Growth and Yield of Sequentially Transplanted Lettuce in an Organically Managed High Tunnel

The use of high tunnels is becoming an integral part of extended and out-of-season vegetable production.  Lettuce is one of the vegetable species that can be grown profitably in high tunnels but the ideal time of planting in North Carolina (headiness zone 7b) has not been studied. The objective was to identify the appropriate planting time of head lettuce for winter/spring and fall/winter production seasons in high tunnels. Organic lettuce varieties Nancy, Spretnak, and Rhazes were seeded in a greenhouse and transplanted into a 30’ x 96’ Quonset high tunnel at three sequential dates (TD1, TD2 and TD3) in spring and again in fall 2014. The three transplanting dates were February 27, March 11, and March 20 in spring; and October 17, October 27, and November 6 in fall.  The experiment was conducted as a completely randomized block design with three replications, 30 plants per replication.  Lettuce heads were harvested at marketable mini head size.  Microclimate data were measured with Spectrum WatchDog inside and outside the high tunnel. In the spring season, the number of days from transplanting to the first harvest for TD1 (44.1) was significantly higher than that for TD2 (35.6) or TD3 (35.1).  For the fall production cycle, days to harvest after transplanting was 44.7 days for TD1, 43.3 days for TD2, and 45 days for TD3.  The highest head weight was TD2 for ‘Nancy’ and ‘Rhazes’ (0.13 kg and 0.12 kg, respectively) and TD3 for ‘Spretnak’ (0.11 kg) in the spring season; while in the fall season it was TD1 for all varieties, which was 0.15 kg for ‘Nancy’, 0.10 kg for ‘Rhazes’, and 0.16 kg for ‘Spretnak’.  Our results suggest that TD2 may be appropriate for spring production to catch early market and TD3 be ideal for fall production to catch late market. Calculation from growing degree days at 4.4 °C base temperature supports this preliminary conclusion.  This study will be replicated in 2015 to draw more comprehensive conclusions.