High tunnel (HT) coverings can reduce light intensity and spectral quality, thus negatively affecting pigmentation and accumulation of several important phytochemicals in lettuce. The objectives of this study were to evaluate the effect of six HT coverings and two growing seasons (spring vs fall) on the red ‘New Red Fire’ and green ‘Two Star’ lettuce at harvest and after 5 days of storage with respect to leaf color and phenolic compound accumulation. Total phenolic content, anthocyanin accumulation, major individual phenolic acids and flavonoid compounds were measured. Chlorogenic and chicoric acid were the most prevalent phenolic compounds in both green and red lettuce. The phenolic compound accumulation in the red lettuce was significantly greater in the spring than in the fall for all measured compounds other than caffeic acid and anthocyanin. In the spring, the flavonoid accumulation was higher under the movable covering for both red and green lettuce. Red lettuce grown under clear poly (clear) and standard poly removed 2 to 3 weeks prior to initial harvest (movable) had darker leaves during both seasons (P <.001), as well as greater anthocyanin accumulation compared to the shade poly (shade) during the spring. During that season, the isoquercetin concentration of the red lettuce was 72% higher under the movable coverings compared to the shade covering (P <0.01). The effect of covering and season on phenolic compound accumulation after 5 days of storage was inconsistent. This study indicates that secondary metabolites can be manipulated due to solar light with the use of various HT coverings.