CULTIVATION, STORAGE & PACKAGING

Preharvest:
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 (45°F at night is considered ideal).

Optimal temperatures for iceberg ranges from 60 to 65°F, though it can tolerate warmer temperatures. Continued heat at temperatures over 70°F can cause plants to bolt and leaves will turn bitter. Hardened plants can resist some extreme temperatures, but late crops should be planted about 80 days before the first 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 with additional applications of nitrogen once or twice during the growing season.

Late seed planting can result in a failed crop if temperatures rise significantly 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. Depending on the region, effective irrigation methods include canals, sprinklers, drip irrigation, or furrow irrigation. 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.

Postharvest:
Butter lettuce can be harvested 60 to 70 days after seeding. 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. Romaine lettuce will be ready 70 to 75 days after seeding. Cut firm, mature heads, leaving a few of the outer leaves as a protective wrapper.

All types of lettuce are harvested and packed in the field; heads are placed into waxed cartons in counts of 20 to 24. To avoid crushing, 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, butter and romaine hearts, 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. Hydrocooling and forced-air cooling are also acceptable. 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.

Grades:
Grades are divided into U.S. Fancy, U.S. No. 1, and U.S. No. 2 for lettuce. All three grades stipulate heads should have similar varietal characteristics, be well trimmed, and free from decay and insects. Lettuce meeting the U.S. No. 1 grade will be less damaged and better trimmed than U.S. No. 2 lettuce, with no softness or bruising. Lettuce meeting the U.S. Fancy grade will also be free from russet spotting and decay, with no injuries caused by tipburn, downy mildew, field freezing, or discoloration.

References: NC State University Cooperative Extension, UC Davis Western Institute for Food Safety and Security, University of Arizona Cooperative Extension, University of Maryland College of Agriculture & Natural Resources, USDA.

GOOD ARRIVAL GUIDELINES

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.

LEAF LETTUCE

U.S. Grade Standards Days Since Shipment % of Defects Allowed Optimum Transit Temp. (°F)
12-6-3 5
4
3
2
1
15-9-5
15-8-4
14-7-4
13-7-3
12-6-3
32

HEAD LETTUCE

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 MOVEMENT & PRICES, USA

Weekly Movements and Prices, USA

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

INSPECTOR’S INSIGHTS

• 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, www.ipt.us.com.

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