period
depending on maturity, quality, and handling condition of the lettuce at
harvest. The storage life
at 38 F is only about half at that at 32 F.
Lettuce is easily damaged by freezing, so all parts of the storage room
must be kept above the
highest freezing point of lettuce (31.6 F).
Controlled atmosphere is of limited benefit to the storage quality of
lettuce. Low oxygen levels of
1 to 8 % can reduce russet spotting in susceptible lots. A 3 percent oxygen
and 1.5 % carbon
dioxide atmosphere maintains the appearance of lettuce and inhibits pink rib
and butt discoloration
better than air, but the effect is not noticeable after the lettuce is held at
50 F in air for 5 days.
Oxygen below 1 % is injurious, as is carbon dioxide above 2.5 percent. High
levels cause brown stain, which may develop after lettuce is transferred to 50
F air. Brown stain
caused by high carbon dioxide is intensified when oxygen is reduced to 2 to 3
%, but the degree
differs with cultivar. If lettuce needs to be in transit over-seas for a
month, an atmosphere of 2 %
carbon dioxide and 3 % oxygen is recommended, because the reduction in decay
achieved by 2 %
carbon dioxide outweighs the danger of injury.
Lettuce should be held at high relative humidity, 98 to 100%. Film
liners or individual
polyethylene head wraps are desirable for attaining high relative humidity;
however they should be
perforated or be permeable to maintain a non-injurious atmosphere and to avoid
100 % relative
humidity on removal from storage. Romaine and leaf lettuce appear to tolerate
a slightly higher
carbon dioxide level when packaged than head lettuce.
Russet spotting, which occasionally causes serious losses, is usually
not a problem at temperatures
below 36 F. Lettuce should not be stored with apples, pears, cantaloupes, or
other products that
give off ethylene, as this gas increases russet spotting. Hard heads are more
susceptible to this
disorder than firm lettuce. Storage in a low-oxygen atmosphere (1 to 8%) is
very effective in
controlling russet spotting.
Soft rot, the most serious disease of lettuce, often starts on bruised
leaves, but it is much less
serious at 32 F than at higher temperatures. Tipburn is also a major market
disease of lettuce. It
is of field origin, but occasionally increases in severity after harvest.
All packaging is done before vacuum cooling. Iceberg or head lettuce
may be closely trimmed
and wrapped in film. Polyethylene films are most common but new PVC films
longer. PVC films are also easier to wrap and result in a neater wrapping.
Iceberg lettuce: commonly packaged in 43 to 48-lb, 24-count, cartons.
Boston lettuce: commonly packaged in 20-lb cartons.
Romaine lettuce: commonly packaged in 24-count cartons.
Leaf lettuce: commonly packaged in 20 to 25-lb or 24-count cartons.
Bibb lettuce: commonly packaged in 10-lb cartons.
Greenhouse lettuce: commonly packaged in 10-lb cartons.
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