Lettuce

Lactuca sativa

Last revised February 11, 2010
 

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.


VARIETIES

See the Vegetable Variety Selection Resources page to find varieties that have been shown to perform well in the Pacific NW.

SOIL

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.


SOIL TEMPERATURE

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.


SEED

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.


SEED TREATMENT

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.


FERTILIZER

A soil test is the most accurate guide to fertilizer requirements. The following recommendations 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.


PRECISION SEEDING

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.


IRRIGATION

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

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 market.

Specialty leaf lettuces and other greens for bag mixes have usually been hand harvested, but harvesters for this purpose are now available.

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. 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 days.

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 corrugated cartons, 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 temperature increases 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 carbon dioxide 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.


PACKAGING

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 maintain freshness 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.

 

Tags:

Fresh Market Vegetable Production, Lettuce