Transplanting lettuce seedlings can insure a nearly perfect stand. In addition, transplanting offers the possibility of bringing the crop to market earlier than a direct-seeded crop, taking advantage of higher early season market prices and allowing marketing over a longer time period. Perhaps more importantly, transplanting may allow for multiple cropping. In the Willamette Valley, for example, transplanting would allow three rather than two crops of lettuce. Transplanting may also take place when soil conditions would not allow direct seeding. However, if weather conditions prevent transplanting on the scheduled date, planting can be delayed until conditions are more favorable.
Transplanting also offers the opportunity to start all plants in the field at the same stage of development, reducing variability in maturity. Greater uniformity at harvest reduces costs by reducing the number of harvests necessary to gather all marketable heads.
Optimal size and shape of transplant blocks for Oregon conditions are not known. The effects of variability in transplant size on variability in head size at harvest or time to maturity have not been widely studied. With the advent of automatic transplanters, it would be helpful to understand how variability at the time of transplanting will affect variability at harvest. This trial examined the effects of size of transplant block and grading of transplants by shoot size on yield and head size variability at harvest.
Methods
'Ithaca' lettuce was seeded approximately 2 millimeters deep in peat-vermiculite (1:1) mix in either 2 x 2 x 3 inch square cross-section plastic pots or in trays with cells of 0.8 x 0.8 x 1.2-inch dimensions (Growers Transplanting, Inc.). Pots were seeded on July 18, 1984, trays on July 18 and July 25, and placed in a glasshouse. Seedlings were watered daily with a solution of 9-45-15 fertilizer containing 100 parts per million nitrogen. On the August 14 transplant date, seedlings in trays seeded on July 18 were judged overmature; seedlings started on July 25 were used for field transplanting. Thus, pot-grown transplants were one week older than the tray-grown when set in the field. The plot area was plowed, disked, and 1,000 pounds/acre of 10-20-10 was broadcast and incorporated with the final tillage operation. Pronamide was applied at 1.5 pounds/acre after planting and irrigated in.
Plants in pots and trays were graded into three size categories (small, medium, large) based on 3 or less, 4-5, and more than 5 leaves present on pot-grown seedlings and 2 or less, 3, and more than 3 for tray-grown seedlings. These six treatments plus ungraded plants from each container type were set out on 3 foot x 1.5 foot spacing, with 10 plants/plot and four replications of each treatment in randomized block design. Plots were irrigated weekly and diazinon was applied once for diabrotica control. All plots were harvested on October 2. Each head was trimmed and weighed separately.
Results and Discussion
Plants grown in pots produced much larger heads than did those grown in trays (Table 1), caused in large part by the seven-day earlier seeding. However, more rapid establishment of the larger-rooted pot-grown plants also may have been a factor. Plants from trays appeared to be more than seven days behind in maturity. Regardless of container size, head size increased with increasing number of leaves present at time of transplant. Small transplants produced smaller mean head size than did ungraded transplants; medium and large transplants produced larger heads than did the ungraded. Most of the difference in seedling size at transplanting could be attributed to 1 or 2 days difference in emergence date. Apparently, these small differences are maintained through maturity of the head.
Variability in lettuce head size is known to decrease with increasing maturity, so it is not surprising that pot-grown seedlings produced less head size variability than did the younger tray-grown seedlings. However, the tray-grown seedlings appeared more uniform at time of transplanting than did the pot-grown seedlings. More plants fell into the medium category with tray-grown seedlings (72% vs. 60%).
Grading the transplant by number of leaves definitely decreased variability at harvest for pot-grown plants (Table 1, column 3). It appears that a mechanism to presort transplants by maturity/size could lead to greater uniformity and greater pack-out at first harvest.
For the immature heads produced from tray-grown transplants, only the largest size grade produced less head size variability that did the ungraded plants. The high degree of variability in the small size grade may indicate that the seedling root balls were too immature to plant.
Table 1. Interaction of seedling block size and grading of transplants on yield and head size variability of 'Ithaca' lettuce. Mean head Total yield Head sizeZ Treatment wt. (lb) (lb/ft2) C. V.(%) 2" pots, ungraded 1.63 0.36 28.6 ? small 1.56 0.34 21.9 ? medium 1.90 0.41 14.0 ? large 1.99 0.43 15.0 trays, ungraded 0.52 0.11 33.8 ? small 0.48 0.10 37.7 ? medium 0.54 0.12 33.1 ? large 0.65 0.14 29.3 LSD(0.05) 0.19 0.14 ZC.V.= coefficient of variation (standard deviation of mean x 100%/mean).