FARMER'S BOOKSHELF

An information system of tropical crops in Hawaii
Department of Tropical Plant & Soil Sciences
University of Hawaii at Manoa



Lettuce

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Climatic Requirements


Head lettuce requires a relatively low average temperature, particularly during the latter part of the growing cycle to form solid heads. Best average temperatures are between 58 degrees - 70 degrees F. Average temperatures above 75 degrees F will produ ce loose, soft-headed lettuce. High temperatures will also cause the head lettuce to bolt before heads mature.

In general, all lettuces and endive would benefit from cool nights (50 degrees F to 60 degrees F), which tends to enhance its mild sweet flavor; whereas high temperatures in general (above 75 degrees F) tend to produce strong flavors (bitterness). Certai n semi-head and leaf lettuce are more tolerant to high temperatures and do not develop strong flavors.

Day length variation does not appear to affect plant/head development to any significant extent.


Basic Cultivars


There are six recognized types of lettuce described by Ryder (1986).


Culture


Soil Preparation

The soil should be deep plowed wherever practicable. If manures are available, it should be plowed in at the rate of 3 to 5 tons per acre 6 - 8 inches deep. The soil should be worked to a fine texture to insure good germination of the small seeds (if di rect seeded).

For most local soils, it should be bedded 4 to 8 inches high and about 2 to 5 feet wide depending on the type of lettuce and cultural practices employed by the grower. Generally, transplanted plantings have narrower beds.

In areas where there may be periods of drought, flat culture may be advantageous in minimizing effects of limited water supply on the crop.


Irrigation

Almost all of our local lettuce is sprinkler irrigated. For optimum growth, a lettuce crop requires a constant and relatively abundant supply of moisture throughout the growing period. Fluctuations in soil moisture especially during the later stages of development are severely detrimental to optimal growth. Too much water during this period along with high temperatures may result in loose, puffy heads in heading types of lettuce. Too dry conditions during this period may induce premature bolting.


Thinning

If the field is direct seeded, it will be necessary to do some thinning. In situations where coated seeds or seed tape are used, thinning may be omitted. However, for regular direct seeded fields, one should first "block" the emerging seedling 10 - 14 days after planting. "Blocking" is to remove all plants using a hoe from the row except small clusters, 12 - 15 inches apart. A few days later all but one of the plants should be removed from each cluster, usually by hand.

Blocking and thinning should not be delayed until the seedlings suffer from crowding. It may be necessary to replace the soil around the plants, after completion of the thinning operation, to avoid injury to the seedlings left growing. If thinning is done, it may be one of the most laborous and expensive operation in the production of lettuce.


Diseases


Tipburn

This disorder is considered to be a physiological breakdown of the cells in the expanding lettuce leaf. No disease organism has been associated with the disorder. In the field, tipburn occurs at the time of harvest and may cause loss of an entire field. It manifests itself as a necrosis of the margins of actively growing inner head leaves.

The basic internal causes and environmental influences are not clearly understood. Calcium nutrition seems to be basic to the events leading to tipburn, but the specific role of calcium and the reason for its critical role in the tipburn syndrome are matters of speculation. Slow calcium mobility in the plant during periods of rapid growth may fail to keep pace with tissue development which leads to tissue weakness.

Some researchers believe that an inadequate supply of calcium restricts protein synthesis and the presence of free amino acids may cause toxicity in affected tissue. Others have linked tipburn to the rupture of the latex-bearing duct system, releasing latex into the surrounding tissue. Whether this rupture is a cause or an effect of tipburn is not entirely clear.


Disease Pests

  1. Spotted wilt virus

  2. Erwina carotovora sub. sp. carotovora, stump rot

  3. Sclerotinia sclerotiorum, drop

  4. Rhizoctonia, bottom rot

  5. Bremia lactucae, downy mildew

  6. Botrytis cinerea, crown and head rot


Fertilizer


Optimum pH= 6.1 - 6.8

Amount of N, P, K taken up by Average Crop (lbs./acre)

N - 95 P - 12 K - 170 Ca - 20

Approximately 70% is taken up in the last three weeks of growth.


Soil Amendment and Fertilization

Have a soil test done, if the soil pH is less than 6.0 or the available calcium is less than 2,000 lbs./acre, apply and incorporate agricultural lime at the rate of 2,000 lbs./acre (4.5 lbs./100 square feet) 8 - 12 weeks before planting.

For soils with less than 50 lbs./acre available phosphate, apply 1,000 lbs./acre (2.25 lbs./100 square feet) of treble super phosphate or its equivalent and incorporate. Both lime and the phosphate could be incorporated together.

For soils with less than 500 lbs./acre of available MgO, apply magnesium sulfate (epsom salt, 9.8% Mg.) at the rate of 200 lbs./acre as a soil application at planting.

For soils low in potassium, murate of potash (0-0-61) at the rate of 300 to 350 lbs./acre should be applied, half of the total at planting and the remainder combined with complete fertilizer and applied post plant.

After adjusting deficiencies for the above major soil conditions or for soils within the acceptable moderate range of available nutrients and pH about 1,500 - 1,800 lbs. (4 - 4.5 lbs./100 square feet) of 10-10-10 or equivalent fertilizer should be suffici ent for a crop of head, Romaine and endive on most Hawaiian soils. This should be applied in three applications (assuming direct seeded) --- one-half at planting, one-fourth 3 - 4 weeks later and one-fourth at 5 - 7 weeks. Supplemental applications of n itrogen at the rate of 100 lbs. per acre of urea (46-0-0) or 200 lbs. per acre sulfate of ammonia (21-0-0) may be applied 4 - 5 weeks after seeding for semi-head and leaf lettuce and 6 - 7 weeks after seeding for head, Romaine and endive. Note for head l ettuce: The application of nitrogen as a side dressing late in the development of the plant may cause too vigorous a growth rate, resulting in large, loose heads or an undesirable ribbiness.

For semi-head or leaf lettuce, 800 - 1,000 lbs. (1.75 - 2.25 lbs./100 square feet) of 10-10-10 or equivalent fertilizer should be sufficient for a crop. One-third could be applied at planting, one-third 2 - 3 weeks later and the remainder 4 - 5 weeks aft er seeding.

Although the uptake of nutrients by lettuce is low compared to that of other vegetables, high fertility is required for good production. This is because of the plants limited root system for absorbing nutrients and the necessity for rapid continuous grow th. Approximately 80% of the growth of lettuce occurs during the 3 - 4 weeks prior to harvest. It is at this time that nutrition is critical and tipburn most common. Tipburn may be connected to nutrition to a limited extent and will be discussed more i n a later section under common physiological disorders in lettuce. Tipburn appears to result from an imbalance in the soil/plant/water system which may also be linked with temperature variation and nitrogen status of the plant. Certain cultivars are mor e tolerant to this disorder. Phosphatic fertilizers encourage the production of firm heads in lettuce.


Postharvest Handling


Lettuce is a perishable commodity and should be handled accordingly. The key to successful delivery of fresh lettuce to the markets depends upon immediate removal of field heat and be kept under proper temperature and humidity conditions. Usually grower s on neighboring islands, away from the Honolulu markets, vacuum cool harvested lettuce. Recommended storage temperature for all lettuces and endive is 32 degrees F and 95% relative humidity. Expected storage life --- 2 - 3 weeks.

The following are some common postharvest disorders in head lettuce, some due to pathogen infection (bacterial soft rot and grey mold), others related to physiological changes:

Most of these postharvest storage disorders can be minimized by carefully managing the storage environment, ventilating to avoid gas buildup, avoiding overmature heads and maintaining uniform temperature and humidity. Use of perforated polyethylene wraps for lettuce in storage or transit reduces weight loss and storage defects, thereby increasing salability.


Seeding to Harvest


Estimated Yield

Harvesting

Head lettuce should be harvested when the heads become hard but yields slightly to pressure. Overmature heads are characterized by yellowing of leaves, cracked ribs, bitter flavors and such heads are more susceptible to postharvest disorders. Immature heads that are spongy may not withstand the marketing process well. Head lettuce do not usually reach maturity uniformly and hand labor is needed to select those ready for harvest.

Semi-head, leaf lettuce and endive should be harvested when the heads or plant size reaches its maximum size close to its suggested days to maturity and before it begins to bolt.

In harvesting, the heads or plants should be cut at the soil surface, leaving as many of the wrapper leaves uninjured as possible. To minimize wrapper leaf damage from harvesting, a crop should not be cut when the heads are wet. The soiled and spoiled l eaves on the base of the head should be removed before packing. Head lettuce is usually field packed in cardboard cartons having two tiers of 12 heads. Semi-head, leaf lettuce and endive are usually packed in containers based on weight rather than count .

If heads are washed to remove soil particles, it should be placed with the butt ends up to allow excess water to run out.

All heads (butts) showing traces of disease infection should be discarded.


Postharvest Handling

Lettuce is a perishable commodity and should be handled accordingly. The key to successful delivery of fresh lettuce to the markets depends upon immediate removal of field heat and be kept under proper temperature and humidity conditions. Usually grower s on neighboring islands, away from the Honolulu markets, vacuum cool harvested lettuce. Recommended storage temperature for all lettuces and endive is 32 degrees F and 95% relative humidity. Expected storage life --- 2 - 3 weeks.

The following are some common postharvest disorders in head lettuce, some due to pathogen infection (bacterial soft rot and grey mold), others related to physiological changes:

Most of these postharvest storage disorders can be minimized by carefully managing the storage environment, ventilating to avoid gas buildup, avoiding overmature heads and maintaining uniform temperature and humidity. Use of perforated polyethylene wraps for lettuce in storage or transit reduces weight loss and storage defects, thereby increasing salability.


Insect Pests


Agrotis ipsilon (Hufnagel), Black Cutworm

Bemisia tabaci (Gennadius), Sweetpotato Whitefly

Chrysodeixis chalcites (Esper), Green Garden Looper

Empoasca solana (DeLong), Southern Garden Leafhopper

Frankliniella occidentalis (Pergande), Western Flower Thrips

Heliothis zea (Boddie), Corn Earworm, Tomato Fruitworm, Bollworm

Hylemya (Delia) platura (Meigen), Seedcorn Maggot

Leucothrips pierci (Morgan), A Thrips

Listroderes costirostris obliquus (Klug), Vegetable Weevil

Macrosiphum euphorbiae (Thomas), Potato Aphid

Melanagromyza splendida Frick, Safflower Stemminer

Myzus persicae (Sulzer) Green Peach Aphid

Pycnoderes quadrimaculatus, Bean Caspid

Spodoptera exigua (Hubner), Beet Armyworm

Tetranychus cinnabarinus (Boisduval), Carmine Spider Mite

Tetranychus neocalidonicus Andre Vegetable Mite

Thrips nigropilosus Uzel, Chrysanthemum Thrips

Trialeurodes vaporariorum (Westwood), Greenhouse Whitefly

Trichoplusia ni (Hubner), Cabbage Looper


Planting


Propagation Method= Mostly transplanted into field from call-type seedling trays at 2 - 3 week stage.

Planting Schedule (See: adapted cultivars)

Spacing (inches)

Between Rows Between Plants
a.Head Type 15 - 18 12 - 15
c.Semi-head Type* 8 - 12 8 - 12
d.Leaf Type(non-heading) 15 - 18 10 - 12
e.Endive Type(Escarole) 15 - 18 8 - 12
* "Anuenue" suggest 12 inch spacing


Amount of Seed

Transplanted fields usually require approximately 1-1/2 lbs.

Direct seeded fields usually require approximately 3 lbs.

Seeds require a period of dry storage before sowing and some cultivars which have been stored at high temperatures have a requirement for exposure to light before dormancy is broken. Good seeds usually germinate in 3 - 5 days under optimum conditions (75 degrees F).


Seeding Depth

1/4 to 1/2 inch, heavier soils require less depth.


Transplants and Transplanting

Seedlings for transplanting used to be those that were started by sowing seeds rather thickly; then, when the first true leaves are fairly well developed, it is then transplanted into another tray spaced uniformly 1 to 2 inches apart. Or seeds germinated rather thickly in flats, allowed to grow about 2 inches then transplanted "bare-rooted" into the field.

Today, most lettuce seedlings are started by sowing in flats that have various set spacings of "cell-type" cavities. The seeds are dropped into each cavity by either a vacuum system or by using a double sheet sliding plexiglass seeder. Four or five days after seeding, each cavity is thinned to have one plant. Usually these trays are suspended on pipe or T-bar racks which allows for each cavity's roots to be air pruned. Air-pruned roots will have an immediate start in establishing a transplan ted seedling.

Transplanting into the field is normally done manually or semi-manually. Semi-manual transplanting includes planters riding on platforms close to the ground that cut furrows in the soil and seedling blocks are set in these furrows or dropped in a timely fashion to establish proper plant spacing.

The establishment of a block transplanted lettuce is basically dependent upon its ability to obtain moisture, its "hardened" condition and its ability to establish a root mass to absorb moisture and available nutrients. It may be beneficial for a grower to apply about four ounces of "starter solution" to the newly transplanted seedling to stimulate rapid establishment. The starter solution is made by dissolving six pounds of a high-analysis water-soluble fertilizer in 100 gallons of water.