Lettuce and Leafy Greens (greenhouse)
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Lettuce, a member of the daisy family Asteraceae, was originally domesticated in Egypt several millennia ago. It proliferated throughout the ancient world and remains popular today, its production second only to potatoes among fresh vegetables in the United States.
Americans annually consume 30 pounds of lettuce per capita, a fivefold increase from the previous century. The ever-increasing selection of bagged salad mixes has added immeasurably to this growth.
Although the majority of lettuce produced and consumed in the United States is field grown, the market for greenhouse lettuce, leafy greens, and herbs continues to expand. A number of states are experiencing growth as significant producers, especially in the Northeast and Midwest. Imports contribute to supply, but to a far lesser degree.
Types & Varieties
Greenhouses are excellent venues to produce a broad array of lettuce, leafy greens, and herbs.
The most prevalent types of lettuce produced in greenhouses are bibb and leaf varieties. Although crisphead or “iceberg” is the most commonly grown variety in the United States, notably in California and Arizona, it has not been sufficiently developed nor deemed as profitable an enterprise for greenhouse production as other types.
Bibb lettuce, also known as butterhead or Boston lettuce, dominates much greenhouse lettuce production. Characterized by its delicacy, mild or sweet flavor, ruffled external appearance, and light green interior, bibb is harvested at full-head size (except for specifically bred baby varieties).
Cegolaine, Rosaine, Roxy, and Mirlo are typical varieties for greenhouse production.
Leaf lettuce includes both red and green varieties, with loose, thin, colorful leaves, ranging from delicate to slightly bitter depending on variety. Waldmann’s Dark Green, Grand Rapids, and Ruby Red are common greenhouse types.
Arugula, kale, spinach, romaine, and many baby varieties are also cultivated in greenhouses to varying degree.
|Greenhouses large and small throughout the United States and in other countries provide a steady supply of year-round lettuce and leafy greens.|
Numerous methods exist for cultivating lettuce in greenhouses. The most popular techniques include raft systems, which involve Styrofoam boards suspended in nutrient solution; aeroponic systems, which entail exposing the plant roots and misting them with nutrient solution to conserve water; and hydroponic systems, which grow plants in water and nutrient solutions absent of soil.
After seeding, lettuce typically requires one month before transplanting. The timeline from seeding to harvest will range from 8 to 10 weeks in spring and 12 to 15 weeks in winter.
For maximum yield, low light intensity and cool temperatures constitute optimal conditions for greenhouse production. Temperature ranges of 60 to 65°F in the day and 50 to 55°F should be maintained, since higher temperatures will stunt development.
Artificial lighting is discouraged because its monetary and energy expenditure surpass the marginal value attained from increased growth rate.
Lettuce requires ample and carefully proscribed nutrition.
The pH of the solution should range from 5.0 to 6.0 and maintain a salt level below 3500 ppm (parts per million).
These two elements most prominently affect greenhouse lettuce development and should consequently be evaluated regularly. Application of the solution should cover the entire root.
Pests & Diseases
Diseases to which greenhouse lettuce is commonly susceptible can be classified into four categories—bacterial, fungal, viral, and non-infectious.
Bacterial diseases include butt (head) rot and stem rot, among myriad other infections. The former produces black coloration on the plant’s stem and lower leaves and the latter brown coloration and streaking on the lower leaves. Proper heating, ventilation, and sanitation constitute the best preventative measures.
Fungal diseases include ring spot, bottom rot, damping-off, downy mildew, drop, grey mold, and powdery mildew. These all result in discoloration and/or lesions on plants in overly humid and/or unsterile conditions, leading to severe damage or death of the plant.
The most common viral diseases are cucumber mosaic (which still can afflict greenhouse lettuce despite its name) and lettuce mosaic. Both viruses stunt plant development and produce yellow mottling and are transmitted by aphids.
Tipburn, which causes leaf browning, is the predominant noninfectious disease for greenhouse lettuce. Studies have debunked the common misunderstanding that tipburn results from calcium deficiency.
For pests, aphids, caterpillars, and slugs pose the greatest threat to greenhouse lettuce through consuming leaves, burrowing into the plants, and circulating other diseases.
Maintaining proper moisture and humidity levels while mitigating insect controls, such as constructing physical barriers to plants provide the optimal means of protection. Temperature control will prevent freezing injury and related conditions that will affect sale.
Shipping & Packaging
Lettuce with discoloration or bruising, foreign material or dirt, and russet spotting should not be included in lots.
Greenhouse leaf lettuce is packed in various sized bags or clamshells, or can be shipped in waxed cartons or crates. Newer packaging using recycled and recyclable materials are gaining favor as well.
References: Texas A&M AgriLife Extension, University of Florida/IFAS Extension, Colorado State University, University of California Davis, University of Kentucky Extension, Alberta Government.
GRADES & GOOD ARRIVAL
Updated grade information for greenhouse leaf lettuces from the USDA was released in September 2016.
There are two grades, U.S. Fancy and U.S. No. 1, and both stipulate lettuce leaves should have similar varietal characteristics, be well grown and well trimmed (fairly well trimmed for U.S. No. 1), free from decay or injury (free from damage for U.S. No. 1), free from blanching or discoloration, as well as wilting, dirt, disease, and insects.
Tolerances for U.S. Fancy and U.S. No. 1 by lot are 10% total defects, including 5% serious damage, and 1% decay. For more than 15 plants per package, the tolerance is 10-5-2; for 15 plants of less per package, it is 20-10-2.
There are not separate good arrival guidelines specific to Canada for greenhouse lettuce.
References: DRC, PACA, USDA.