Lettuce Transplanting – A Potential Late Summer Planting Strategy for Yuma Iceberg Lettuce Production

Transplanting is a widely used strategy for rapid establishment of the stand in the field under unfavorable climate conditions and to achieve earlier yields than that with direct seeding. Yuma Valley, Arizona, is the major winter lettuce production site in the US (15,978 ha) and producers currently use direct seedling followed by manual thinning as a standard practice. Direct seeding earlier than September is problematic as it likely results in undesirable bolting due to exposure to high temperature and long photoperiod. Therefore, use of seedlings grown under relatively cooler climate inside a controlled environment facility or in a selected geographical location may delay bolting so that lettuce heads could be harvestable before plants exhibit bolting.  A preliminary experiment was conducted for August 15 – December 2, 2014 using a half-acre field plot at Yuma Agriculture Center. Iceberg lettuce seeds of two cultivars (Syngenta 7101A and Vanguard 1221) were sown in 128 cell-trays containing peat-based substrate on August 15; grown to develop 4-5 true leaves inside a walk-in growth room (20.4±1.8°C average air temperature, 245±35 micro-mol·m^-2·s^-1 photosynthetic photon flux with white LEDs, and 12-h photoperiod); and transplanted in the soil beds conventionally prepared in the experimental field plot on September 12, 2014 (T-815 plants, hereafter).  Direct seeding was performed on August 15 and September 12 (D-815 and D-912 plants, hereafter), followed by thinning three weeks after seeding. Standard field lettuce cultivation practices were applied afterwards.  All T-815 plants survived to harvest while some loss was observed for D-815 plants at the end of September. D-815 plants and T-815 plants were matured to harvest on November 7, approximately one month earlier than the conventional D-912 plants (harvested on December 2). Both D-815 and T-815 lettuces exhibited core extension at the time of harvest but D-815 had significantly greater core length (5.5±0.25 cm) than T-815 (4.6±0.09 cm). A small percentage of D-815 plants showed advanced stage of bolting with extended flower stalks. Harvested lettuce fresh weight was also significantly greater for T-815 (4.6±0.07 kg for a box of 12 heads) than D-815 plants (3.8±0.12 kg per box).  However, while transplanting showed some merit in achieving earlier yield and mitigating bolting at the early seeding date (August 15) examined in this experiment, the highest yield and head quality were achieved for the D-912 plants, suggesting that use of transplants need to be evaluated over various planting dates.