Vegetable Seed Production: Lettuce

You are here: Seed Production: Asteraceae : Lettuce

Common Name: Lettuce Scientific Name: Lactuca sativa Family: Asteraceae

In addition to reading the information in this section, please view this narrated video on Lettuce (The video will open in a new window.)

Botany

Lettuce is a cool season annual crop, tolerant of frost (as low as 25°F for well-hardened seedlings) when immature but damaged by frost as it nears harvest maturity. Growth is extremely slow when temperatures are below 45°F. Optimum daily temperatures for growth are 65 to 70°F and night temperatures should be in the range of 45-50°F. Romaine lettuce and leaf lettuce are slightly more tolerant of high temperatures than crisphead and butterhead types. High temperatures cause tip burn (necrosis of the leaf margins which leads to head rot), bolting in some cultivars, small poor quality heads that form prematurely and lack density.

Genus Latuca

Tall annual and perennial herbs, mostly from the northern hemisphere, about 100 species, most are weeds; Leaves are alternate and variable and flower heads borne on small long irregular panicles. Achenes oval to linear, flat, plainly 3-5 ribbed on either side, a soft, thin white or brown pappus is elevated on a beak.

Annual, erect smooth herb grown for its crisp edible highly developed radical leaves, which appear before the flower stalk; Flower heads are erect with 12-16 flowers with yellow rays, open in forenoon. The achenes are oblong, broadest towards the apex, and straw-colored or black.

Var. asparagina - Asparagus or Stem Lettuce

Grown for its thick edible stem, not forming a head (not a major crop in the US)

Var. capitata - Head Lettuce

(Crisphead and butterhead types) rosette very dense and cabbage-like

Var. crispa - Curled Lettuce

(Loose-leaf lettuce) loose rosette of cut, fringed or crisped leaves

Leaf Lettuce

Var. longifolia - Cos (Romaine) Lettuce

Upright head and narrow or columnar, leaves ovate to oblong, obtuse 8-12 inches with broad midrib

Romaine Lettuce

Soil Nutrition

Lettuce can be grown on a wide range of soil types. It does best on muck soils, or well drained sandy or silt loam soils that have high organic matter, are not prone to crusting, and have a pH from 6.0 to 6.8. Lettuce should be rotated with unrelated crops to prevent the buildup of soil borne diseases and pests.

Isolation

For the production of certified seed a period of three years should have elapsed from a previous lettuce seed crop produced on the same site or two years from a previous market crop. Although up to 5 per cent cross-pollination has been observed in lettuce in some areas most authorities regard it as a self-pollinating crop and only specify a physical barrier (e.g. adjacent sections of greenhouses) or a minimum of 2 m between different cultivars.

Planting

Lettuce is established by both transplants or direct seeding.

In California, much of the lettuce production is direct-seeded using precision seeders and pelleted seeds. In many cases the crop can be precision seeded to a final stand but thinning may also be necessary to achieve the optimum plant population in some situations. In many other locations like New York, Ohio, and Michigan, lettuce is grown from transplants.

Stand establishment is a major production concern. Lettuce seeds are initially dormant at harvest and should not be planted for at least 2 months or until the seeds have ripened and are fully germinable and vigorous.

In addition, the germination of lettuce seed can be inhibited by high temperatures. Exposure to temperatures above 75°F for as little as 24 hours inhibit seed germination. Generally the higher the temperature the greater number of seeds in the population are effected and the deeper the dormancy.

The Grand Rapids cultivar has a phytochrome triggered dormancy mechanism (red light induces dormancy). However, this dormancy has been bred out of most modern cultivars.

Lettuce is a self-pollinated crop with tiny flowers that produce only one seed (achene) per flower. Because of this, no F-1 hybrid cultivars have been developed. All lettuce cultivars are non-hybrid, self-pollinated purelines.

For commercial lettuce production in the West, seed is usually sown on beds that are centered 40 inches apart. The top of the bed is generally 24 inches wide and the rows are spaced at the edges of the bed. In this system, double rows are spaced 12 to 20 inches apart depending on the type of lettuce and cultivar grown. Leaf lettuces can be spaced closer together than heading types. In-row spacing varies with cultivar and ranges from about 2 inches for leaf types to 12 to 18 inches for crisphead cultivars.

Irrigation

Since lettuce has a shallow root system plants must receive a consistent supply of water throughout the growth period.

Drought stress particularly in combination with high temperature can cause leaf tip burn caused by a calcium deficiency. Too much water can cause soft puffy heads with reduced storage life. Excessive irrigation near harvest can also result in splitting of crisphead lettuce. The recommendation for one inch of water every 7 to 10 day applies.

In Western states, all lettuce is irrigated. Early in the season, the crop is sprinkler irrigated to activate the fertilizer in the bed, prevent crusting, and insure uniform hydration for good seed germination or transplant establishment. Once the crop is established, furrow irrigation is generally used. On muck soils in the Midwest and east sprinkler irrigation is generally used. In Florida, both seepage irrigation and fertigation are used to grow the crop.

Roguing

There are three main stages for roguing and selection, which are:

1. The young plant during the four- to six-leaf stage.
2. The mature plant at time of heading.
3. After bolting has commenced.

There is an additional earlier stage in the life of the plant when it is still a seedling. In practice this is only used when the plants are propagated prior to planting out, or if relatively few seedlings are being examined.

The most important stage is at the time of heading; in practice this is usually the only stage at which commercial seed crops are rogued.

The features used in the assessment of trueness to type of lettuce seed crops are mainly based on morphological characters observed during the vegetative stages of plant growth up to and including hearting.

Seed Harvest

The succession of inflorescences during anthesis subsequently provides a steady sequence of ripe seed. The length of time from flowering to ripe seed produced on an individual capitulum is between twelve and twenty-one days, depending on the environment. High temperatures speed up this relatively fast rate of seed development.

If the seed producer waits for the development and ripening of the seeds from the later inflorescences, the earlier ripened ones will probably have been lost. It is therefore general practice to cut and harvest the seed when an estimated 50 per cent of seed heads are ready on a typical sample plant. The stage of ripeness at which the pappus is fully developed and dry is referred to as "feathering".

Lettuce seeds

The standing lettuce seed crop can be hand or machine cut and put in windrows. A machine which causes the minimum of shattering should be used for the cutting process. There is less shattering if the plants are cut with dew on them. The cut material is normally left in windrows for up to five days, but in very arid areas the seed is extracted the same day as cutting. After windrowing the seed is extracted in stationary threshers or passed through a combine. If the seed is combined direct from the standing crop many small pieces of wet plant debris, such as fragments of leaf bract, will be mixed with it; this causes an undesirable increase in seed moisture unless the material is dried quickly.

In production areas with plenty of available labor, the seed can be harvested from single plants by shaking their heads into a canvas bag or sack. If this is done every two or three days the maximum seed yield is collected.

Cleaning

Pre-cleaning vegetable seeds of Compositae is an important operation because the weight of chaff in mechanically harvested seed can be as high as one and a half times the weight of the seed itself. The initial cleaning can be done with an air-screen machine. There is usually some plant material such as small pieces of flower stem remaining in the sample; these can be removed by passing the seed through a disc separator or an indent cylinder.

Seed Yield

A satisfactory seed yield for hearting lettuce under good conditions is between 0.5 and 1 ton per hectare. (446 pounds per acre)

Seed Identification: