Seed to Seed

Lettuce, a member of the Compositae family, is believed to be one of the first vegetables brought to the new world by explorer Christopher Columbus and has been grown in the United States since colonial times. Today, lettuce is the leading vegetable crop in the United States with more than 95 percent of the winter U.S. lettuce production located in the desert Southwest. In Yuma, lettuce production has risen 25% since 1992, and the soaring popularity of romaine lettuce (a staple of Caesar salads and bagged salad mixes) has led to a 162% production increase in the past 12 years. Iceberg lettuce is grown from seed for its succulent leaves and forms a distinctive head that is harvested several months after planting. While most link Iceberg lettuce with a salad or sandwich accessory, have you ever wondered about the nature of its growth and development?

Head lettuce passes through 8 botanically distinct stages of growth: seed, cotyledon, seedling, rosette, cupping, heading, flowering and seed maturation periods.
• The seed stage occurs from pre-planting to seedling emergence. Exposed to water and the appropriate environmental conditions, lettuce seeds will begin to germinate. Depending on soil temperature, germination can occur in as little as 12 hours (fall lettuce) to as long as 1 week for lettuce planted in the colder winter months.
• Once the plant sheds the seed coat and emerges from the soil, it enters the cotyledon stage. The cotyledon stage will last while the plant establishes its root system, from 7 to 20 days depending on soil temperature. sustain
• The seedling stage coincides with the first true leaf and continues until the plant develops a distinct circular cluster of leaves known as a rosette. It is at this stage when the lettuce is thinned to spacing of between 10 and 12 inches.
• The rosette stage for fall planted lettuce will generally last 25 days, but may last as long as 50 days for winter planted lettuce. Plants can be fertilized by sidedressing equipment at this developmental stage.
• Cupping begins when the tips of the inner leaves begin to curl inwards on the edges. Cupping signifies that the beginning of head formation is near, and will usually last about 7 days for fall planted lettuce and 14 days for winter planted varieties.
• Heading begins once the cupped leaves begin to overlap and cover the growing point of the plant and will continue until the crop is ready for harvest. Fall planted lettuce may require as little as 65 days from the beginning of germination to harvest, while winter planted lettuce will require as long as 120 days.
• About a month after the head forms, if lettuce is not harvested or when the correct environmental conditions are in place, the stem within the head bolts or elongates and branches to produce the inflorescence, which can range from 2 to 4 feet high.
• The seed is produced by the flowers of the inflorescence. The terminal portion of the inflorescence is primarily a panicle or cluster of yellow flowering heads which, after pollination, later form seeds with an attached pappus. In its natural setting, the pappus aids in wind dispersal like many of its Compositae relatives. Commercial lettuce seeds are pelleted with inert materials to enhance its size and shape for improved plantability.

All lettuce stages of lettuce growth are part of the delicate balance between timing, production and the environment. By knowing how any management activity will affect the plant at a particular growth stage, growers can make proper decisions that result in the greatest yields and the highest quality.