Lettuce is difficult to plant to stand, particularly in early spring and again during mid-summer periods of high soil temperature. Transplanting greenhouse-grown seedlings may offer the advantage of nearly ideal stands and even permit one extra crop during the growing season. Other possible advantages which may offset the cost of raising transplants include elimination of thinning equipment or labor, reduced herbicide use or cultivation, and production of a more uniform head of lettuce.
Methods
Ithaca crisphead lettuce was seeded in Speedling trays with one inch cells on March 15, 1979. The plot area was prepared for seeding on April 24. Pronamide at one pound/acre and 1,000 pounds/acre of 10-20-10 were incorporated into the surface three inches of soil. Seeding and transplanting took place on May 1. The direct seeding was with a Planet Jr. and transplanting was with a Jetspeed HydroSynchron three-row transplanter. Direct-seeded plots were thinned by hand to obtain the best stand possible. Transplants were not graded before planting but were slightly overmature and were topped at three inches with a rotary blade, resulting in a high degree of uniformity at time of transplanting. Plots were arranged in a randomized block design with six replications per treatment. Plot size was 20 feet x three rows with two feet between rows and nine inch ideal in-row-spacing. Transplants were harvested on June 15 and direct-seeded plants were harvested on July 12.
Results
Table 10. Yield of Ithaca Lettuce as Affected by Direct Seeding vs. Transplanting Treatment Yield, tons/acre Mean head weight, pounds C.V.Z ____________________________________________________________________________________________________________ Transplant 22.5 1.65 14.8 Direct seed 18.2 1.22 31.8 ZC.V. = std. deviation of mean x 100%/mean head weight.
Transplanting with the HydroSynchron machine and Speedlings resulted in a 100 percent stand. Direct-seeded plants were thinned to the same number of plants/plot but between neighbor distances were irregular. Transplants out-yielded direct-seeded by over four tons/acre but this was primarily because of greater average head weight at harvest. The difference in head weight was from greater maturity of transplants when harvested.
The coefficient of variation (C.V.) for transplanted heads was far smaller than for direct-seeded heads, reflecting a far greater degree of uniformity in head size for transplants. However, head size variability is known to decrease with increasing maturity, and the C.V. for direct-seeded lettuce might have been the same as for transplants if the direct-seeded heads had achieved the same mean size. Transplants were harvested 17 days earlier than direct-seeded plants and were at least seven days more mature. Thus, transplanting requires at least three weeks less field time per crop and could allow three crops per season rather than two.