Overall, lettuce is hardy, growing well in cooler temperatures and resisting frost and cold. Leaf varieties grow quickly, maturing within 50 days, and grow well in cool weather. Some butter varieties take up to 75 days to mature and can tolerate different soil and weather conditions. Romaine, because of its vertical growth, grows well in tighter rows and can also tolerate a variety of weather conditions. Iceberg, which takes up to 80 days to mature, is the least tolerant. To cultivate firm heads, cooler conditions are needed.

Optimal temperatures range from 60 to 65°F. Although lettuce can tolerate periodic temperatures in the 80s, plants will tend to seed and bolt at temperatures over 70°F and leaves can turn bitter. Hardened plants can resist some extreme temperatures, however, late crops should be planted about 80 days before the first expected hard freeze.

Seeds will germinate at temperatures as low as 35°F, but perform best at about 70°F. Lettuce is best planted in early spring in loose, fertilized soil with a pH of 6.0 to 6.7. Fertilizer should be applied before planting and additional applications of 20 to 25 pounds of nitrogen per acre can be applied once or twice during the growing season. Late seed planting can result in a failed crop if the temperature rises too high before harvest. Seeds can be planted in rows 30 to 42 inches apart and covered with topsoil. Start to thin the rows when the plants are about 2 inches in height, 8 to 12 inches apart for most varieties.

Summer and fall crops need to be protected from heat and too much sunlight. With shallow roots, lettuce plants need sufficient water. Mulch and groundcovers can help conserve moisture in the soil. Insufficient water will impact quality, so irrigation is recommended. Beds should be weeded to reduce competitors for water and the risk of pests and diseases. Also, late crops, because of soil movement in the field, can get soil into the heads and leaves.

Leaf lettuce will be ready for harvest 50 to 60 days after seeding or 30 to 45 days after transplanting. Iceberg can be harvested 70 to 80 days after seeding or 60 to 70 days after transplanting. Cut firm, mature heads, leaving a few of the outer leaves as a protective wrapper. Iceberg harvest is done in the field and heads are packed into a waxed carton in counts of 20 to 24. To avoid crushing, flat pack cartons in two layers: a bottom layer stem down and a top layer stem up to keep the stem milk from damaging the leaves. Some varieties, including leaf and butter, are put in bags and packed 24 to 36 per carton.

Depending on the crop, harvest mature heads about every 2 to 3 days. Good crops yield about 400 to 500 crates of iceberg or 800 to 1,000 crates of leaf lettuce per acre. Harvested lettuce must be cooled prior to shipping. Vacuum cooling to quickly lower head temperatures to 34 to 38°F is recommended. Cartons of iceberg stored at 32°F and 95% relative humidity will last 2 to 3 weeks, sometimes longer if the head is bagged and the leaves are dry. Leaf and butter lettuces will last 1 to 2 weeks in cold storage.

References: University of Arizona Cooperative Extension; University of Maryland College of Agriculture & Natural Resources; NC State University Cooperative Extension.


Generally speaking, the percentage of defects shown on a timely government inspection certificate should not exceed the percentage of allowable defects, provided: (1) transportation conditions were normal; (2) the U.S. Department of Agriculture (USDA) or Canadian Food Inspection Agency (CFIA) inspection was timely; and (3) the entire lot was inspected.


U.S. Grade Standards Days Since Shipment % of Defects Allowed Optimum Transit Temp. (°F)
12-6-3 5


U.S. Grade Standards Days Since Shipment % of Defects Allowed Optimum Transit Temp. (°F)
12-6-3 (all days) 15-9-5 32

Canadian good arrival guidelines (unless otherwise noted) are broken down into five parts as follows: maximum percentage of defects, maximum percentage of permanent defects, maximum percentage for any single permanent defect, maximum percentage for any single condition defect, and maximum for decay. Canadian destination guidelines for head lettuce are 15-10-10-15-5.

References: DRC, PACA, USDA.


Weekly Movements and Prices, USA

Source: Chart by Gallo Torrez Agricultural Price Trends (GTAPT),, compiled from USDA data.


• Tipburn (external or internal) is scored as a defect if affecting an area greater than a half-inch by one inch
• Russet spotting is scored as a defect when present on more than two head leaves, or when more than five spots are found on any individual leaf
• Yellow to brown discoloration from field freezing is scored as a defect when affecting more than 50% of the surface over the crown
• A head of lettuce is scored as a defect if one live or dead worm is found in the head leaves; yet up to five live or dead aphids or similar insects are allowed in the headleaves before being scored as a defect.

Source: Tom Yawman, International Produce Training,

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